[dev.typeparams] all: merge master (fdab5be) into dev.typeparams

[gostls13.git] / src / runtime / traceback.go

// Copyright 2009 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 runtime

import (
        "internal/bytealg"
        "runtime/internal/atomic"
        "runtime/internal/sys"
        "unsafe"
)

// The code in this file implements stack trace walking for all architectures.
// The most important fact about a given architecture is whether it uses a link register.
// On systems with link registers, the prologue for a non-leaf function stores the
// incoming value of LR at the bottom of the newly allocated stack frame.
// On systems without link registers (x86), the architecture pushes a return PC during
// the call instruction, so the return PC ends up above the stack frame.
// In this file, the return PC is always called LR, no matter how it was found.

const usesLR = sys.MinFrameSize > 0

// Generic traceback. Handles runtime stack prints (pcbuf == nil),
// the runtime.Callers function (pcbuf != nil), as well as the garbage
// collector (callback != nil).  A little clunky to merge these, but avoids
// duplicating the code and all its subtlety.
//
// The skip argument is only valid with pcbuf != nil and counts the number
// of logical frames to skip rather than physical frames (with inlining, a
// PC in pcbuf can represent multiple calls).
func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max int, callback func(*stkframe, unsafe.Pointer) bool, v unsafe.Pointer, flags uint) int {
        if skip > 0 && callback != nil {
                throw("gentraceback callback cannot be used with non-zero skip")
        }

        // Don't call this "g"; it's too easy get "g" and "gp" confused.
        if ourg := getg(); ourg == gp && ourg == ourg.m.curg {
                // The starting sp has been passed in as a uintptr, and the caller may
                // have other uintptr-typed stack references as well.
                // If during one of the calls that got us here or during one of the
                // callbacks below the stack must be grown, all these uintptr references
                // to the stack will not be updated, and gentraceback will continue
                // to inspect the old stack memory, which may no longer be valid.
                // Even if all the variables were updated correctly, it is not clear that
                // we want to expose a traceback that begins on one stack and ends
                // on another stack. That could confuse callers quite a bit.
                // Instead, we require that gentraceback and any other function that
                // accepts an sp for the current goroutine (typically obtained by
                // calling getcallersp) must not run on that goroutine's stack but
                // instead on the g0 stack.
                throw("gentraceback cannot trace user goroutine on its own stack")
        }
        level, _, _ := gotraceback()

        var ctxt *funcval // Context pointer for unstarted goroutines. See issue #25897.

        if pc0 == ^uintptr(0) && sp0 == ^uintptr(0) { // Signal to fetch saved values from gp.
                if gp.syscallsp != 0 {
                        pc0 = gp.syscallpc
                        sp0 = gp.syscallsp
                        if usesLR {
                                lr0 = 0
                        }
                } else {
                        pc0 = gp.sched.pc
                        sp0 = gp.sched.sp
                        if usesLR {
                                lr0 = gp.sched.lr
                        }
                        ctxt = (*funcval)(gp.sched.ctxt)
                }
        }

        nprint := 0
        var frame stkframe
        frame.pc = pc0
        frame.sp = sp0
        if usesLR {
                frame.lr = lr0
        }
        waspanic := false
        cgoCtxt := gp.cgoCtxt
        printing := pcbuf == nil && callback == nil

        // If the PC is zero, it's likely a nil function call.
        // Start in the caller's frame.
        if frame.pc == 0 {
                if usesLR {
                        frame.pc = *(*uintptr)(unsafe.Pointer(frame.sp))
                        frame.lr = 0
                } else {
                        frame.pc = uintptr(*(*uintptr)(unsafe.Pointer(frame.sp)))
                        frame.sp += sys.PtrSize
                }
        }

        f := findfunc(frame.pc)
        if !f.valid() {
                if callback != nil || printing {
                        print("runtime: unknown pc ", hex(frame.pc), "\n")
                        tracebackHexdump(gp.stack, &frame, 0)
                }
                if callback != nil {
                        throw("unknown pc")
                }
                return 0
        }
        frame.fn = f

        var cache pcvalueCache

        lastFuncID := funcID_normal
        n := 0
        for n < max {
                // Typically:
                //      pc is the PC of the running function.
                //      sp is the stack pointer at that program counter.
                //      fp is the frame pointer (caller's stack pointer) at that program counter, or nil if unknown.
                //      stk is the stack containing sp.
                //      The caller's program counter is lr, unless lr is zero, in which case it is *(uintptr*)sp.
                f = frame.fn
                if f.pcsp == 0 {
                        // No frame information, must be external function, like race support.
                        // See golang.org/issue/13568.
                        break
                }

                // Compute function info flags.
                flag := f.flag
                if f.funcID == funcID_cgocallback {
                        // cgocallback does write SP to switch from the g0 to the curg stack,
                        // but it carefully arranges that during the transition BOTH stacks
                        // have cgocallback frame valid for unwinding through.
                        // So we don't need to exclude it with the other SP-writing functions.
                        flag &^= funcFlag_SPWRITE
                }
                if frame.pc == pc0 && frame.sp == sp0 && pc0 == gp.syscallpc && sp0 == gp.syscallsp {
                        // Some Syscall functions write to SP, but they do so only after
                        // saving the entry PC/SP using entersyscall.
                        // Since we are using the entry PC/SP, the later SP write doesn't matter.
                        flag &^= funcFlag_SPWRITE
                }

