cmd/link: merge branch 'dev.link' into master

[gostls13.git] / src / cmd / link / internal / loader / loader.go

// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package loader

import (
        "bytes"
        "cmd/internal/bio"
        "cmd/internal/goobj"
        "cmd/internal/obj"
        "cmd/internal/objabi"
        "cmd/internal/sys"
        "cmd/link/internal/sym"
        "debug/elf"
        "fmt"
        "log"
        "math/bits"
        "os"
        "sort"
        "strings"
)

var _ = fmt.Print

// Sym encapsulates a global symbol index, used to identify a specific
// Go symbol. The 0-valued Sym is corresponds to an invalid symbol.
type Sym int

// Relocs encapsulates the set of relocations on a given symbol; an
// instance of this type is returned by the Loader Relocs() method.
type Relocs struct {
        rs []goobj.Reloc

        li uint32   // local index of symbol whose relocs we're examining
        r  *oReader // object reader for containing package
        l  *Loader  // loader
}

// ExtReloc contains the payload for an external relocation.
type ExtReloc struct {
        Xsym Sym
        Xadd int64
        Type objabi.RelocType
        Size uint8
}

// Reloc holds a "handle" to access a relocation record from an
// object file.
type Reloc struct {
        *goobj.Reloc
        r *oReader
        l *Loader

        // External reloc types may not fit into a uint8 which the Go object file uses.
        // Store it here, instead of in the byte of goobj.Reloc.
        // For Go symbols this will always be zero.
        // goobj.Reloc.Type() + typ is always the right type, for both Go and external
        // symbols.
        typ objabi.RelocType
}

func (rel Reloc) Type() objabi.RelocType { return objabi.RelocType(rel.Reloc.Type()) + rel.typ }
func (rel Reloc) Sym() Sym               { return rel.l.resolve(rel.r, rel.Reloc.Sym()) }
func (rel Reloc) SetSym(s Sym)           { rel.Reloc.SetSym(goobj.SymRef{PkgIdx: 0, SymIdx: uint32(s)}) }
func (rel Reloc) IsMarker() bool         { return rel.Siz() == 0 }

func (rel Reloc) SetType(t objabi.RelocType) {
        if t != objabi.RelocType(uint8(t)) {
                panic("SetType: type doesn't fit into Reloc")
        }
        rel.Reloc.SetType(uint8(t))
        if rel.typ != 0 {
                // should use SymbolBuilder.SetRelocType
                panic("wrong method to set reloc type")
        }
}

// Aux holds a "handle" to access an aux symbol record from an
// object file.
type Aux struct {
        *goobj.Aux
        r *oReader
        l *Loader
}

func (a Aux) Sym() Sym { return a.l.resolve(a.r, a.Aux.Sym()) }

// oReader is a wrapper type of obj.Reader, along with some
// extra information.
type oReader struct {
        *goobj.Reader
        unit         *sym.CompilationUnit
        version      int    // version of static symbol
        flags        uint32 // read from object file
        pkgprefix    string
        syms         []Sym    // Sym's global index, indexed by local index
        pkg          []uint32 // indices of referenced package by PkgIdx (index into loader.objs array)
        ndef         int      // cache goobj.Reader.NSym()
        nhashed64def int      // cache goobj.Reader.NHashed64Def()
        nhasheddef   int      // cache goobj.Reader.NHashedDef()
        objidx       uint32   // index of this reader in the objs slice
}

// Total number of defined symbols (package symbols, hashed symbols, and
// non-package symbols).
func (r *oReader) NAlldef() int { return r.ndef + r.nhashed64def + r.nhasheddef + r.NNonpkgdef() }

type objIdx struct {
        r *oReader
        i Sym // start index
}

// objSym represents a symbol in an object file. It is a tuple of
// the object and the symbol's local index.
// For external symbols, objidx is the index of l.extReader (extObj),
// s is its index into the payload array.
// {0, 0} represents the nil symbol.
type objSym struct {
        objidx uint32 // index of the object (in l.objs array)
        s      uint32 // local index
}

type nameVer struct {
        name string
        v    int
}

type Bitmap []uint32

// set the i-th bit.
func (bm Bitmap) Set(i Sym) {
        n, r := uint(i)/32, uint(i)%32
        bm[n] |= 1 << r
}

// unset the i-th bit.
func (bm Bitmap) Unset(i Sym) {
        n, r := uint(i)/32, uint(i)%32
        bm[n] &^= (1 << r)
}

// whether the i-th bit is set.
func (bm Bitmap) Has(i Sym) bool {
        n, r := uint(i)/32, uint(i)%32
        return bm[n]&(1<<r) != 0
}

// return current length of bitmap in bits.
func (bm Bitmap) Len() int {
        return len(bm) * 32
}

// return the number of bits set.
func (bm Bitmap) Count() int {
        s := 0
        for _, x := range bm {
                s += bits.OnesCount32(x)
        }
        return s
}

func MakeBitmap(n int) Bitmap {
        return make(Bitmap, (n+31)/32)
}

// growBitmap insures that the specified bitmap has enough capacity,
// reallocating (doubling the size) if needed.
func growBitmap(reqLen int, b Bitmap) Bitmap {
        curLen := b.Len()
        if reqLen > curLen {
                b = append(b, MakeBitmap(reqLen+1-curLen)...)
        }
        return b
}

type symAndSize struct {
        sym  Sym
        size uint32
}

// A Loader loads new object files and resolves indexed symbol references.
//
// Notes on the layout of global symbol index space:
//
// - Go object files are read before host object files; each Go object
//   read adds its defined package symbols to the global index space.
//   Nonpackage symbols are not yet added.
//
// - In loader.LoadNonpkgSyms, add non-package defined symbols and
//   references in all object files to the global index space.
//
// - Host object file loading happens; the host object loader does a
//   name/version lookup for each symbol it finds; this can wind up
//   extending the external symbol index space range. The host object
//   loader stores symbol payloads in loader.payloads using SymbolBuilder.
//
// - Each symbol gets a unique global index. For duplicated and
//   overwriting/overwritten symbols, the second (or later) appearance
//   of the symbol gets the same global index as the first appearance.
type Loader struct {
        start       map[*oReader]Sym // map from object file to its start index
        objs        []objIdx         // sorted by start index (i.e. objIdx.i)
        extStart    Sym              // from this index on, the symbols are externally defined
        builtinSyms []Sym            // global index of builtin symbols

        objSyms []objSym // global index mapping to local index

        symsByName    [2]map[string]Sym // map symbol name to index, two maps are for ABI0 and ABIInternal
        extStaticSyms map[nameVer]Sym   // externally defined static symbols, keyed by name

        extReader    *oReader // a dummy oReader, for external symbols
        payloadBatch []extSymPayload
        payloads     []*extSymPayload // contents of linker-materialized external syms
        values       []int64          // symbol values, indexed by global sym index

        sects    []*sym.Section // sections
        symSects []uint16       // symbol's section, index to sects array

        align []uint8 // symbol 2^N alignment, indexed by global index

        deferReturnTramp map[Sym]bool // whether the symbol is a trampoline of a deferreturn call

        objByPkg map[string]uint32 // map package path to the index of its Go object reader

        anonVersion int // most recently assigned ext static sym pseudo-version

        // Bitmaps and other side structures used to store data used to store
        // symbol flags/attributes; these are to be accessed via the
        // corresponding loader "AttrXXX" and "SetAttrXXX" methods. Please
        // visit the comments on these methods for more details on the
        // semantics / interpretation of the specific flags or attribute.
        attrReachable        Bitmap // reachable symbols, indexed by global index
        attrOnList           Bitmap // "on list" symbols, indexed by global index
        attrLocal            Bitmap // "local" symbols, indexed by global index
        attrNotInSymbolTable Bitmap // "not in symtab" symbols, indexed by global idx
        attrUsedInIface      Bitmap // "used in interface" symbols, indexed by global idx
        attrVisibilityHidden Bitmap // hidden symbols, indexed by ext sym index
        attrDuplicateOK      Bitmap // dupOK symbols, indexed by ext sym index
        attrShared           Bitmap // shared symbols, indexed by ext sym index
        attrExternal         Bitmap // external symbols, indexed by ext sym index

        attrReadOnly         map[Sym]bool     // readonly data for this sym
        attrTopFrame         map[Sym]struct{} // top frame symbols
        attrSpecial          map[Sym]struct{} // "special" frame symbols
        attrCgoExportDynamic map[Sym]struct{} // "cgo_export_dynamic" symbols
        attrCgoExportStatic  map[Sym]struct{} // "cgo_export_static" symbols
        generatedSyms        map[Sym]struct{} // symbols that generate their content

        // Outer and Sub relations for symbols.
        // TODO: figure out whether it's more efficient to just have these
        // as fields on extSymPayload (note that this won't be a viable
        // strategy if somewhere in the linker we set sub/outer for a
        // non-external sym).
        outer map[Sym]Sym
        sub   map[Sym]Sym

        dynimplib   map[Sym]string      // stores Dynimplib symbol attribute
        dynimpvers  map[Sym]string      // stores Dynimpvers symbol attribute
        localentry  map[Sym]uint8       // stores Localentry symbol attribute
        extname     map[Sym]string      // stores Extname symbol attribute
        elfType     map[Sym]elf.SymType // stores elf type symbol property
        elfSym      map[Sym]int32       // stores elf sym symbol property
        localElfSym map[Sym]int32       // stores "local" elf sym symbol property
        symPkg      map[Sym]string      // stores package for symbol, or library for shlib-derived syms
        plt         map[Sym]int32       // stores dynimport for pe objects
        got         map[Sym]int32       // stores got for pe objects
        dynid       map[Sym]int32       // stores Dynid for symbol

        relocVariant map[relocId]sym.RelocVariant // stores variant relocs

        // Used to implement field tracking; created during deadcode if
        // field tracking is enabled. Reachparent[K] contains the index of
        // the symbol that triggered the marking of symbol K as live.
        Reachparent []Sym

        flags uint32

        hasUnknownPkgPath bool // if any Go object has unknown package path

        strictDupMsgs int // number of strict-dup warning/errors, when FlagStrictDups is enabled

        elfsetstring elfsetstringFunc

        errorReporter *ErrorReporter

        npkgsyms    int // number of package symbols, for accounting
        nhashedsyms int // number of hashed symbols, for accounting
}

const (
        pkgDef = iota
        hashed64Def
        hashedDef
        nonPkgDef
        nonPkgRef
)

// objidx
const (
        nilObj = iota
        extObj
        goObjStart
)

type elfsetstringFunc func(str string, off int)