                // Found an actual function.
                // Derive frame pointer and link register.
                if frame.fp == 0 {
                        // Jump over system stack transitions. If we're on g0 and there's a user
                        // goroutine, try to jump. Otherwise this is a regular call.
                        if flags&_TraceJumpStack != 0 && gp == gp.m.g0 && gp.m.curg != nil {
                                switch f.funcID {
                                case funcID_morestack:
                                        // morestack does not return normally -- newstack()
                                        // gogo's to curg.sched. Match that.
                                        // This keeps morestack() from showing up in the backtrace,
                                        // but that makes some sense since it'll never be returned
                                        // to.
                                        frame.pc = gp.m.curg.sched.pc
                                        frame.fn = findfunc(frame.pc)
                                        f = frame.fn
                                        flag = f.flag
                                        frame.sp = gp.m.curg.sched.sp
                                        cgoCtxt = gp.m.curg.cgoCtxt
                                case funcID_systemstack:
                                        // systemstack returns normally, so just follow the
                                        // stack transition.
                                        frame.sp = gp.m.curg.sched.sp
                                        cgoCtxt = gp.m.curg.cgoCtxt
                                        flag &^= funcFlag_SPWRITE
                                }
                        }
                        frame.fp = frame.sp + uintptr(funcspdelta(f, frame.pc, &cache))
                        if !usesLR {
                                // On x86, call instruction pushes return PC before entering new function.
                                frame.fp += sys.PtrSize
                        }
                }
                var flr funcInfo
                if flag&funcFlag_TOPFRAME != 0 {
                        // This function marks the top of the stack. Stop the traceback.
                        frame.lr = 0
                        flr = funcInfo{}
                } else if flag&funcFlag_SPWRITE != 0 && (callback == nil || n > 0) {
                        // The function we are in does a write to SP that we don't know
                        // how to encode in the spdelta table. Examples include context
                        // switch routines like runtime.gogo but also any code that switches
                        // to the g0 stack to run host C code. Since we can't reliably unwind
                        // the SP (we might not even be on the stack we think we are),
                        // we stop the traceback here.
                        // This only applies for profiling signals (callback == nil).
                        //
                        // For a GC stack traversal (callback != nil), we should only see
                        // a function when it has voluntarily preempted itself on entry
                        // during the stack growth check. In that case, the function has
                        // not yet had a chance to do any writes to SP and is safe to unwind.
                        // isAsyncSafePoint does not allow assembly functions to be async preempted,
                        // and preemptPark double-checks that SPWRITE functions are not async preempted.
                        // So for GC stack traversal we leave things alone (this if body does not execute for n == 0)
                        // at the bottom frame of the stack. But farther up the stack we'd better not
                        // find any.
                        if callback != nil {
                                println("traceback: unexpected SPWRITE function", funcname(f))
                                throw("traceback")
                        }
                        frame.lr = 0
                        flr = funcInfo{}
                } else {
                        var lrPtr uintptr
                        if usesLR {
                                if n == 0 && frame.sp < frame.fp || frame.lr == 0 {
                                        lrPtr = frame.sp
                                        frame.lr = *(*uintptr)(unsafe.Pointer(lrPtr))
                                }
                        } else {
                                if frame.lr == 0 {
                                        lrPtr = frame.fp - sys.PtrSize
                                        frame.lr = uintptr(*(*uintptr)(unsafe.Pointer(lrPtr)))
                                }
                        }
                        flr = findfunc(frame.lr)
                        if !flr.valid() {
                                // This happens if you get a profiling interrupt at just the wrong time.
                                // In that context it is okay to stop early.
                                // But if callback is set, we're doing a garbage collection and must
                                // get everything, so crash loudly.
                                doPrint := printing
                                if doPrint && gp.m.incgo && f.funcID == funcID_sigpanic {
                                        // We can inject sigpanic
                                        // calls directly into C code,
                                        // in which case we'll see a C
                                        // return PC. Don't complain.
                                        doPrint = false
                                }
                                if callback != nil || doPrint {
                                        print("runtime: unexpected return pc for ", funcname(f), " called from ", hex(frame.lr), "\n")
                                        tracebackHexdump(gp.stack, &frame, lrPtr)
                                }
                                if callback != nil {
                                        throw("unknown caller pc")
                                }
                        }
                }

                frame.varp = frame.fp
                if !usesLR {
                        // On x86, call instruction pushes return PC before entering new function.
                        frame.varp -= sys.PtrSize
                }

                // For architectures with frame pointers, if there's
                // a frame, then there's a saved frame pointer here.
                //
                // NOTE: This code is not as general as it looks.
                // On x86, the ABI is to save the frame pointer word at the
                // top of the stack frame, so we have to back down over it.
                // On arm64, the frame pointer should be at the bottom of
                // the stack (with R29 (aka FP) = RSP), in which case we would
                // not want to do the subtraction here. But we started out without
                // any frame pointer, and when we wanted to add it, we didn't
                // want to break all the assembly doing direct writes to 8(RSP)
                // to set the first parameter to a called function.
                // So we decided to write the FP link *below* the stack pointer
                // (with R29 = RSP - 8 in Go functions).
                // This is technically ABI-compatible but not standard.
                // And it happens to end up mimicking the x86 layout.
                // Other architectures may make different decisions.
                if frame.varp > frame.sp && framepointer_enabled {
                        frame.varp -= sys.PtrSize
                }

                // Derive size of arguments.
                // Most functions have a fixed-size argument block,
                // so we can use metadata about the function f.
                // Not all, though: there are some variadic functions
                // in package runtime and reflect, and for those we use call-specific
                // metadata recorded by f's caller.
                if callback != nil || printing {
                        frame.argp = frame.fp + sys.MinFrameSize
                        var ok bool
                        frame.arglen, frame.argmap, ok = getArgInfoFast(f, callback != nil)
                        if !ok {
                                frame.arglen, frame.argmap = getArgInfo(&frame, f, callback != nil, ctxt)
                        }
                }
                ctxt = nil // ctxt is only needed to get arg maps for the topmost frame

                // Determine frame's 'continuation PC', where it can continue.
                // Normally this is the return address on the stack, but if sigpanic
                // is immediately below this function on the stack, then the frame
                // stopped executing due to a trap, and frame.pc is probably not
                // a safe point for looking up liveness information. In this panicking case,
                // the function either doesn't return at all (if it has no defers or if the
                // defers do not recover) or it returns from one of the calls to
                // deferproc a second time (if the corresponding deferred func recovers).
                // In the latter case, use a deferreturn call site as the continuation pc.
                frame.continpc = frame.pc
                if waspanic {
                        if frame.fn.deferreturn != 0 {
                                frame.continpc = frame.fn.entry + uintptr(frame.fn.deferreturn) + 1
                                // Note: this may perhaps keep return variables alive longer than
                                // strictly necessary, as we are using "function has a defer statement"
                                // as a proxy for "function actually deferred something". It seems
                                // to be a minor drawback. (We used to actually look through the
                                // gp._defer for a defer corresponding to this function, but that
                                // is hard to do with defer records on the stack during a stack copy.)
                                // Note: the +1 is to offset the -1 that
                                // stack.go:getStackMap does to back up a return
                                // address make sure the pc is in the CALL instruction.
                        } else {
                                frame.continpc = 0
                        }
                }

                if callback != nil {
                        if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
                                return n
                        }
                }

                if pcbuf != nil {
                        pc := frame.pc
                        // backup to CALL instruction to read inlining info (same logic as below)
                        tracepc := pc
                        // Normally, pc is a return address. In that case, we want to look up
                        // file/line information using pc-1, because that is the pc of the
                        // call instruction (more precisely, the last byte of the call instruction).
                        // Callers expect the pc buffer to contain return addresses and do the
                        // same -1 themselves, so we keep pc unchanged.
                        // When the pc is from a signal (e.g. profiler or segv) then we want
                        // to look up file/line information using pc, and we store pc+1 in the
                        // pc buffer so callers can unconditionally subtract 1 before looking up.
                        // See issue 34123.
                        // The pc can be at function entry when the frame is initialized without
                        // actually running code, like runtime.mstart.
                        if (n == 0 && flags&_TraceTrap != 0) || waspanic || pc == f.entry {
                                pc++
                        } else {
                                tracepc--
                        }

                        // If there is inlining info, record the inner frames.
                        if inldata := funcdata(f, _FUNCDATA_InlTree); inldata != nil {
                                inltree := (*[1 << 20]inlinedCall)(inldata)
                                for {
                                        ix := pcdatavalue(f, _PCDATA_InlTreeIndex, tracepc, &cache)
                                        if ix < 0 {
                                                break
                                        }
                                        if inltree[ix].funcID == funcID_wrapper && elideWrapperCalling(lastFuncID) {
                                                // ignore wrappers
                                        } else if skip > 0 {
                                                skip--
                                        } else if n < max {
                                                (*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = pc
                                                n++
                                        }
                                        lastFuncID = inltree[ix].funcID
                                        // Back up to an instruction in the "caller".
                                        tracepc = frame.fn.entry + uintptr(inltree[ix].parentPc)
                                        pc = tracepc + 1
                                }
                        }
                        // Record the main frame.
                        if f.funcID == funcID_wrapper && elideWrapperCalling(lastFuncID) {
                                // Ignore wrapper functions (except when they trigger panics).
                        } else if skip > 0 {
                                skip--
                        } else if n < max {
                                (*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = pc
                                n++
                        }
                        lastFuncID = f.funcID
                        n-- // offset n++ below
                }

                if printing {
                        // assume skip=0 for printing.
                        //
                        // Never elide wrappers if we haven't printed
                        // any frames. And don't elide wrappers that
                        // called panic rather than the wrapped
                        // function. Otherwise, leave them out.