// extSymPayload holds the payload (data + relocations) for linker-synthesized
// external symbols (note that symbol value is stored in a separate slice).
type extSymPayload struct {
        name     string // TODO: would this be better as offset into str table?
        size     int64
        ver      int
        kind     sym.SymKind
        objidx   uint32 // index of original object if sym made by cloneToExternal
        relocs   []goobj.Reloc
        reltypes []objabi.RelocType // relocation types
        data     []byte
        auxs     []goobj.Aux
}

const (
        // Loader.flags
        FlagStrictDups = 1 << iota
)

func NewLoader(flags uint32, elfsetstring elfsetstringFunc, reporter *ErrorReporter) *Loader {
        nbuiltin := goobj.NBuiltin()
        extReader := &oReader{objidx: extObj}
        ldr := &Loader{
                start:                make(map[*oReader]Sym),
                objs:                 []objIdx{{}, {extReader, 0}}, // reserve index 0 for nil symbol, 1 for external symbols
                objSyms:              make([]objSym, 1, 1),         // This will get overwritten later.
                extReader:            extReader,
                symsByName:           [2]map[string]Sym{make(map[string]Sym, 80000), make(map[string]Sym, 50000)}, // preallocate ~2MB for ABI0 and ~1MB for ABI1 symbols
                objByPkg:             make(map[string]uint32),
                outer:                make(map[Sym]Sym),
                sub:                  make(map[Sym]Sym),
                dynimplib:            make(map[Sym]string),
                dynimpvers:           make(map[Sym]string),
                localentry:           make(map[Sym]uint8),
                extname:              make(map[Sym]string),
                attrReadOnly:         make(map[Sym]bool),
                elfType:              make(map[Sym]elf.SymType),
                elfSym:               make(map[Sym]int32),
                localElfSym:          make(map[Sym]int32),
                symPkg:               make(map[Sym]string),
                plt:                  make(map[Sym]int32),
                got:                  make(map[Sym]int32),
                dynid:                make(map[Sym]int32),
                attrTopFrame:         make(map[Sym]struct{}),
                attrSpecial:          make(map[Sym]struct{}),
                attrCgoExportDynamic: make(map[Sym]struct{}),
                attrCgoExportStatic:  make(map[Sym]struct{}),
                generatedSyms:        make(map[Sym]struct{}),
                deferReturnTramp:     make(map[Sym]bool),
                extStaticSyms:        make(map[nameVer]Sym),
                builtinSyms:          make([]Sym, nbuiltin),
                flags:                flags,
                elfsetstring:         elfsetstring,
                errorReporter:        reporter,
                sects:                []*sym.Section{nil}, // reserve index 0 for nil section
        }
        reporter.ldr = ldr
        return ldr
}

// Add object file r, return the start index.
func (l *Loader) addObj(pkg string, r *oReader) Sym {
        if _, ok := l.start[r]; ok {
                panic("already added")
        }
        pkg = objabi.PathToPrefix(pkg) // the object file contains escaped package path
        if _, ok := l.objByPkg[pkg]; !ok {
                l.objByPkg[pkg] = r.objidx
        }
        i := Sym(len(l.objSyms))
        l.start[r] = i
        l.objs = append(l.objs, objIdx{r, i})
        if r.NeedNameExpansion() && !r.FromAssembly() {
                l.hasUnknownPkgPath = true
        }
        return i
}

// Add a symbol from an object file, return the global index.
// If the symbol already exist, it returns the index of that symbol.
func (st *loadState) addSym(name string, ver int, r *oReader, li uint32, kind int, osym *goobj.Sym) Sym {
        l := st.l
        if l.extStart != 0 {
                panic("addSym called after external symbol is created")
        }
        i := Sym(len(l.objSyms))
        addToGlobal := func() {
                l.objSyms = append(l.objSyms, objSym{r.objidx, li})
        }
        if name == "" && kind != hashed64Def && kind != hashedDef {
                addToGlobal()
                return i // unnamed aux symbol
        }
        if ver == r.version {
                // Static symbol. Add its global index but don't
                // add to name lookup table, as it cannot be
                // referenced by name.
                addToGlobal()
                return i
        }
        switch kind {
        case pkgDef:
                // Defined package symbols cannot be dup to each other.
                // We load all the package symbols first, so we don't need
                // to check dup here.
                // We still add it to the lookup table, as it may still be
                // referenced by name (e.g. through linkname).
                l.symsByName[ver][name] = i
                addToGlobal()
                return i
        case hashed64Def, hashedDef:
                // Hashed (content-addressable) symbol. Check the hash
                // but don't add to name lookup table, as they are not
                // referenced by name. Also no need to do overwriting
                // check, as same hash indicates same content.
                var checkHash func() (symAndSize, bool)
                var addToHashMap func(symAndSize)
                var h64 uint64        // only used for hashed64Def
                var h *goobj.HashType // only used for hashedDef
                if kind == hashed64Def {
                        checkHash = func() (symAndSize, bool) {
                                h64 = r.Hash64(li - uint32(r.ndef))
                                s, existed := st.hashed64Syms[h64]
                                return s, existed
                        }
                        addToHashMap = func(ss symAndSize) { st.hashed64Syms[h64] = ss }
                } else {
                        checkHash = func() (symAndSize, bool) {
                                h = r.Hash(li - uint32(r.ndef+r.nhashed64def))
                                s, existed := st.hashedSyms[*h]
                                return s, existed
                        }
                        addToHashMap = func(ss symAndSize) { st.hashedSyms[*h] = ss }
                }
                siz := osym.Siz()
                if s, existed := checkHash(); existed {
                        // The content hash is built from symbol data and relocations. In the
                        // object file, the symbol data may not always contain trailing zeros,
                        // e.g. for [5]int{1,2,3} and [100]int{1,2,3}, the data is same
                        // (although the size is different).
                        // Also, for short symbols, the content hash is the identity function of
                        // the 8 bytes, and trailing zeros doesn't change the hash value, e.g.
                        // hash("A") == hash("A\0\0\0").
                        // So when two symbols have the same hash, we need to use the one with
                        // larger size.
                        if siz > s.size {
                                // New symbol has larger size, use the new one. Rewrite the index mapping.
                                l.objSyms[s.sym] = objSym{r.objidx, li}
                                addToHashMap(symAndSize{s.sym, siz})
                        }
                        return s.sym
                }
                addToHashMap(symAndSize{i, siz})
                addToGlobal()
                return i
        }

        // Non-package (named) symbol. Check if it already exists.
        oldi, existed := l.symsByName[ver][name]
        if !existed {
                l.symsByName[ver][name] = i
                addToGlobal()
                return i
        }
        // symbol already exists
        if osym.Dupok() {
                if l.flags&FlagStrictDups != 0 {
                        l.checkdup(name, r, li, oldi)
                }
                return oldi
        }
        oldr, oldli := l.toLocal(oldi)
        oldsym := oldr.Sym(oldli)
        if oldsym.Dupok() {
                return oldi
        }
        overwrite := r.DataSize(li) != 0
        if overwrite {
                // new symbol overwrites old symbol.
                oldtyp := sym.AbiSymKindToSymKind[objabi.SymKind(oldsym.Type())]
                if !(oldtyp.IsData() && oldr.DataSize(oldli) == 0) {
                        log.Fatalf("duplicated definition of symbol " + name)
                }
                l.objSyms[oldi] = objSym{r.objidx, li}
        } else {
                // old symbol overwrites new symbol.
                typ := sym.AbiSymKindToSymKind[objabi.SymKind(oldsym.Type())]
                if !typ.IsData() { // only allow overwriting data symbol
                        log.Fatalf("duplicated definition of symbol " + name)
                }
        }
        return oldi
}

// newExtSym creates a new external sym with the specified
// name/version.
func (l *Loader) newExtSym(name string, ver int) Sym {
        i := Sym(len(l.objSyms))
        if l.extStart == 0 {
                l.extStart = i
        }
        l.growValues(int(i) + 1)
        l.growAttrBitmaps(int(i) + 1)
        pi := l.newPayload(name, ver)
        l.objSyms = append(l.objSyms, objSym{l.extReader.objidx, uint32(pi)})
        l.extReader.syms = append(l.extReader.syms, i)
        return i
}

// LookupOrCreateSym looks up the symbol with the specified name/version,
// returning its Sym index if found. If the lookup fails, a new external
// Sym will be created, entered into the lookup tables, and returned.
func (l *Loader) LookupOrCreateSym(name string, ver int) Sym {
        i := l.Lookup(name, ver)
        if i != 0 {
                return i
        }
        i = l.newExtSym(name, ver)
        static := ver >= sym.SymVerStatic || ver < 0
        if static {
                l.extStaticSyms[nameVer{name, ver}] = i
        } else {
                l.symsByName[ver][name] = i
        }
        return i
}

func (l *Loader) IsExternal(i Sym) bool {
        r, _ := l.toLocal(i)
        return l.isExtReader(r)
}

func (l *Loader) isExtReader(r *oReader) bool {
        return r == l.extReader
}

// For external symbol, return its index in the payloads array.
// XXX result is actually not a global index. We (ab)use the Sym type
// so we don't need conversion for accessing bitmaps.
func (l *Loader) extIndex(i Sym) Sym {
        _, li := l.toLocal(i)
        return Sym(li)
}

// Get a new payload for external symbol, return its index in
// the payloads array.
func (l *Loader) newPayload(name string, ver int) int {
        pi := len(l.payloads)
        pp := l.allocPayload()
        pp.name = name
        pp.ver = ver
        l.payloads = append(l.payloads, pp)
        l.growExtAttrBitmaps()
        return pi
}

// getPayload returns a pointer to the extSymPayload struct for an
// external symbol if the symbol has a payload. Will panic if the
// symbol in question is bogus (zero or not an external sym).
func (l *Loader) getPayload(i Sym) *extSymPayload {
        if !l.IsExternal(i) {
                panic(fmt.Sprintf("bogus symbol index %d in getPayload", i))
        }
        pi := l.extIndex(i)
        return l.payloads[pi]
}

// allocPayload allocates a new payload.
func (l *Loader) allocPayload() *extSymPayload {
        batch := l.payloadBatch
        if len(batch) == 0 {
                batch = make([]extSymPayload, 1000)
        }
        p := &batch[0]
        l.payloadBatch = batch[1:]
        return p
}

func (ms *extSymPayload) Grow(siz int64) {
        if int64(int(siz)) != siz {
                log.Fatalf("symgrow size %d too long", siz)
        }
        if int64(len(ms.data)) >= siz {
                return
        }
        if cap(ms.data) < int(siz) {
                cl := len(ms.data)
                ms.data = append(ms.data, make([]byte, int(siz)+1-cl)...)
                ms.data = ms.data[0:cl]
        }
        ms.data = ms.data[:siz]
}

// Convert a local index to a global index.
func (l *Loader) toGlobal(r *oReader, i uint32) Sym {
        return r.syms[i]
}

// Convert a global index to a local index.
func (l *Loader) toLocal(i Sym) (*oReader, uint32) {
        return l.objs[l.objSyms[i].objidx].r, l.objSyms[i].s
}

// Resolve a local symbol reference. Return global index.
func (l *Loader) resolve(r *oReader, s goobj.SymRef) Sym {
        var rr *oReader
        switch p := s.PkgIdx; p {
        case goobj.PkgIdxInvalid:
                // {0, X} with non-zero X is never a valid sym reference from a Go object.
                // We steal this space for symbol references from external objects.
                // In this case, X is just the global index.
                if l.isExtReader(r) {
                        return Sym(s.SymIdx)
                }
                if s.SymIdx != 0 {
                        panic("bad sym ref")
                }
                return 0
        case goobj.PkgIdxHashed64:
                i := int(s.SymIdx) + r.ndef
                return r.syms[i]
        case goobj.PkgIdxHashed:
                i := int(s.SymIdx) + r.ndef + r.nhashed64def
                return r.syms[i]
        case goobj.PkgIdxNone:
                i := int(s.SymIdx) + r.ndef + r.nhashed64def + r.nhasheddef
                return r.syms[i]
        case goobj.PkgIdxBuiltin:
                return l.builtinSyms[s.SymIdx]
        case goobj.PkgIdxSelf:
                rr = r
        default:
                rr = l.objs[r.pkg[p]].r
        }
        return l.toGlobal(rr, s.SymIdx)
}

// Look up a symbol by name, return global index, or 0 if not found.
// This is more like Syms.ROLookup than Lookup -- it doesn't create
// new symbol.
func (l *Loader) Lookup(name string, ver int) Sym {
        if ver >= sym.SymVerStatic || ver < 0 {
                return l.extStaticSyms[nameVer{name, ver}]
        }
        return l.symsByName[ver][name]
}

// Check that duplicate symbols have same contents.
func (l *Loader) checkdup(name string, r *oReader, li uint32, dup Sym) {
        p := r.Data(li)
        rdup, ldup := l.toLocal(dup)
        pdup := rdup.Data(ldup)
        if bytes.Equal(p, pdup) {
                return
        }
        reason := "same length but different contents"
        if len(p) != len(pdup) {
                reason = fmt.Sprintf("new length %d != old length %d", len(p), len(pdup))
        }
        fmt.Fprintf(os.Stderr, "cmd/link: while reading object for '%v': duplicate symbol '%s', previous def at '%v', with mismatched payload: %s\n", r.unit.Lib, name, rdup.unit.Lib, reason)

        // For the moment, allow DWARF subprogram DIEs for
        // auto-generated wrapper functions. What seems to happen
        // here is that we get different line numbers on formal
        // params; I am guessing that the pos is being inherited
        // from the spot where the wrapper is needed.
        allowed := strings.HasPrefix(name, "go.info.go.interface") ||
                strings.HasPrefix(name, "go.info.go.builtin") ||
                strings.HasPrefix(name, "go.debuglines")
        if !allowed {
                l.strictDupMsgs++
        }
}

func (l *Loader) NStrictDupMsgs() int { return l.strictDupMsgs }

// Number of total symbols.
func (l *Loader) NSym() int {
        return len(l.objSyms)
}

// Number of defined Go symbols.
func (l *Loader) NDef() int {
        return int(l.extStart)
}

// Number of reachable symbols.
func (l *Loader) NReachableSym() int {
        return l.attrReachable.Count()
}

// SymNameLen returns the length of the symbol name, trying hard not to load
// the name.
func (l *Loader) SymNameLen(i Sym) int {
        // Not much we can do about external symbols.
        if l.IsExternal(i) {
                return len(l.SymName(i))
        }
        r, li := l.toLocal(i)
        le := r.Sym(li).NameLen(r.Reader)
        if !r.NeedNameExpansion() {
                return le
        }
        // Just load the symbol name. We don't know how expanded it'll be.
        return len(l.SymName(i))
}

// Returns the raw (unpatched) name of the i-th symbol.
func (l *Loader) RawSymName(i Sym) string {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                return pp.name
        }
        r, li := l.toLocal(i)
        return r.Sym(li).Name(r.Reader)
}

// Returns the (patched) name of the i-th symbol.
func (l *Loader) SymName(i Sym) string {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                return pp.name
        }
        r, li := l.toLocal(i)
        name := r.Sym(li).Name(r.Reader)
        if !r.NeedNameExpansion() {
                return name
        }
        return strings.Replace(name, "\"\".", r.pkgprefix, -1)
}

// Returns the version of the i-th symbol.
func (l *Loader) SymVersion(i Sym) int {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                return pp.ver
        }
        r, li := l.toLocal(i)
        return int(abiToVer(r.Sym(li).ABI(), r.version))
}

func (l *Loader) IsFileLocal(i Sym) bool {
        return l.SymVersion(i) >= sym.SymVerStatic
}

// IsFromAssembly returns true if this symbol is derived from an
// object file generated by the Go assembler.
func (l *Loader) IsFromAssembly(i Sym) bool {
        if l.IsExternal(i) {
                return false
        }
        r, _ := l.toLocal(i)
        return r.FromAssembly()
}

// Returns the type of the i-th symbol.
func (l *Loader) SymType(i Sym) sym.SymKind {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                if pp != nil {
                        return pp.kind
                }
                return 0
        }
        r, li := l.toLocal(i)
        return sym.AbiSymKindToSymKind[objabi.SymKind(r.Sym(li).Type())]
}

// Returns the attributes of the i-th symbol.
func (l *Loader) SymAttr(i Sym) uint8 {
        if l.IsExternal(i) {
                // TODO: do something? External symbols have different representation of attributes.
                // For now, ReflectMethod, NoSplit, GoType, and Typelink are used and they cannot be
                // set by external symbol.
                return 0
        }
        r, li := l.toLocal(i)
        return r.Sym(li).Flag()
}

// Returns the size of the i-th symbol.
func (l *Loader) SymSize(i Sym) int64 {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                return pp.size
        }
        r, li := l.toLocal(i)
        return int64(r.Sym(li).Siz())
}

// AttrReachable returns true for symbols that are transitively
// referenced from the entry points. Unreachable symbols are not
// written to the output.
func (l *Loader) AttrReachable(i Sym) bool {
        return l.attrReachable.Has(i)
}

// SetAttrReachable sets the reachability property for a symbol (see
// AttrReachable).
func (l *Loader) SetAttrReachable(i Sym, v bool) {
        if v {
                l.attrReachable.Set(i)
        } else {
                l.attrReachable.Unset(i)
        }
}

// AttrOnList returns true for symbols that are on some list (such as
// the list of all text symbols, or one of the lists of data symbols)
// and is consulted to avoid bugs where a symbol is put on a list
// twice.
func (l *Loader) AttrOnList(i Sym) bool {
        return l.attrOnList.Has(i)
}

// SetAttrOnList sets the "on list" property for a symbol (see
// AttrOnList).
func (l *Loader) SetAttrOnList(i Sym, v bool) {
        if v {
                l.attrOnList.Set(i)
        } else {
                l.attrOnList.Unset(i)
        }
}

// AttrLocal returns true for symbols that are only visible within the
// module (executable or shared library) being linked. This attribute
// is applied to thunks and certain other linker-generated symbols.
func (l *Loader) AttrLocal(i Sym) bool {
        return l.attrLocal.Has(i)
}

// SetAttrLocal the "local" property for a symbol (see AttrLocal above).
func (l *Loader) SetAttrLocal(i Sym, v bool) {
        if v {
                l.attrLocal.Set(i)
        } else {
                l.attrLocal.Unset(i)
        }
}

// AttrUsedInIface returns true for a type symbol that is used in
// an interface.
func (l *Loader) AttrUsedInIface(i Sym) bool {
        return l.attrUsedInIface.Has(i)
}

func (l *Loader) SetAttrUsedInIface(i Sym, v bool) {
        if v {
                l.attrUsedInIface.Set(i)
        } else {
                l.attrUsedInIface.Unset(i)
        }
}

// SymAddr checks that a symbol is reachable, and returns its value.
func (l *Loader) SymAddr(i Sym) int64 {
        if !l.AttrReachable(i) {
                panic("unreachable symbol in symaddr")
        }
        return l.values[i]
}

// AttrNotInSymbolTable returns true for symbols that should not be
// added to the symbol table of the final generated load module.
func (l *Loader) AttrNotInSymbolTable(i Sym) bool {
        return l.attrNotInSymbolTable.Has(i)
}

// SetAttrNotInSymbolTable the "not in symtab" property for a symbol
// (see AttrNotInSymbolTable above).
func (l *Loader) SetAttrNotInSymbolTable(i Sym, v bool) {
        if v {
                l.attrNotInSymbolTable.Set(i)
        } else {
                l.attrNotInSymbolTable.Unset(i)
        }
}

// AttrVisibilityHidden symbols returns true for ELF symbols with
// visibility set to STV_HIDDEN. They become local symbols in
// the final executable. Only relevant when internally linking
// on an ELF platform.
func (l *Loader) AttrVisibilityHidden(i Sym) bool {
        if !l.IsExternal(i) {
                return false
        }
        return l.attrVisibilityHidden.Has(l.extIndex(i))
}

// SetAttrVisibilityHidden sets the "hidden visibility" property for a
// symbol (see AttrVisibilityHidden).
func (l *Loader) SetAttrVisibilityHidden(i Sym, v bool) {
        if !l.IsExternal(i) {
                panic("tried to set visibility attr on non-external symbol")
        }
        if v {
                l.attrVisibilityHidden.Set(l.extIndex(i))
        } else {
                l.attrVisibilityHidden.Unset(l.extIndex(i))
        }
}

// AttrDuplicateOK returns true for a symbol that can be present in
// multiple object files.
func (l *Loader) AttrDuplicateOK(i Sym) bool {
        if !l.IsExternal(i) {
                // TODO: if this path winds up being taken frequently, it
                // might make more sense to copy the flag value out of the object
                // into a larger bitmap during preload.
                r, li := l.toLocal(i)
                return r.Sym(li).Dupok()
        }
        return l.attrDuplicateOK.Has(l.extIndex(i))
}

// SetAttrDuplicateOK sets the "duplicate OK" property for an external
// symbol (see AttrDuplicateOK).
func (l *Loader) SetAttrDuplicateOK(i Sym, v bool) {
        if !l.IsExternal(i) {
                panic("tried to set dupok attr on non-external symbol")
        }
        if v {
                l.attrDuplicateOK.Set(l.extIndex(i))
        } else {
                l.attrDuplicateOK.Unset(l.extIndex(i))
        }
}

// AttrShared returns true for symbols compiled with the -shared option.
func (l *Loader) AttrShared(i Sym) bool {
        if !l.IsExternal(i) {
                // TODO: if this path winds up being taken frequently, it
                // might make more sense to copy the flag value out of the
                // object into a larger bitmap during preload.
                r, _ := l.toLocal(i)
                return r.Shared()
        }
        return l.attrShared.Has(l.extIndex(i))
}

// SetAttrShared sets the "shared" property for an external
// symbol (see AttrShared).
func (l *Loader) SetAttrShared(i Sym, v bool) {
        if !l.IsExternal(i) {
                panic(fmt.Sprintf("tried to set shared attr on non-external symbol %d %s", i, l.SymName(i)))
        }
        if v {
                l.attrShared.Set(l.extIndex(i))
        } else {
                l.attrShared.Unset(l.extIndex(i))
        }
}

// AttrExternal returns true for function symbols loaded from host
// object files.
func (l *Loader) AttrExternal(i Sym) bool {
        if !l.IsExternal(i) {
                return false
        }
        return l.attrExternal.Has(l.extIndex(i))
}

// SetAttrExternal sets the "external" property for an host object
// symbol (see AttrExternal).
func (l *Loader) SetAttrExternal(i Sym, v bool) {
        if !l.IsExternal(i) {
                panic(fmt.Sprintf("tried to set external attr on non-external symbol %q", l.RawSymName(i)))
        }
        if v {
                l.attrExternal.Set(l.extIndex(i))
        } else {
                l.attrExternal.Unset(l.extIndex(i))
        }
}

// AttrTopFrame returns true for a function symbol that is an entry
// point, meaning that unwinders should stop when they hit this
// function.
func (l *Loader) AttrTopFrame(i Sym) bool {
        _, ok := l.attrTopFrame[i]
        return ok
}