                        // backup to CALL instruction to read inlining info (same logic as below)
                        tracepc := frame.pc
                        if (n > 0 || flags&_TraceTrap == 0) && frame.pc > f.entry && !waspanic {
                                tracepc--
                        }
                        // If there is inlining info, print the inner frames.
                        if inldata := funcdata(f, _FUNCDATA_InlTree); inldata != nil {
                                inltree := (*[1 << 20]inlinedCall)(inldata)
                                var inlFunc _func
                                inlFuncInfo := funcInfo{&inlFunc, f.datap}
                                for {
                                        ix := pcdatavalue(f, _PCDATA_InlTreeIndex, tracepc, nil)
                                        if ix < 0 {
                                                break
                                        }

                                        // Create a fake _func for the
                                        // inlined function.
                                        inlFunc.nameoff = inltree[ix].func_
                                        inlFunc.funcID = inltree[ix].funcID

                                        if (flags&_TraceRuntimeFrames) != 0 || showframe(inlFuncInfo, gp, nprint == 0, inlFuncInfo.funcID, lastFuncID) {
                                                name := funcname(inlFuncInfo)
                                                file, line := funcline(f, tracepc)
                                                print(name, "(...)\n")
                                                print("\t", file, ":", line, "\n")
                                                nprint++
                                        }
                                        lastFuncID = inltree[ix].funcID
                                        // Back up to an instruction in the "caller".
                                        tracepc = frame.fn.entry + uintptr(inltree[ix].parentPc)
                                }
                        }
                        if (flags&_TraceRuntimeFrames) != 0 || showframe(f, gp, nprint == 0, f.funcID, lastFuncID) {
                                // Print during crash.
                                //      main(0x1, 0x2, 0x3)
                                //              /home/rsc/go/src/runtime/x.go:23 +0xf
                                //
                                name := funcname(f)
                                file, line := funcline(f, tracepc)
                                if name == "runtime.gopanic" {
                                        name = "panic"
                                }
                                print(name, "(")
                                argp := unsafe.Pointer(frame.argp)
                                printArgs(f, argp)
                                print(")\n")
                                print("\t", file, ":", line)
                                if frame.pc > f.entry {
                                        print(" +", hex(frame.pc-f.entry))
                                }
                                if gp.m != nil && gp.m.throwing > 0 && gp == gp.m.curg || level >= 2 {
                                        print(" fp=", hex(frame.fp), " sp=", hex(frame.sp), " pc=", hex(frame.pc))
                                }
                                print("\n")
                                nprint++
                        }
                        lastFuncID = f.funcID
                }
                n++

                if f.funcID == funcID_cgocallback && len(cgoCtxt) > 0 {
                        ctxt := cgoCtxt[len(cgoCtxt)-1]
                        cgoCtxt = cgoCtxt[:len(cgoCtxt)-1]

                        // skip only applies to Go frames.
                        // callback != nil only used when we only care
                        // about Go frames.
                        if skip == 0 && callback == nil {
                                n = tracebackCgoContext(pcbuf, printing, ctxt, n, max)
                        }
                }

                waspanic = f.funcID == funcID_sigpanic
                injectedCall := waspanic || f.funcID == funcID_asyncPreempt

                // Do not unwind past the bottom of the stack.
                if !flr.valid() {
                        break
                }

                // Unwind to next frame.
                frame.fn = flr
                frame.pc = frame.lr
                frame.lr = 0
                frame.sp = frame.fp
                frame.fp = 0
                frame.argmap = nil

                // On link register architectures, sighandler saves the LR on stack
                // before faking a call.
                if usesLR && injectedCall {
                        x := *(*uintptr)(unsafe.Pointer(frame.sp))
                        frame.sp += alignUp(sys.MinFrameSize, sys.StackAlign)
                        f = findfunc(frame.pc)
                        frame.fn = f
                        if !f.valid() {
                                frame.pc = x
                        } else if funcspdelta(f, frame.pc, &cache) == 0 {
                                frame.lr = x
                        }
                }
        }

        if printing {
                n = nprint
        }

        // Note that panic != nil is okay here: there can be leftover panics,
        // because the defers on the panic stack do not nest in frame order as
        // they do on the defer stack. If you have:
        //
        //      frame 1 defers d1
        //      frame 2 defers d2
        //      frame 3 defers d3
        //      frame 4 panics
        //      frame 4's panic starts running defers
        //      frame 5, running d3, defers d4
        //      frame 5 panics
        //      frame 5's panic starts running defers
        //      frame 6, running d4, garbage collects
        //      frame 6, running d2, garbage collects
        //
        // During the execution of d4, the panic stack is d4 -> d3, which
        // is nested properly, and we'll treat frame 3 as resumable, because we
        // can find d3. (And in fact frame 3 is resumable. If d4 recovers
        // and frame 5 continues running, d3, d3 can recover and we'll
        // resume execution in (returning from) frame 3.)
        //
        // During the execution of d2, however, the panic stack is d2 -> d3,
        // which is inverted. The scan will match d2 to frame 2 but having
        // d2 on the stack until then means it will not match d3 to frame 3.
        // This is okay: if we're running d2, then all the defers after d2 have
        // completed and their corresponding frames are dead. Not finding d3
        // for frame 3 means we'll set frame 3's continpc == 0, which is correct
        // (frame 3 is dead). At the end of the walk the panic stack can thus
        // contain defers (d3 in this case) for dead frames. The inversion here
        // always indicates a dead frame, and the effect of the inversion on the
        // scan is to hide those dead frames, so the scan is still okay:
        // what's left on the panic stack are exactly (and only) the dead frames.
        //
        // We require callback != nil here because only when callback != nil
        // do we know that gentraceback is being called in a "must be correct"
        // context as opposed to a "best effort" context. The tracebacks with
        // callbacks only happen when everything is stopped nicely.
        // At other times, such as when gathering a stack for a profiling signal
        // or when printing a traceback during a crash, everything may not be
        // stopped nicely, and the stack walk may not be able to complete.
        if callback != nil && n < max && frame.sp != gp.stktopsp {
                print("runtime: g", gp.goid, ": frame.sp=", hex(frame.sp), " top=", hex(gp.stktopsp), "\n")
                print("\tstack=[", hex(gp.stack.lo), "-", hex(gp.stack.hi), "] n=", n, " max=", max, "\n")
                throw("traceback did not unwind completely")
        }