// SetAttrTopFrame sets the "top frame" property for a symbol (see
// AttrTopFrame).
func (l *Loader) SetAttrTopFrame(i Sym, v bool) {
        if v {
                l.attrTopFrame[i] = struct{}{}
        } else {
                delete(l.attrTopFrame, i)
        }
}

// AttrSpecial returns true for a symbols that do not have their
// address (i.e. Value) computed by the usual mechanism of
// data.go:dodata() & data.go:address().
func (l *Loader) AttrSpecial(i Sym) bool {
        _, ok := l.attrSpecial[i]
        return ok
}

// SetAttrSpecial sets the "special" property for a symbol (see
// AttrSpecial).
func (l *Loader) SetAttrSpecial(i Sym, v bool) {
        if v {
                l.attrSpecial[i] = struct{}{}
        } else {
                delete(l.attrSpecial, i)
        }
}

// AttrCgoExportDynamic returns true for a symbol that has been
// specially marked via the "cgo_export_dynamic" compiler directive
// written by cgo (in response to //export directives in the source).
func (l *Loader) AttrCgoExportDynamic(i Sym) bool {
        _, ok := l.attrCgoExportDynamic[i]
        return ok
}

// SetAttrCgoExportDynamic sets the "cgo_export_dynamic" for a symbol
// (see AttrCgoExportDynamic).
func (l *Loader) SetAttrCgoExportDynamic(i Sym, v bool) {
        if v {
                l.attrCgoExportDynamic[i] = struct{}{}
        } else {
                delete(l.attrCgoExportDynamic, i)
        }
}

// AttrCgoExportStatic returns true for a symbol that has been
// specially marked via the "cgo_export_static" directive
// written by cgo.
func (l *Loader) AttrCgoExportStatic(i Sym) bool {
        _, ok := l.attrCgoExportStatic[i]
        return ok
}

// SetAttrCgoExportStatic sets the "cgo_export_static" for a symbol
// (see AttrCgoExportStatic).
func (l *Loader) SetAttrCgoExportStatic(i Sym, v bool) {
        if v {
                l.attrCgoExportStatic[i] = struct{}{}
        } else {
                delete(l.attrCgoExportStatic, i)
        }
}

// IsGeneratedSym returns true if a symbol's been previously marked as a
// generator symbol through the SetIsGeneratedSym. The functions for generator
// symbols are kept in the Link context.
func (l *Loader) IsGeneratedSym(i Sym) bool {
        _, ok := l.generatedSyms[i]
        return ok
}

// SetIsGeneratedSym marks symbols as generated symbols. Data shouldn't be
// stored in generated symbols, and a function is registered and called for
// each of these symbols.
func (l *Loader) SetIsGeneratedSym(i Sym, v bool) {
        if !l.IsExternal(i) {
                panic("only external symbols can be generated")
        }
        if v {
                l.generatedSyms[i] = struct{}{}
        } else {
                delete(l.generatedSyms, i)
        }
}

func (l *Loader) AttrCgoExport(i Sym) bool {
        return l.AttrCgoExportDynamic(i) || l.AttrCgoExportStatic(i)
}

// AttrReadOnly returns true for a symbol whose underlying data
// is stored via a read-only mmap.
func (l *Loader) AttrReadOnly(i Sym) bool {
        if v, ok := l.attrReadOnly[i]; ok {
                return v
        }
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                if pp.objidx != 0 {
                        return l.objs[pp.objidx].r.ReadOnly()
                }
                return false
        }
        r, _ := l.toLocal(i)
        return r.ReadOnly()
}

// SetAttrReadOnly sets the "data is read only" property for a symbol
// (see AttrReadOnly).
func (l *Loader) SetAttrReadOnly(i Sym, v bool) {
        l.attrReadOnly[i] = v
}

// AttrSubSymbol returns true for symbols that are listed as a
// sub-symbol of some other outer symbol. The sub/outer mechanism is
// used when loading host objects (sections from the host object
// become regular linker symbols and symbols go on the Sub list of
// their section) and for constructing the global offset table when
// internally linking a dynamic executable.
//
// Note that in later stages of the linker, we set Outer(S) to some
// container symbol C, but don't set Sub(C). Thus we have two
// distinct scenarios:
//
// - Outer symbol covers the address ranges of its sub-symbols.
//   Outer.Sub is set in this case.
// - Outer symbol doesn't conver the address ranges. It is zero-sized
//   and doesn't have sub-symbols. In the case, the inner symbol is
//   not actually a "SubSymbol". (Tricky!)
//
// This method returns TRUE only for sub-symbols in the first scenario.
//
// FIXME: would be better to do away with this and have a better way
// to represent container symbols.

func (l *Loader) AttrSubSymbol(i Sym) bool {
        // we don't explicitly store this attribute any more -- return
        // a value based on the sub-symbol setting.
        o := l.OuterSym(i)
        if o == 0 {
                return false
        }
        return l.SubSym(o) != 0
}

// Note that we don't have a 'SetAttrSubSymbol' method in the loader;
// clients should instead use the AddInteriorSym method to establish
// containment relationships for host object symbols.

// Returns whether the i-th symbol has ReflectMethod attribute set.
func (l *Loader) IsReflectMethod(i Sym) bool {
        return l.SymAttr(i)&goobj.SymFlagReflectMethod != 0
}

// Returns whether the i-th symbol is nosplit.
func (l *Loader) IsNoSplit(i Sym) bool {
        return l.SymAttr(i)&goobj.SymFlagNoSplit != 0
}

// Returns whether this is a Go type symbol.
func (l *Loader) IsGoType(i Sym) bool {
        return l.SymAttr(i)&goobj.SymFlagGoType != 0
}

// Returns whether this symbol should be included in typelink.
func (l *Loader) IsTypelink(i Sym) bool {
        return l.SymAttr(i)&goobj.SymFlagTypelink != 0
}

// Returns whether this symbol is an itab symbol.
func (l *Loader) IsItab(i Sym) bool {
        if l.IsExternal(i) {
                return false
        }
        r, li := l.toLocal(i)
        return r.Sym(li).IsItab()
}

// Return whether this is a trampoline of a deferreturn call.
func (l *Loader) IsDeferReturnTramp(i Sym) bool {
        return l.deferReturnTramp[i]
}

// Set that i is a trampoline of a deferreturn call.
func (l *Loader) SetIsDeferReturnTramp(i Sym, v bool) {
        l.deferReturnTramp[i] = v
}

// growValues grows the slice used to store symbol values.
func (l *Loader) growValues(reqLen int) {
        curLen := len(l.values)
        if reqLen > curLen {
                l.values = append(l.values, make([]int64, reqLen+1-curLen)...)
        }
}

// SymValue returns the value of the i-th symbol. i is global index.
func (l *Loader) SymValue(i Sym) int64 {
        return l.values[i]
}

// SetSymValue sets the value of the i-th symbol. i is global index.
func (l *Loader) SetSymValue(i Sym, val int64) {
        l.values[i] = val
}

// AddToSymValue adds to the value of the i-th symbol. i is the global index.
func (l *Loader) AddToSymValue(i Sym, val int64) {
        l.values[i] += val
}

// Returns the symbol content of the i-th symbol. i is global index.
func (l *Loader) Data(i Sym) []byte {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                if pp != nil {
                        return pp.data
                }
                return nil
        }
        r, li := l.toLocal(i)
        return r.Data(li)
}

// FreeData clears the symbol data of an external symbol, allowing the memory
// to be freed earlier. No-op for non-external symbols.
// i is global index.
func (l *Loader) FreeData(i Sym) {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                if pp != nil {
                        pp.data = nil
                }
        }
}

// SymAlign returns the alignment for a symbol.
func (l *Loader) SymAlign(i Sym) int32 {
        if int(i) >= len(l.align) {
                // align is extended lazily -- it the sym in question is
                // outside the range of the existing slice, then we assume its
                // alignment has not yet been set.
                return 0
        }
        // TODO: would it make sense to return an arch-specific
        // alignment depending on section type? E.g. STEXT => 32,
        // SDATA => 1, etc?
        abits := l.align[i]
        if abits == 0 {
                return 0
        }
        return int32(1 << (abits - 1))
}

// SetSymAlign sets the alignment for a symbol.
func (l *Loader) SetSymAlign(i Sym, align int32) {
        // Reject nonsense alignments.
        if align < 0 || align&(align-1) != 0 {
                panic("bad alignment value")
        }
        if int(i) >= len(l.align) {
                l.align = append(l.align, make([]uint8, l.NSym()-len(l.align))...)
        }
        if align == 0 {
                l.align[i] = 0
        }
        l.align[i] = uint8(bits.Len32(uint32(align)))
}

// SymValue returns the section of the i-th symbol. i is global index.
func (l *Loader) SymSect(i Sym) *sym.Section {
        if int(i) >= len(l.symSects) {
                // symSects is extended lazily -- it the sym in question is
                // outside the range of the existing slice, then we assume its
                // section has not yet been set.
                return nil
        }
        return l.sects[l.symSects[i]]
}

// SetSymSect sets the section of the i-th symbol. i is global index.
func (l *Loader) SetSymSect(i Sym, sect *sym.Section) {
        if int(i) >= len(l.symSects) {
                l.symSects = append(l.symSects, make([]uint16, l.NSym()-len(l.symSects))...)
        }
        l.symSects[i] = sect.Index
}

// growSects grows the slice used to store symbol sections.
func (l *Loader) growSects(reqLen int) {
        curLen := len(l.symSects)
        if reqLen > curLen {
                l.symSects = append(l.symSects, make([]uint16, reqLen+1-curLen)...)
        }
}

// NewSection creates a new (output) section.
func (l *Loader) NewSection() *sym.Section {
        sect := new(sym.Section)
        idx := len(l.sects)
        if idx != int(uint16(idx)) {
                panic("too many sections created")
        }
        sect.Index = uint16(idx)
        l.sects = append(l.sects, sect)
        return sect
}

// SymDynImplib returns the "dynimplib" attribute for the specified
// symbol, making up a portion of the info for a symbol specified
// on a "cgo_import_dynamic" compiler directive.
func (l *Loader) SymDynimplib(i Sym) string {
        return l.dynimplib[i]
}

// SetSymDynimplib sets the "dynimplib" attribute for a symbol.
func (l *Loader) SetSymDynimplib(i Sym, value string) {
        // reject bad symbols
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol index in SetDynimplib")
        }
        if value == "" {
                delete(l.dynimplib, i)
        } else {
                l.dynimplib[i] = value
        }
}

// SymDynimpvers returns the "dynimpvers" attribute for the specified
// symbol, making up a portion of the info for a symbol specified
// on a "cgo_import_dynamic" compiler directive.
func (l *Loader) SymDynimpvers(i Sym) string {
        return l.dynimpvers[i]
}

// SetSymDynimpvers sets the "dynimpvers" attribute for a symbol.
func (l *Loader) SetSymDynimpvers(i Sym, value string) {
        // reject bad symbols
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol index in SetDynimpvers")
        }
        if value == "" {
                delete(l.dynimpvers, i)
        } else {
                l.dynimpvers[i] = value
        }
}

// SymExtname returns the "extname" value for the specified
// symbol.
func (l *Loader) SymExtname(i Sym) string {
        if s, ok := l.extname[i]; ok {
                return s
        }
        return l.SymName(i)
}