        return n
}

// printArgs prints function arguments in traceback.
func printArgs(f funcInfo, argp unsafe.Pointer) {
        // The "instruction" of argument printing is encoded in _FUNCDATA_ArgInfo.
        // See cmd/compile/internal/ssagen.emitArgInfo for the description of the
        // encoding.
        // These constants need to be in sync with the compiler.
        const (
                _endSeq         = 0xff
                _startAgg       = 0xfe
                _endAgg         = 0xfd
                _dotdotdot      = 0xfc
                _offsetTooLarge = 0xfb
        )

        const (
                limit    = 10                       // print no more than 10 args/components
                maxDepth = 5                        // no more than 5 layers of nesting
                maxLen   = (maxDepth*3+2)*limit + 1 // max length of _FUNCDATA_ArgInfo (see the compiler side for reasoning)
        )

        p := (*[maxLen]uint8)(funcdata(f, _FUNCDATA_ArgInfo))
        if p == nil {
                return
        }

        print1 := func(off, sz uint8) {
                x := readUnaligned64(add(argp, uintptr(off)))
                // mask out irrelavant bits
                if sz < 8 {
                        shift := 64 - sz*8
                        if sys.BigEndian {
                                x = x >> shift
                        } else {
                                x = x << shift >> shift
                        }
                }
                print(hex(x))
        }

        start := true
        printcomma := func() {
                if !start {
                        print(", ")
                }
        }
        pi := 0
printloop:
        for {
                o := p[pi]
                pi++
                switch o {
                case _endSeq:
                        break printloop
                case _startAgg:
                        printcomma()
                        print("{")
                        start = true
                        continue
                case _endAgg:
                        print("}")
                case _dotdotdot:
                        printcomma()
                        print("...")
                case _offsetTooLarge:
                        printcomma()
                        print("_")
                default:
                        printcomma()
                        sz := p[pi]
                        pi++
                        print1(o, sz)
                }
                start = false
        }
}

// reflectMethodValue is a partial duplicate of reflect.makeFuncImpl
// and reflect.methodValue.
type reflectMethodValue struct {
        fn     uintptr
        stack  *bitvector // ptrmap for both args and results
        argLen uintptr    // just args
}

// getArgInfoFast returns the argument frame information for a call to f.
// It is short and inlineable. However, it does not handle all functions.
// If ok reports false, you must call getArgInfo instead.
// TODO(josharian): once we do mid-stack inlining,
// call getArgInfo directly from getArgInfoFast and stop returning an ok bool.
func getArgInfoFast(f funcInfo, needArgMap bool) (arglen uintptr, argmap *bitvector, ok bool) {
        return uintptr(f.args), nil, !(needArgMap && f.args == _ArgsSizeUnknown)
}

// getArgInfo returns the argument frame information for a call to f
// with call frame frame.
//
// This is used for both actual calls with active stack frames and for
// deferred calls or goroutines that are not yet executing. If this is an actual
// call, ctxt must be nil (getArgInfo will retrieve what it needs from
// the active stack frame). If this is a deferred call or unstarted goroutine,
// ctxt must be the function object that was deferred or go'd.
func getArgInfo(frame *stkframe, f funcInfo, needArgMap bool, ctxt *funcval) (arglen uintptr, argmap *bitvector) {
        arglen = uintptr(f.args)
        if needArgMap && f.args == _ArgsSizeUnknown {
                // Extract argument bitmaps for reflect stubs from the calls they made to reflect.
                switch funcname(f) {
                case "reflect.makeFuncStub", "reflect.methodValueCall":
                        // These take a *reflect.methodValue as their
                        // context register.
                        var mv *reflectMethodValue
                        var retValid bool
                        if ctxt != nil {
                                // This is not an actual call, but a
                                // deferred call or an unstarted goroutine.
                                // The function value is itself the *reflect.methodValue.
                                mv = (*reflectMethodValue)(unsafe.Pointer(ctxt))
                        } else {
                                // This is a real call that took the
                                // *reflect.methodValue as its context
                                // register and immediately saved it
                                // to 0(SP). Get the methodValue from
                                // 0(SP).
                                arg0 := frame.sp + sys.MinFrameSize
                                mv = *(**reflectMethodValue)(unsafe.Pointer(arg0))
                                // Figure out whether the return values are valid.
                                // Reflect will update this value after it copies
                                // in the return values.
                                retValid = *(*bool)(unsafe.Pointer(arg0 + 4*sys.PtrSize))
                        }
                        if mv.fn != f.entry {
                                print("runtime: confused by ", funcname(f), "\n")
                                throw("reflect mismatch")
                        }
                        bv := mv.stack
                        arglen = uintptr(bv.n * sys.PtrSize)
                        if !retValid {
                                arglen = uintptr(mv.argLen) &^ (sys.PtrSize - 1)
                        }
                        argmap = bv
                }
        }
        return
}

// tracebackCgoContext handles tracing back a cgo context value, from
// the context argument to setCgoTraceback, for the gentraceback
// function. It returns the new value of n.
func tracebackCgoContext(pcbuf *uintptr, printing bool, ctxt uintptr, n, max int) int {
        var cgoPCs [32]uintptr
        cgoContextPCs(ctxt, cgoPCs[:])
        var arg cgoSymbolizerArg
        anySymbolized := false
        for _, pc := range cgoPCs {
                if pc == 0 || n >= max {
                        break
                }
                if pcbuf != nil {
                        (*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = pc
                }
                if printing {
                        if cgoSymbolizer == nil {
                                print("non-Go function at pc=", hex(pc), "\n")
                        } else {
                                c := printOneCgoTraceback(pc, max-n, &arg)
                                n += c - 1 // +1 a few lines down
                                anySymbolized = true
                        }
                }
                n++
        }
        if anySymbolized {
                arg.pc = 0
                callCgoSymbolizer(&arg)
        }
        return n
}

func printcreatedby(gp *g) {
        // Show what created goroutine, except main goroutine (goid 1).
        pc := gp.gopc
        f := findfunc(pc)
        if f.valid() && showframe(f, gp, false, funcID_normal, funcID_normal) && gp.goid != 1 {
                printcreatedby1(f, pc)
        }
}

func printcreatedby1(f funcInfo, pc uintptr) {
        print("created by ", funcname(f), "\n")
        tracepc := pc // back up to CALL instruction for funcline.
        if pc > f.entry {
                tracepc -= sys.PCQuantum
        }
        file, line := funcline(f, tracepc)
        print("\t", file, ":", line)
        if pc > f.entry {
                print(" +", hex(pc-f.entry))
        }
        print("\n")
}

func traceback(pc, sp, lr uintptr, gp *g) {
        traceback1(pc, sp, lr, gp, 0)
}

// tracebacktrap is like traceback but expects that the PC and SP were obtained
// from a trap, not from gp->sched or gp->syscallpc/gp->syscallsp or getcallerpc/getcallersp.
// Because they are from a trap instead of from a saved pair,
// the initial PC must not be rewound to the previous instruction.
// (All the saved pairs record a PC that is a return address, so we
// rewind it into the CALL instruction.)
// If gp.m.libcall{g,pc,sp} information is available, it uses that information in preference to
// the pc/sp/lr passed in.
func tracebacktrap(pc, sp, lr uintptr, gp *g) {
        if gp.m.libcallsp != 0 {
                // We're in C code somewhere, traceback from the saved position.
                traceback1(gp.m.libcallpc, gp.m.libcallsp, 0, gp.m.libcallg.ptr(), 0)
                return
        }
        traceback1(pc, sp, lr, gp, _TraceTrap)
}

func traceback1(pc, sp, lr uintptr, gp *g, flags uint) {
        // If the goroutine is in cgo, and we have a cgo traceback, print that.
        if iscgo && gp.m != nil && gp.m.ncgo > 0 && gp.syscallsp != 0 && gp.m.cgoCallers != nil && gp.m.cgoCallers[0] != 0 {
                // Lock cgoCallers so that a signal handler won't
                // change it, copy the array, reset it, unlock it.
                // We are locked to the thread and are not running
                // concurrently with a signal handler.
                // We just have to stop a signal handler from interrupting
                // in the middle of our copy.
                atomic.Store(&gp.m.cgoCallersUse, 1)
                cgoCallers := *gp.m.cgoCallers
                gp.m.cgoCallers[0] = 0
                atomic.Store(&gp.m.cgoCallersUse, 0)

                printCgoTraceback(&cgoCallers)
        }

        var n int
        if readgstatus(gp)&^_Gscan == _Gsyscall {
                // Override registers if blocked in system call.
                pc = gp.syscallpc
                sp = gp.syscallsp
                flags &^= _TraceTrap
        }
        // Print traceback. By default, omits runtime frames.
        // If that means we print nothing at all, repeat forcing all frames printed.
        n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, flags)
        if n == 0 && (flags&_TraceRuntimeFrames) == 0 {
                n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, flags|_TraceRuntimeFrames)
        }
        if n == _TracebackMaxFrames {
                print("...additional frames elided...\n")
        }
        printcreatedby(gp)

        if gp.ancestors == nil {
                return
        }
        for _, ancestor := range *gp.ancestors {
                printAncestorTraceback(ancestor)
        }
}