// SetSymExtname sets the  "extname" attribute for a symbol.
func (l *Loader) SetSymExtname(i Sym, value string) {
        // reject bad symbols
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol index in SetExtname")
        }
        if value == "" {
                delete(l.extname, i)
        } else {
                l.extname[i] = value
        }
}

// SymElfType returns the previously recorded ELF type for a symbol
// (used only for symbols read from shared libraries by ldshlibsyms).
// It is not set for symbols defined by the packages being linked or
// by symbols read by ldelf (and so is left as elf.STT_NOTYPE).
func (l *Loader) SymElfType(i Sym) elf.SymType {
        if et, ok := l.elfType[i]; ok {
                return et
        }
        return elf.STT_NOTYPE
}

// SetSymElfType sets the elf type attribute for a symbol.
func (l *Loader) SetSymElfType(i Sym, et elf.SymType) {
        // reject bad symbols
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol index in SetSymElfType")
        }
        if et == elf.STT_NOTYPE {
                delete(l.elfType, i)
        } else {
                l.elfType[i] = et
        }
}

// SymElfSym returns the ELF symbol index for a given loader
// symbol, assigned during ELF symtab generation.
func (l *Loader) SymElfSym(i Sym) int32 {
        return l.elfSym[i]
}

// SetSymElfSym sets the elf symbol index for a symbol.
func (l *Loader) SetSymElfSym(i Sym, es int32) {
        if i == 0 {
                panic("bad sym index")
        }
        if es == 0 {
                delete(l.elfSym, i)
        } else {
                l.elfSym[i] = es
        }
}

// SymLocalElfSym returns the "local" ELF symbol index for a given loader
// symbol, assigned during ELF symtab generation.
func (l *Loader) SymLocalElfSym(i Sym) int32 {
        return l.localElfSym[i]
}

// SetSymLocalElfSym sets the "local" elf symbol index for a symbol.
func (l *Loader) SetSymLocalElfSym(i Sym, es int32) {
        if i == 0 {
                panic("bad sym index")
        }
        if es == 0 {
                delete(l.localElfSym, i)
        } else {
                l.localElfSym[i] = es
        }
}

// SymPlt returns the plt value for pe symbols.
func (l *Loader) SymPlt(s Sym) int32 {
        if v, ok := l.plt[s]; ok {
                return v
        }
        return -1
}

// SetPlt sets the plt value for pe symbols.
func (l *Loader) SetPlt(i Sym, v int32) {
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol for SetPlt")
        }
        if v == -1 {
                delete(l.plt, i)
        } else {
                l.plt[i] = v
        }
}

// SymGot returns the got value for pe symbols.
func (l *Loader) SymGot(s Sym) int32 {
        if v, ok := l.got[s]; ok {
                return v
        }
        return -1
}

// SetGot sets the got value for pe symbols.
func (l *Loader) SetGot(i Sym, v int32) {
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol for SetGot")
        }
        if v == -1 {
                delete(l.got, i)
        } else {
                l.got[i] = v
        }
}

// SymDynid returns the "dynid" property for the specified symbol.
func (l *Loader) SymDynid(i Sym) int32 {
        if s, ok := l.dynid[i]; ok {
                return s
        }
        return -1
}

// SetSymDynid sets the "dynid" property for a symbol.
func (l *Loader) SetSymDynid(i Sym, val int32) {
        // reject bad symbols
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol index in SetSymDynid")
        }
        if val == -1 {
                delete(l.dynid, i)
        } else {
                l.dynid[i] = val
        }
}

// DynIdSyms returns the set of symbols for which dynID is set to an
// interesting (non-default) value. This is expected to be a fairly
// small set.
func (l *Loader) DynidSyms() []Sym {
        sl := make([]Sym, 0, len(l.dynid))
        for s := range l.dynid {
                sl = append(sl, s)
        }
        sort.Slice(sl, func(i, j int) bool { return sl[i] < sl[j] })
        return sl
}

// SymGoType returns the 'Gotype' property for a given symbol (set by
// the Go compiler for variable symbols). This version relies on
// reading aux symbols for the target sym -- it could be that a faster
// approach would be to check for gotype during preload and copy the
// results in to a map (might want to try this at some point and see
// if it helps speed things up).
func (l *Loader) SymGoType(i Sym) Sym {
        var r *oReader
        var auxs []goobj.Aux
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                r = l.objs[pp.objidx].r
                auxs = pp.auxs
        } else {
                var li uint32
                r, li = l.toLocal(i)
                auxs = r.Auxs(li)
        }
        for j := range auxs {
                a := &auxs[j]
                switch a.Type() {
                case goobj.AuxGotype:
                        return l.resolve(r, a.Sym())
                }
        }
        return 0
}

// SymUnit returns the compilation unit for a given symbol (which will
// typically be nil for external or linker-manufactured symbols).
func (l *Loader) SymUnit(i Sym) *sym.CompilationUnit {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                if pp.objidx != 0 {
                        r := l.objs[pp.objidx].r
                        return r.unit
                }
                return nil
        }
        r, _ := l.toLocal(i)
        return r.unit
}

// SymPkg returns the package where the symbol came from (for
// regular compiler-generated Go symbols), but in the case of
// building with "-linkshared" (when a symbol is read from a
// shared library), will hold the library name.
// NOTE: this correspondes to sym.Symbol.File field.
func (l *Loader) SymPkg(i Sym) string {
        if f, ok := l.symPkg[i]; ok {
                return f
        }
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                if pp.objidx != 0 {
                        r := l.objs[pp.objidx].r
                        return r.unit.Lib.Pkg
                }
                return ""
        }
        r, _ := l.toLocal(i)
        return r.unit.Lib.Pkg
}

// SetSymPkg sets the package/library for a symbol. This is
// needed mainly for external symbols, specifically those imported
// from shared libraries.
func (l *Loader) SetSymPkg(i Sym, pkg string) {
        // reject bad symbols
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol index in SetSymPkg")
        }
        l.symPkg[i] = pkg
}

// SymLocalentry returns the "local entry" value for the specified
// symbol.
func (l *Loader) SymLocalentry(i Sym) uint8 {
        return l.localentry[i]
}

// SetSymLocalentry sets the "local entry" attribute for a symbol.
func (l *Loader) SetSymLocalentry(i Sym, value uint8) {
        // reject bad symbols
        if i >= Sym(len(l.objSyms)) || i == 0 {
                panic("bad symbol index in SetSymLocalentry")
        }
        if value == 0 {
                delete(l.localentry, i)
        } else {
                l.localentry[i] = value
        }
}

// Returns the number of aux symbols given a global index.
func (l *Loader) NAux(i Sym) int {
        if l.IsExternal(i) {
                return 0
        }
        r, li := l.toLocal(i)
        return r.NAux(li)
}

// Returns the "handle" to the j-th aux symbol of the i-th symbol.
func (l *Loader) Aux(i Sym, j int) Aux {
        if l.IsExternal(i) {
                return Aux{}
        }
        r, li := l.toLocal(i)
        if j >= r.NAux(li) {
                return Aux{}
        }
        return Aux{r.Aux(li, j), r, l}
}

// GetFuncDwarfAuxSyms collects and returns the auxiliary DWARF
// symbols associated with a given function symbol.  Prior to the
// introduction of the loader, this was done purely using name
// lookups, e.f. for function with name XYZ we would then look up
// go.info.XYZ, etc.
func (l *Loader) GetFuncDwarfAuxSyms(fnSymIdx Sym) (auxDwarfInfo, auxDwarfLoc, auxDwarfRanges, auxDwarfLines Sym) {
        if l.SymType(fnSymIdx) != sym.STEXT {
                log.Fatalf("error: non-function sym %d/%s t=%s passed to GetFuncDwarfAuxSyms", fnSymIdx, l.SymName(fnSymIdx), l.SymType(fnSymIdx).String())
        }
        if l.IsExternal(fnSymIdx) {
                // Current expectation is that any external function will
                // not have auxsyms.
                return
        }
        r, li := l.toLocal(fnSymIdx)
        auxs := r.Auxs(li)
        for i := range auxs {
                a := &auxs[i]
                switch a.Type() {
                case goobj.AuxDwarfInfo:
                        auxDwarfInfo = l.resolve(r, a.Sym())
                        if l.SymType(auxDwarfInfo) != sym.SDWARFFCN {
                                panic("aux dwarf info sym with wrong type")
                        }
                case goobj.AuxDwarfLoc:
                        auxDwarfLoc = l.resolve(r, a.Sym())
                        if l.SymType(auxDwarfLoc) != sym.SDWARFLOC {
                                panic("aux dwarf loc sym with wrong type")
                        }
                case goobj.AuxDwarfRanges:
                        auxDwarfRanges = l.resolve(r, a.Sym())
                        if l.SymType(auxDwarfRanges) != sym.SDWARFRANGE {
                                panic("aux dwarf ranges sym with wrong type")
                        }
                case goobj.AuxDwarfLines:
                        auxDwarfLines = l.resolve(r, a.Sym())
                        if l.SymType(auxDwarfLines) != sym.SDWARFLINES {
                                panic("aux dwarf lines sym with wrong type")
                        }
                }
        }
        return
}

// AddInteriorSym sets up 'interior' as an interior symbol of
// container/payload symbol 'container'. An interior symbol does not
// itself have data, but gives a name to a subrange of the data in its
// container symbol. The container itself may or may not have a name.
// This method is intended primarily for use in the host object
// loaders, to capture the semantics of symbols and sections in an
// object file. When reading a host object file, we'll typically
// encounter a static section symbol (ex: ".text") containing content
// for a collection of functions, then a series of ELF (or macho, etc)
// symbol table entries each of which points into a sub-section
// (offset and length) of its corresponding container symbol. Within
// the go linker we create a loader.Sym for the container (which is
// expected to have the actual content/payload) and then a set of
// interior loader.Sym's that point into a portion of the container.
func (l *Loader) AddInteriorSym(container Sym, interior Sym) {
        // Container symbols are expected to have content/data.
        // NB: this restriction may turn out to be too strict (it's possible
        // to imagine a zero-sized container with an interior symbol pointing
        // into it); it's ok to relax or remove it if we counter an
        // oddball host object that triggers this.
        if l.SymSize(container) == 0 && len(l.Data(container)) == 0 {
                panic("unexpected empty container symbol")
        }
        // The interior symbols for a container are not expected to have
        // content/data or relocations.
        if len(l.Data(interior)) != 0 {
                panic("unexpected non-empty interior symbol")
        }
        // Interior symbol is expected to be in the symbol table.
        if l.AttrNotInSymbolTable(interior) {
                panic("interior symbol must be in symtab")
        }
        // Only a single level of containment is allowed.
        if l.OuterSym(container) != 0 {
                panic("outer has outer itself")
        }
        // Interior sym should not already have a sibling.
        if l.SubSym(interior) != 0 {
                panic("sub set for subsym")
        }
        // Interior sym should not already point at a container.
        if l.OuterSym(interior) != 0 {
                panic("outer already set for subsym")
        }
        l.sub[interior] = l.sub[container]
        l.sub[container] = interior
        l.outer[interior] = container
}

// OuterSym gets the outer symbol for host object loaded symbols.
func (l *Loader) OuterSym(i Sym) Sym {
        // FIXME: add check for isExternal?
        return l.outer[i]
}