// printAncestorTraceback prints the traceback of the given ancestor.
// TODO: Unify this with gentraceback and CallersFrames.
func printAncestorTraceback(ancestor ancestorInfo) {
        print("[originating from goroutine ", ancestor.goid, "]:\n")
        for fidx, pc := range ancestor.pcs {
                f := findfunc(pc) // f previously validated
                if showfuncinfo(f, fidx == 0, funcID_normal, funcID_normal) {
                        printAncestorTracebackFuncInfo(f, pc)
                }
        }
        if len(ancestor.pcs) == _TracebackMaxFrames {
                print("...additional frames elided...\n")
        }
        // Show what created goroutine, except main goroutine (goid 1).
        f := findfunc(ancestor.gopc)
        if f.valid() && showfuncinfo(f, false, funcID_normal, funcID_normal) && ancestor.goid != 1 {
                printcreatedby1(f, ancestor.gopc)
        }
}

// printAncestorTraceback prints the given function info at a given pc
// within an ancestor traceback. The precision of this info is reduced
// due to only have access to the pcs at the time of the caller
// goroutine being created.
func printAncestorTracebackFuncInfo(f funcInfo, pc uintptr) {
        name := funcname(f)
        if inldata := funcdata(f, _FUNCDATA_InlTree); inldata != nil {
                inltree := (*[1 << 20]inlinedCall)(inldata)
                ix := pcdatavalue(f, _PCDATA_InlTreeIndex, pc, nil)
                if ix >= 0 {
                        name = funcnameFromNameoff(f, inltree[ix].func_)
                }
        }
        file, line := funcline(f, pc)
        if name == "runtime.gopanic" {
                name = "panic"
        }
        print(name, "(...)\n")
        print("\t", file, ":", line)
        if pc > f.entry {
                print(" +", hex(pc-f.entry))
        }
        print("\n")
}

func callers(skip int, pcbuf []uintptr) int {
        sp := getcallersp()
        pc := getcallerpc()
        gp := getg()
        var n int
        systemstack(func() {
                n = gentraceback(pc, sp, 0, gp, skip, &pcbuf[0], len(pcbuf), nil, nil, 0)
        })
        return n
}

func gcallers(gp *g, skip int, pcbuf []uintptr) int {
        return gentraceback(^uintptr(0), ^uintptr(0), 0, gp, skip, &pcbuf[0], len(pcbuf), nil, nil, 0)
}

// showframe reports whether the frame with the given characteristics should
// be printed during a traceback.
func showframe(f funcInfo, gp *g, firstFrame bool, funcID, childID funcID) bool {
        g := getg()
        if g.m.throwing > 0 && gp != nil && (gp == g.m.curg || gp == g.m.caughtsig.ptr()) {
                return true
        }
        return showfuncinfo(f, firstFrame, funcID, childID)
}

// showfuncinfo reports whether a function with the given characteristics should
// be printed during a traceback.
func showfuncinfo(f funcInfo, firstFrame bool, funcID, childID funcID) bool {
        // Note that f may be a synthesized funcInfo for an inlined
        // function, in which case only nameoff and funcID are set.

        level, _, _ := gotraceback()
        if level > 1 {
                // Show all frames.
                return true
        }

        if !f.valid() {
                return false
        }

        if funcID == funcID_wrapper && elideWrapperCalling(childID) {
                return false
        }

        name := funcname(f)

        // Special case: always show runtime.gopanic frame
        // in the middle of a stack trace, so that we can
        // see the boundary between ordinary code and
        // panic-induced deferred code.
        // See golang.org/issue/5832.
        if name == "runtime.gopanic" && !firstFrame {
                return true
        }

        return bytealg.IndexByteString(name, '.') >= 0 && (!hasPrefix(name, "runtime.") || isExportedRuntime(name))
}

// isExportedRuntime reports whether name is an exported runtime function.
// It is only for runtime functions, so ASCII A-Z is fine.
func isExportedRuntime(name string) bool {
        const n = len("runtime.")
        return len(name) > n && name[:n] == "runtime." && 'A' <= name[n] && name[n] <= 'Z'
}

// elideWrapperCalling reports whether a wrapper function that called
// function id should be elided from stack traces.
func elideWrapperCalling(id funcID) bool {
        // If the wrapper called a panic function instead of the
        // wrapped function, we want to include it in stacks.
        return !(id == funcID_gopanic || id == funcID_sigpanic || id == funcID_panicwrap)
}

var gStatusStrings = [...]string{
        _Gidle:      "idle",
        _Grunnable:  "runnable",
        _Grunning:   "running",
        _Gsyscall:   "syscall",
        _Gwaiting:   "waiting",
        _Gdead:      "dead",
        _Gcopystack: "copystack",
        _Gpreempted: "preempted",
}

func goroutineheader(gp *g) {
        gpstatus := readgstatus(gp)

        isScan := gpstatus&_Gscan != 0
        gpstatus &^= _Gscan // drop the scan bit

        // Basic string status
        var status string
        if 0 <= gpstatus && gpstatus < uint32(len(gStatusStrings)) {
                status = gStatusStrings[gpstatus]
        } else {
                status = "???"
        }

        // Override.
        if gpstatus == _Gwaiting && gp.waitreason != waitReasonZero {
                status = gp.waitreason.String()
        }