// SubSym gets the subsymbol for host object loaded symbols.
func (l *Loader) SubSym(i Sym) Sym {
        // NB: note -- no check for l.isExternal(), since I am pretty sure
        // that later phases in the linker set subsym for "type." syms
        return l.sub[i]
}

// SetCarrierSym declares that 'c' is the carrier or container symbol
// for 's'. Carrier symbols are used in the linker to as a container
// for a collection of sub-symbols where the content of the
// sub-symbols is effectively concatenated to form the content of the
// carrier. The carrier is given a name in the output symbol table
// while the sub-symbol names are not. For example, the Go compiler
// emits named string symbols (type SGOSTRING) when compiling a
// package; after being deduplicated, these symbols are collected into
// a single unit by assigning them a new carrier symbol named
// "go.string.*" (which appears in the final symbol table for the
// output load module).
func (l *Loader) SetCarrierSym(s Sym, c Sym) {
        if c == 0 {
                panic("invalid carrier in SetCarrierSym")
        }
        if s == 0 {
                panic("invalid sub-symbol in SetCarrierSym")
        }
        // Carrier symbols are not expected to have content/data. It is
        // ok for them to have non-zero size (to allow for use of generator
        // symbols).
        if len(l.Data(c)) != 0 {
                panic("unexpected non-empty carrier symbol")
        }
        l.outer[s] = c
        // relocsym's foldSubSymbolOffset requires that we only
        // have a single level of containment-- enforce here.
        if l.outer[c] != 0 {
                panic("invalid nested carrier sym")
        }
}

// Initialize Reachable bitmap and its siblings for running deadcode pass.
func (l *Loader) InitReachable() {
        l.growAttrBitmaps(l.NSym() + 1)
}

type symWithVal struct {
        s Sym
        v int64
}
type bySymValue []symWithVal

func (s bySymValue) Len() int           { return len(s) }
func (s bySymValue) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
func (s bySymValue) Less(i, j int) bool { return s[i].v < s[j].v }

// SortSub walks through the sub-symbols for 's' and sorts them
// in place by increasing value. Return value is the new
// sub symbol for the specified outer symbol.
func (l *Loader) SortSub(s Sym) Sym {

        if s == 0 || l.sub[s] == 0 {
                return s
        }

        // Sort symbols using a slice first. Use a stable sort on the off
        // chance that there's more than once symbol with the same value,
        // so as to preserve reproducible builds.
        sl := []symWithVal{}
        for ss := l.sub[s]; ss != 0; ss = l.sub[ss] {
                sl = append(sl, symWithVal{s: ss, v: l.SymValue(ss)})
        }
        sort.Stable(bySymValue(sl))

        // Then apply any changes needed to the sub map.
        ns := Sym(0)
        for i := len(sl) - 1; i >= 0; i-- {
                s := sl[i].s
                l.sub[s] = ns
                ns = s
        }

        // Update sub for outer symbol, then return
        l.sub[s] = sl[0].s
        return sl[0].s
}

// Insure that reachable bitmap and its siblings have enough size.
func (l *Loader) growAttrBitmaps(reqLen int) {
        if reqLen > l.attrReachable.Len() {
                // These are indexed by global symbol
                l.attrReachable = growBitmap(reqLen, l.attrReachable)
                l.attrOnList = growBitmap(reqLen, l.attrOnList)
                l.attrLocal = growBitmap(reqLen, l.attrLocal)
                l.attrNotInSymbolTable = growBitmap(reqLen, l.attrNotInSymbolTable)
                l.attrUsedInIface = growBitmap(reqLen, l.attrUsedInIface)
        }
        l.growExtAttrBitmaps()
}

func (l *Loader) growExtAttrBitmaps() {
        // These are indexed by external symbol index (e.g. l.extIndex(i))
        extReqLen := len(l.payloads)
        if extReqLen > l.attrVisibilityHidden.Len() {
                l.attrVisibilityHidden = growBitmap(extReqLen, l.attrVisibilityHidden)
                l.attrDuplicateOK = growBitmap(extReqLen, l.attrDuplicateOK)
                l.attrShared = growBitmap(extReqLen, l.attrShared)
                l.attrExternal = growBitmap(extReqLen, l.attrExternal)
        }
}

func (relocs *Relocs) Count() int { return len(relocs.rs) }

// At returns the j-th reloc for a global symbol.
func (relocs *Relocs) At(j int) Reloc {
        if relocs.l.isExtReader(relocs.r) {
                pp := relocs.l.payloads[relocs.li]
                return Reloc{&relocs.rs[j], relocs.r, relocs.l, pp.reltypes[j]}
        }
        return Reloc{&relocs.rs[j], relocs.r, relocs.l, 0}
}

// Relocs returns a Relocs object for the given global sym.
func (l *Loader) Relocs(i Sym) Relocs {
        r, li := l.toLocal(i)
        if r == nil {
                panic(fmt.Sprintf("trying to get oreader for invalid sym %d\n\n", i))
        }
        return l.relocs(r, li)
}

// Relocs returns a Relocs object given a local sym index and reader.
func (l *Loader) relocs(r *oReader, li uint32) Relocs {
        var rs []goobj.Reloc
        if l.isExtReader(r) {
                pp := l.payloads[li]
                rs = pp.relocs
        } else {
                rs = r.Relocs(li)
        }
        return Relocs{
                rs: rs,
                li: li,
                r:  r,
                l:  l,
        }
}

// FuncInfo provides hooks to access goobj.FuncInfo in the objects.
type FuncInfo struct {
        l       *Loader
        r       *oReader
        data    []byte
        auxs    []goobj.Aux
        lengths goobj.FuncInfoLengths
}

func (fi *FuncInfo) Valid() bool { return fi.r != nil }

func (fi *FuncInfo) Args() int {
        return int((*goobj.FuncInfo)(nil).ReadArgs(fi.data))
}

func (fi *FuncInfo) Locals() int {
        return int((*goobj.FuncInfo)(nil).ReadLocals(fi.data))
}

func (fi *FuncInfo) FuncID() objabi.FuncID {
        return objabi.FuncID((*goobj.FuncInfo)(nil).ReadFuncID(fi.data))
}

func (fi *FuncInfo) Pcsp() Sym {
        sym := (*goobj.FuncInfo)(nil).ReadPcsp(fi.data)
        return fi.l.resolve(fi.r, sym)
}

func (fi *FuncInfo) Pcfile() Sym {
        sym := (*goobj.FuncInfo)(nil).ReadPcfile(fi.data)
        return fi.l.resolve(fi.r, sym)
}

func (fi *FuncInfo) Pcline() Sym {
        sym := (*goobj.FuncInfo)(nil).ReadPcline(fi.data)
        return fi.l.resolve(fi.r, sym)
}

func (fi *FuncInfo) Pcinline() Sym {
        sym := (*goobj.FuncInfo)(nil).ReadPcinline(fi.data)
        return fi.l.resolve(fi.r, sym)
}

// Preload has to be called prior to invoking the various methods
// below related to pcdata, funcdataoff, files, and inltree nodes.
func (fi *FuncInfo) Preload() {
        fi.lengths = (*goobj.FuncInfo)(nil).ReadFuncInfoLengths(fi.data)
}

func (fi *FuncInfo) Pcdata() []Sym {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        syms := (*goobj.FuncInfo)(nil).ReadPcdata(fi.data)
        ret := make([]Sym, len(syms))
        for i := range ret {
                ret[i] = fi.l.resolve(fi.r, syms[i])
        }
        return ret
}

func (fi *FuncInfo) NumFuncdataoff() uint32 {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        return fi.lengths.NumFuncdataoff
}

func (fi *FuncInfo) Funcdataoff(k int) int64 {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        return (*goobj.FuncInfo)(nil).ReadFuncdataoff(fi.data, fi.lengths.FuncdataoffOff, uint32(k))
}

func (fi *FuncInfo) Funcdata(syms []Sym) []Sym {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        if int(fi.lengths.NumFuncdataoff) > cap(syms) {
                syms = make([]Sym, 0, fi.lengths.NumFuncdataoff)
        } else {
                syms = syms[:0]
        }
        for j := range fi.auxs {
                a := &fi.auxs[j]
                if a.Type() == goobj.AuxFuncdata {
                        syms = append(syms, fi.l.resolve(fi.r, a.Sym()))
                }
        }
        return syms
}

func (fi *FuncInfo) NumFile() uint32 {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        return fi.lengths.NumFile
}

func (fi *FuncInfo) File(k int) goobj.CUFileIndex {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        return (*goobj.FuncInfo)(nil).ReadFile(fi.data, fi.lengths.FileOff, uint32(k))
}

type InlTreeNode struct {
        Parent   int32
        File     goobj.CUFileIndex
        Line     int32
        Func     Sym
        ParentPC int32
}

func (fi *FuncInfo) NumInlTree() uint32 {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        return fi.lengths.NumInlTree
}

func (fi *FuncInfo) InlTree(k int) InlTreeNode {
        if !fi.lengths.Initialized {
                panic("need to call Preload first")
        }
        node := (*goobj.FuncInfo)(nil).ReadInlTree(fi.data, fi.lengths.InlTreeOff, uint32(k))
        return InlTreeNode{
                Parent:   node.Parent,
                File:     node.File,
                Line:     node.Line,
                Func:     fi.l.resolve(fi.r, node.Func),
                ParentPC: node.ParentPC,
        }
}

func (l *Loader) FuncInfo(i Sym) FuncInfo {
        var r *oReader
        var auxs []goobj.Aux
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                if pp.objidx == 0 {
                        return FuncInfo{}
                }
                r = l.objs[pp.objidx].r
                auxs = pp.auxs
        } else {
                var li uint32
                r, li = l.toLocal(i)
                auxs = r.Auxs(li)
        }
        for j := range auxs {
                a := &auxs[j]
                if a.Type() == goobj.AuxFuncInfo {
                        b := r.Data(a.Sym().SymIdx)
                        return FuncInfo{l, r, b, auxs, goobj.FuncInfoLengths{}}
                }
        }
        return FuncInfo{}
}

// Preload a package: adds autolib.
// Does not add defined package or non-packaged symbols to the symbol table.
// These are done in LoadSyms.
// Does not read symbol data.
// Returns the fingerprint of the object.
func (l *Loader) Preload(localSymVersion int, f *bio.Reader, lib *sym.Library, unit *sym.CompilationUnit, length int64) goobj.FingerprintType {
        roObject, readonly, err := f.Slice(uint64(length)) // TODO: no need to map blocks that are for tools only (e.g. RefName)
        if err != nil {
                log.Fatal("cannot read object file:", err)
        }
        r := goobj.NewReaderFromBytes(roObject, readonly)
        if r == nil {
                if len(roObject) >= 8 && bytes.Equal(roObject[:8], []byte("\x00go114ld")) {
                        log.Fatalf("found object file %s in old format", f.File().Name())
                }
                panic("cannot read object file")
        }
        pkgprefix := objabi.PathToPrefix(lib.Pkg) + "."
        ndef := r.NSym()
        nhashed64def := r.NHashed64def()
        nhasheddef := r.NHasheddef()
        or := &oReader{
                Reader:       r,
                unit:         unit,
                version:      localSymVersion,
                flags:        r.Flags(),
                pkgprefix:    pkgprefix,
                syms:         make([]Sym, ndef+nhashed64def+nhasheddef+r.NNonpkgdef()+r.NNonpkgref()),
                ndef:         ndef,
                nhasheddef:   nhasheddef,
                nhashed64def: nhashed64def,
                objidx:       uint32(len(l.objs)),
        }

        // Autolib
        lib.Autolib = append(lib.Autolib, r.Autolib()...)

        // DWARF file table
        nfile := r.NFile()
        unit.FileTable = make([]string, nfile)
        for i := range unit.FileTable {
                unit.FileTable[i] = r.File(i)
        }

        l.addObj(lib.Pkg, or)

        // The caller expects us consuming all the data
        f.MustSeek(length, os.SEEK_CUR)

        return r.Fingerprint()
}

// Holds the loader along with temporary states for loading symbols.
type loadState struct {
        l            *Loader
        hashed64Syms map[uint64]symAndSize         // short hashed (content-addressable) symbols, keyed by content hash
        hashedSyms   map[goobj.HashType]symAndSize // hashed (content-addressable) symbols, keyed by content hash
}

// Preload symbols of given kind from an object.
func (st *loadState) preloadSyms(r *oReader, kind int) {
        l := st.l
        var start, end uint32
        switch kind {
        case pkgDef:
                start = 0
                end = uint32(r.ndef)
        case hashed64Def:
                start = uint32(r.ndef)
                end = uint32(r.ndef + r.nhashed64def)
        case hashedDef:
                start = uint32(r.ndef + r.nhashed64def)
                end = uint32(r.ndef + r.nhashed64def + r.nhasheddef)
                if l.hasUnknownPkgPath {
                        // The content hash depends on symbol name expansion. If any package is
                        // built without fully expanded names, the content hash is unreliable.
                        // Treat them as named symbols.
                        // This is rare.
                        // (We don't need to do this for hashed64Def case, as there the hash
                        // function is simply the identity function, which doesn't depend on
                        // name expansion.)
                        kind = nonPkgDef
                }
        case nonPkgDef:
                start = uint32(r.ndef + r.nhashed64def + r.nhasheddef)
                end = uint32(r.ndef + r.nhashed64def + r.nhasheddef + r.NNonpkgdef())
        default:
                panic("preloadSyms: bad kind")
        }
        l.growAttrBitmaps(len(l.objSyms) + int(end-start))
        needNameExpansion := r.NeedNameExpansion()
        loadingRuntimePkg := r.unit.Lib.Pkg == "runtime"
        for i := start; i < end; i++ {
                osym := r.Sym(i)
                var name string
                var v int
                if kind != hashed64Def && kind != hashedDef { // we don't need the name, etc. for hashed symbols
                        name = osym.Name(r.Reader)
                        if needNameExpansion {
                                name = strings.Replace(name, "\"\".", r.pkgprefix, -1)
                        }
                        v = abiToVer(osym.ABI(), r.version)
                }
                gi := st.addSym(name, v, r, i, kind, osym)
                r.syms[i] = gi
                if osym.TopFrame() {
                        l.SetAttrTopFrame(gi, true)
                }
                if osym.Local() {
                        l.SetAttrLocal(gi, true)
                }
                if osym.UsedInIface() {
                        l.SetAttrUsedInIface(gi, true)
                }
                if strings.HasPrefix(name, "runtime.") ||
                        (loadingRuntimePkg && strings.HasPrefix(name, "type.")) {
                        if bi := goobj.BuiltinIdx(name, v); bi != -1 {
                                // This is a definition of a builtin symbol. Record where it is.
                                l.builtinSyms[bi] = gi
                        }
                }
                if a := int32(osym.Align()); a != 0 && a > l.SymAlign(gi) {
                        l.SetSymAlign(gi, a)
                }
        }
}

// Add syms, hashed (content-addressable) symbols, non-package symbols, and
// references to external symbols (which are always named).
func (l *Loader) LoadSyms(arch *sys.Arch) {
        // Allocate space for symbols, making a guess as to how much space we need.
        // This function was determined empirically by looking at the cmd/compile on
        // Darwin, and picking factors for hashed and hashed64 syms.
        var symSize, hashedSize, hashed64Size int
        for _, o := range l.objs[goObjStart:] {
                symSize += o.r.ndef + o.r.nhasheddef/2 + o.r.nhashed64def/2 + o.r.NNonpkgdef()
                hashedSize += o.r.nhasheddef / 2
                hashed64Size += o.r.nhashed64def / 2
        }
        // Index 0 is invalid for symbols.
        l.objSyms = make([]objSym, 1, symSize)

        l.npkgsyms = l.NSym()
        st := loadState{
                l:            l,
                hashed64Syms: make(map[uint64]symAndSize, hashed64Size),
                hashedSyms:   make(map[goobj.HashType]symAndSize, hashedSize),
        }

        for _, o := range l.objs[goObjStart:] {
                st.preloadSyms(o.r, pkgDef)
        }
        for _, o := range l.objs[goObjStart:] {
                st.preloadSyms(o.r, hashed64Def)
                st.preloadSyms(o.r, hashedDef)
                st.preloadSyms(o.r, nonPkgDef)
        }
        l.nhashedsyms = len(st.hashed64Syms) + len(st.hashedSyms)
        for _, o := range l.objs[goObjStart:] {
                loadObjRefs(l, o.r, arch)
        }
        l.values = make([]int64, l.NSym(), l.NSym()+1000) // +1000 make some room for external symbols
}

func loadObjRefs(l *Loader, r *oReader, arch *sys.Arch) {
        // load non-package refs
        ndef := uint32(r.NAlldef())
        needNameExpansion := r.NeedNameExpansion()
        for i, n := uint32(0), uint32(r.NNonpkgref()); i < n; i++ {
                osym := r.Sym(ndef + i)
                name := osym.Name(r.Reader)
                if needNameExpansion {
                        name = strings.Replace(name, "\"\".", r.pkgprefix, -1)
                }
                v := abiToVer(osym.ABI(), r.version)
                r.syms[ndef+i] = l.LookupOrCreateSym(name, v)
                gi := r.syms[ndef+i]
                if osym.Local() {
                        l.SetAttrLocal(gi, true)
                }
                if osym.UsedInIface() {
                        l.SetAttrUsedInIface(gi, true)
                }
        }

        // referenced packages
        npkg := r.NPkg()
        r.pkg = make([]uint32, npkg)
        for i := 1; i < npkg; i++ { // PkgIdx 0 is a dummy invalid package
                pkg := r.Pkg(i)
                objidx, ok := l.objByPkg[pkg]
                if !ok {
                        log.Fatalf("reference of nonexisted package %s, from %v", pkg, r.unit.Lib)
                }
                r.pkg[i] = objidx
        }

        // load flags of package refs
        for i, n := 0, r.NRefFlags(); i < n; i++ {
                rf := r.RefFlags(i)
                gi := l.resolve(r, rf.Sym())
                if rf.Flag2()&goobj.SymFlagUsedInIface != 0 {
                        l.SetAttrUsedInIface(gi, true)
                }
        }
}

func abiToVer(abi uint16, localSymVersion int) int {
        var v int
        if abi == goobj.SymABIstatic {
                // Static
                v = localSymVersion
        } else if abiver := sym.ABIToVersion(obj.ABI(abi)); abiver != -1 {
                // Note that data symbols are "ABI0", which maps to version 0.
                v = abiver
        } else {
                log.Fatalf("invalid symbol ABI: %d", abi)
        }
        return v
}

// ResolveABIAlias given a symbol returns the ABI alias target of that
// symbol. If the sym in question is not an alias, the sym itself is
// returned.
func (l *Loader) ResolveABIAlias(s Sym) Sym {
        if s == 0 {
                return 0
        }
        if l.SymType(s) != sym.SABIALIAS {
                return s
        }
        relocs := l.Relocs(s)
        target := relocs.At(0).Sym()
        if l.SymType(target) == sym.SABIALIAS {
                panic(fmt.Sprintf("ABI alias %s references another ABI alias %s", l.SymName(s), l.SymName(target)))
        }
        return target
}

// TopLevelSym tests a symbol (by name and kind) to determine whether
// the symbol first class sym (participating in the link) or is an
// anonymous aux or sub-symbol containing some sub-part or payload of
// another symbol.
func (l *Loader) TopLevelSym(s Sym) bool {
        return topLevelSym(l.RawSymName(s), l.SymType(s))
}

// topLevelSym tests a symbol name and kind to determine whether
// the symbol first class sym (participating in the link) or is an
// anonymous aux or sub-symbol containing some sub-part or payload of
// another symbol.
func topLevelSym(sname string, skind sym.SymKind) bool {
        if sname != "" {
                return true
        }
        switch skind {
        case sym.SDWARFFCN, sym.SDWARFABSFCN, sym.SDWARFTYPE, sym.SDWARFCONST, sym.SDWARFCUINFO, sym.SDWARFRANGE, sym.SDWARFLOC, sym.SDWARFLINES, sym.SGOFUNC:
                return true
        default:
                return false
        }
}

// cloneToExternal takes the existing object file symbol (symIdx)
// and creates a new external symbol payload that is a clone with
// respect to name, version, type, relocations, etc. The idea here
// is that if the linker decides it wants to update the contents of
// a symbol originally discovered as part of an object file, it's
// easier to do this if we make the updates to an external symbol
// payload.
func (l *Loader) cloneToExternal(symIdx Sym) {
        if l.IsExternal(symIdx) {
                panic("sym is already external, no need for clone")
        }

        // Read the particulars from object.
        r, li := l.toLocal(symIdx)
        osym := r.Sym(li)
        sname := osym.Name(r.Reader)
        if r.NeedNameExpansion() {
                sname = strings.Replace(sname, "\"\".", r.pkgprefix, -1)
        }
        sver := abiToVer(osym.ABI(), r.version)
        skind := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type())]

        // Create new symbol, update version and kind.
        pi := l.newPayload(sname, sver)
        pp := l.payloads[pi]
        pp.kind = skind
        pp.ver = sver
        pp.size = int64(osym.Siz())
        pp.objidx = r.objidx

        // If this is a def, then copy the guts. We expect this case
        // to be very rare (one case it may come up is with -X).
        if li < uint32(r.NAlldef()) {

                // Copy relocations
                relocs := l.Relocs(symIdx)
                pp.relocs = make([]goobj.Reloc, relocs.Count())
                pp.reltypes = make([]objabi.RelocType, relocs.Count())
                for i := range pp.relocs {
                        // Copy the relocs slice.
                        // Convert local reference to global reference.
                        rel := relocs.At(i)
                        pp.relocs[i].Set(rel.Off(), rel.Siz(), 0, rel.Add(), goobj.SymRef{PkgIdx: 0, SymIdx: uint32(rel.Sym())})
                        pp.reltypes[i] = rel.Type()
                }

                // Copy data
                pp.data = r.Data(li)
        }

        // If we're overriding a data symbol, collect the associated
        // Gotype, so as to propagate it to the new symbol.
        auxs := r.Auxs(li)
        pp.auxs = auxs

        // Install new payload to global index space.
        // (This needs to happen at the end, as the accessors above
        // need to access the old symbol content.)
        l.objSyms[symIdx] = objSym{l.extReader.objidx, uint32(pi)}
        l.extReader.syms = append(l.extReader.syms, symIdx)
}

// Copy the payload of symbol src to dst. Both src and dst must be external
// symbols.
// The intended use case is that when building/linking against a shared library,
// where we do symbol name mangling, the Go object file may have reference to
// the original symbol name whereas the shared library provides a symbol with
// the mangled name. When we do mangling, we copy payload of mangled to original.
func (l *Loader) CopySym(src, dst Sym) {
        if !l.IsExternal(dst) {
                panic("dst is not external") //l.newExtSym(l.SymName(dst), l.SymVersion(dst))
        }
        if !l.IsExternal(src) {
                panic("src is not external") //l.cloneToExternal(src)
        }
        l.payloads[l.extIndex(dst)] = l.payloads[l.extIndex(src)]
        l.SetSymPkg(dst, l.SymPkg(src))
        // TODO: other attributes?
}

// CopyAttributes copies over all of the attributes of symbol 'src' to
// symbol 'dst'.
func (l *Loader) CopyAttributes(src Sym, dst Sym) {
        l.SetAttrReachable(dst, l.AttrReachable(src))
        l.SetAttrOnList(dst, l.AttrOnList(src))
        l.SetAttrLocal(dst, l.AttrLocal(src))
        l.SetAttrNotInSymbolTable(dst, l.AttrNotInSymbolTable(src))
        if l.IsExternal(dst) {
                l.SetAttrVisibilityHidden(dst, l.AttrVisibilityHidden(src))
                l.SetAttrDuplicateOK(dst, l.AttrDuplicateOK(src))
                l.SetAttrShared(dst, l.AttrShared(src))
                l.SetAttrExternal(dst, l.AttrExternal(src))
        } else {
                // Some attributes are modifiable only for external symbols.
                // In such cases, don't try to transfer over the attribute
                // from the source even if there is a clash. This comes up
                // when copying attributes from a dupOK ABI wrapper symbol to
                // the real target symbol (which may not be marked dupOK).
        }
        l.SetAttrTopFrame(dst, l.AttrTopFrame(src))
        l.SetAttrSpecial(dst, l.AttrSpecial(src))
        l.SetAttrCgoExportDynamic(dst, l.AttrCgoExportDynamic(src))
        l.SetAttrCgoExportStatic(dst, l.AttrCgoExportStatic(src))
        l.SetAttrReadOnly(dst, l.AttrReadOnly(src))
}

// CreateExtSym creates a new external symbol with the specified name
// without adding it to any lookup tables, returning a Sym index for it.
func (l *Loader) CreateExtSym(name string, ver int) Sym {
        return l.newExtSym(name, ver)
}

// CreateStaticSym creates a new static symbol with the specified name
// without adding it to any lookup tables, returning a Sym index for it.
func (l *Loader) CreateStaticSym(name string) Sym {
        // Assign a new unique negative version -- this is to mark the
        // symbol so that it is not included in the name lookup table.
        l.anonVersion--
        return l.newExtSym(name, l.anonVersion)
}

func (l *Loader) FreeSym(i Sym) {
        if l.IsExternal(i) {
                pp := l.getPayload(i)
                *pp = extSymPayload{}
        }
}

// relocId is essentially a <S,R> tuple identifying the Rth
// relocation of symbol S.
type relocId struct {
        sym  Sym
        ridx int
}

// SetRelocVariant sets the 'variant' property of a relocation on
// some specific symbol.
func (l *Loader) SetRelocVariant(s Sym, ri int, v sym.RelocVariant) {
        // sanity check
        if relocs := l.Relocs(s); ri >= relocs.Count() {
                panic("invalid relocation ID")
        }
        if l.relocVariant == nil {
                l.relocVariant = make(map[relocId]sym.RelocVariant)
        }
        if v != 0 {
                l.relocVariant[relocId{s, ri}] = v
        } else {
                delete(l.relocVariant, relocId{s, ri})
        }
}

// RelocVariant returns the 'variant' property of a relocation on
// some specific symbol.
func (l *Loader) RelocVariant(s Sym, ri int) sym.RelocVariant {
        return l.relocVariant[relocId{s, ri}]
}

// UndefinedRelocTargets iterates through the global symbol index
// space, looking for symbols with relocations targeting undefined
// references. The linker's loadlib method uses this to determine if
// there are unresolved references to functions in system libraries
// (for example, libgcc.a), presumably due to CGO code. Return
// value is a list of loader.Sym's corresponding to the undefined
// cross-refs. The "limit" param controls the maximum number of
// results returned; if "limit" is -1, then all undefs are returned.
func (l *Loader) UndefinedRelocTargets(limit int) []Sym {
        result := []Sym{}
        for si := Sym(1); si < Sym(len(l.objSyms)); si++ {
                relocs := l.Relocs(si)
                for ri := 0; ri < relocs.Count(); ri++ {
                        r := relocs.At(ri)
                        rs := r.Sym()
                        if rs != 0 && l.SymType(rs) == sym.SXREF && l.RawSymName(rs) != ".got" {
                                result = append(result, rs)
                                if limit != -1 && len(result) >= limit {
                                        break
                                }
                        }
                }
        }
        return result
}

// AssignTextSymbolOrder populates the Textp slices within each
// library and compilation unit, insuring that packages are laid down
// in dependency order (internal first, then everything else). Return value
// is a slice of all text syms.
func (l *Loader) AssignTextSymbolOrder(libs []*sym.Library, intlibs []bool, extsyms []Sym) []Sym {

        // Library Textp lists should be empty at this point.
        for _, lib := range libs {
                if len(lib.Textp) != 0 {
                        panic("expected empty Textp slice for library")
                }
                if len(lib.DupTextSyms) != 0 {
                        panic("expected empty DupTextSyms slice for library")
                }
        }

        // Used to record which dupok symbol we've assigned to a unit.
        // Can't use the onlist attribute here because it will need to
        // clear for the later assignment of the sym.Symbol to a unit.
        // NB: we can convert to using onList once we no longer have to
        // call the regular addToTextp.
        assignedToUnit := MakeBitmap(l.NSym() + 1)

        // Start off textp with reachable external syms.
        textp := []Sym{}
        for _, sym := range extsyms {
                if !l.attrReachable.Has(sym) {
                        continue
                }
                textp = append(textp, sym)
        }

        // Walk through all text symbols from Go object files and append
        // them to their corresponding library's textp list.
        for _, o := range l.objs[goObjStart:] {
                r := o.r
                lib := r.unit.Lib
                for i, n := uint32(0), uint32(r.NAlldef()); i < n; i++ {
                        gi := l.toGlobal(r, i)
                        if !l.attrReachable.Has(gi) {
                                continue
                        }
                        osym := r.Sym(i)
                        st := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type())]
                        if st != sym.STEXT {
                                continue
                        }
                        dupok := osym.Dupok()
                        if r2, i2 := l.toLocal(gi); r2 != r || i2 != i {
                                // A dupok text symbol is resolved to another package.
                                // We still need to record its presence in the current
                                // package, as the trampoline pass expects packages
                                // are laid out in dependency order.
                                lib.DupTextSyms = append(lib.DupTextSyms, sym.LoaderSym(gi))
                                continue // symbol in different object
                        }
                        if dupok {
                                lib.DupTextSyms = append(lib.DupTextSyms, sym.LoaderSym(gi))
                                continue
                        }

                        lib.Textp = append(lib.Textp, sym.LoaderSym(gi))
                }
        }

        // Now assemble global textp, and assign text symbols to units.
        for _, doInternal := range [2]bool{true, false} {
                for idx, lib := range libs {
                        if intlibs[idx] != doInternal {
                                continue
                        }
                        lists := [2][]sym.LoaderSym{lib.Textp, lib.DupTextSyms}
                        for i, list := range lists {
                                for _, s := range list {
                                        sym := Sym(s)
                                        if l.attrReachable.Has(sym) && !assignedToUnit.Has(sym) {
                                                textp = append(textp, sym)
                                                unit := l.SymUnit(sym)
                                                if unit != nil {
                                                        unit.Textp = append(unit.Textp, s)
                                                        assignedToUnit.Set(sym)
                                                }
                                                // Dupok symbols may be defined in multiple packages; the
                                                // associated package for a dupok sym is chosen sort of
                                                // arbitrarily (the first containing package that the linker
                                                // loads). Canonicalizes its Pkg to the package with which
                                                // it will be laid down in text.
                                                if i == 1 /* DupTextSyms2 */ && l.SymPkg(sym) != lib.Pkg {
                                                        l.SetSymPkg(sym, lib.Pkg)
                                                }
                                        }
                                }
                        }
                        lib.Textp = nil
                        lib.DupTextSyms = nil
                }
        }

        return textp
}

// ErrorReporter is a helper class for reporting errors.
type ErrorReporter struct {
        ldr              *Loader
        AfterErrorAction func()
}

// Errorf method logs an error message.
//
// After each error, the error actions function will be invoked; this
// will either terminate the link immediately (if -h option given)
// or it will keep a count and exit if more than 20 errors have been printed.
//
// Logging an error means that on exit cmd/link will delete any
// output file and return a non-zero error code.
//
func (reporter *ErrorReporter) Errorf(s Sym, format string, args ...interface{}) {
        if s != 0 && reporter.ldr.SymName(s) != "" {
                format = reporter.ldr.SymName(s) + ": " + format
        } else {
                format = fmt.Sprintf("sym %d: %s", s, format)
        }
        format += "\n"
        fmt.Fprintf(os.Stderr, format, args...)
        reporter.AfterErrorAction()
}

// GetErrorReporter returns the loader's associated error reporter.
func (l *Loader) GetErrorReporter() *ErrorReporter {
        return l.errorReporter
}

// Errorf method logs an error message. See ErrorReporter.Errorf for details.
func (l *Loader) Errorf(s Sym, format string, args ...interface{}) {
        l.errorReporter.Errorf(s, format, args...)
}

// Symbol statistics.
func (l *Loader) Stat() string {
        s := fmt.Sprintf("%d symbols, %d reachable\n", l.NSym(), l.NReachableSym())
        s += fmt.Sprintf("\t%d package symbols, %d hashed symbols, %d non-package symbols, %d external symbols\n",
                l.npkgsyms, l.nhashedsyms, int(l.extStart)-l.npkgsyms-l.nhashedsyms, l.NSym()-int(l.extStart))
        return s
}

// For debugging.
func (l *Loader) Dump() {
        fmt.Println("objs")
        for _, obj := range l.objs[goObjStart:] {
                if obj.r != nil {
                        fmt.Println(obj.i, obj.r.unit.Lib)
                }
        }
        fmt.Println("extStart:", l.extStart)
        fmt.Println("Nsyms:", len(l.objSyms))
        fmt.Println("syms")
        for i := Sym(1); i < Sym(len(l.objSyms)); i++ {
                pi := interface{}("")
                if l.IsExternal(i) {
                        pi = fmt.Sprintf("<ext %d>", l.extIndex(i))
                }
                fmt.Println(i, l.SymName(i), l.SymType(i), pi)
        }
        fmt.Println("symsByName")
        for name, i := range l.symsByName[0] {
                fmt.Println(i, name, 0)
        }
        for name, i := range l.symsByName[1] {
                fmt.Println(i, name, 1)
        }
        fmt.Println("payloads:")
        for i := range l.payloads {
                pp := l.payloads[i]
                fmt.Println(i, pp.name, pp.ver, pp.kind)
        }
}