        // approx time the G is blocked, in minutes
        var waitfor int64
        if (gpstatus == _Gwaiting || gpstatus == _Gsyscall) && gp.waitsince != 0 {
                waitfor = (nanotime() - gp.waitsince) / 60e9
        }
        print("goroutine ", gp.goid, " [", status)
        if isScan {
                print(" (scan)")
        }
        if waitfor >= 1 {
                print(", ", waitfor, " minutes")
        }
        if gp.lockedm != 0 {
                print(", locked to thread")
        }
        print("]:\n")
}

func tracebackothers(me *g) {
        level, _, _ := gotraceback()

        // Show the current goroutine first, if we haven't already.
        curgp := getg().m.curg
        if curgp != nil && curgp != me {
                print("\n")
                goroutineheader(curgp)
                traceback(^uintptr(0), ^uintptr(0), 0, curgp)
        }

        // We can't call locking forEachG here because this may be during fatal
        // throw/panic, where locking could be out-of-order or a direct
        // deadlock.
        //
        // Instead, use forEachGRace, which requires no locking. We don't lock
        // against concurrent creation of new Gs, but even with allglock we may
        // miss Gs created after this loop.
        forEachGRace(func(gp *g) {
                if gp == me || gp == curgp || readgstatus(gp) == _Gdead || isSystemGoroutine(gp, false) && level < 2 {
                        return
                }
                print("\n")
                goroutineheader(gp)
                // Note: gp.m == g.m occurs when tracebackothers is
                // called from a signal handler initiated during a
                // systemstack call. The original G is still in the
                // running state, and we want to print its stack.
                if gp.m != getg().m && readgstatus(gp)&^_Gscan == _Grunning {
                        print("\tgoroutine running on other thread; stack unavailable\n")
                        printcreatedby(gp)
                } else {
                        traceback(^uintptr(0), ^uintptr(0), 0, gp)
                }
        })
}

// tracebackHexdump hexdumps part of stk around frame.sp and frame.fp
// for debugging purposes. If the address bad is included in the
// hexdumped range, it will mark it as well.
func tracebackHexdump(stk stack, frame *stkframe, bad uintptr) {
        const expand = 32 * sys.PtrSize
        const maxExpand = 256 * sys.PtrSize
        // Start around frame.sp.
        lo, hi := frame.sp, frame.sp
        // Expand to include frame.fp.
        if frame.fp != 0 && frame.fp < lo {
                lo = frame.fp
        }
        if frame.fp != 0 && frame.fp > hi {
                hi = frame.fp
        }
        // Expand a bit more.
        lo, hi = lo-expand, hi+expand
        // But don't go too far from frame.sp.
        if lo < frame.sp-maxExpand {
                lo = frame.sp - maxExpand
        }
        if hi > frame.sp+maxExpand {
                hi = frame.sp + maxExpand
        }
        // And don't go outside the stack bounds.
        if lo < stk.lo {
                lo = stk.lo
        }
        if hi > stk.hi {
                hi = stk.hi
        }

        // Print the hex dump.
        print("stack: frame={sp:", hex(frame.sp), ", fp:", hex(frame.fp), "} stack=[", hex(stk.lo), ",", hex(stk.hi), ")\n")
        hexdumpWords(lo, hi, func(p uintptr) byte {
                switch p {
                case frame.fp:
                        return '>'
                case frame.sp:
                        return '<'
                case bad:
                        return '!'
                }
                return 0
        })
}

// isSystemGoroutine reports whether the goroutine g must be omitted
// in stack dumps and deadlock detector. This is any goroutine that
// starts at a runtime.* entry point, except for runtime.main,
// runtime.handleAsyncEvent (wasm only) and sometimes runtime.runfinq.
//
// If fixed is true, any goroutine that can vary between user and
// system (that is, the finalizer goroutine) is considered a user
// goroutine.
func isSystemGoroutine(gp *g, fixed bool) bool {
        // Keep this in sync with cmd/trace/trace.go:isSystemGoroutine.
        f := findfunc(gp.startpc)
        if !f.valid() {
                return false
        }
        if f.funcID == funcID_runtime_main || f.funcID == funcID_handleAsyncEvent {
                return false
        }
        if f.funcID == funcID_runfinq {
                // We include the finalizer goroutine if it's calling
                // back into user code.
                if fixed {
                        // This goroutine can vary. In fixed mode,
                        // always consider it a user goroutine.
                        return false
                }
                return !fingRunning
        }
        return hasPrefix(funcname(f), "runtime.")
}

// SetCgoTraceback records three C functions to use to gather
// traceback information from C code and to convert that traceback
// information into symbolic information. These are used when printing
// stack traces for a program that uses cgo.
//
// The traceback and context functions may be called from a signal
// handler, and must therefore use only async-signal safe functions.
// The symbolizer function may be called while the program is
// crashing, and so must be cautious about using memory.  None of the
// functions may call back into Go.
//
// The context function will be called with a single argument, a
// pointer to a struct:
//
//      struct {
//              Context uintptr
//      }
//
// In C syntax, this struct will be
//
//      struct {
//              uintptr_t Context;
//      };
//
// If the Context field is 0, the context function is being called to
// record the current traceback context. It should record in the
// Context field whatever information is needed about the current
// point of execution to later produce a stack trace, probably the
// stack pointer and PC. In this case the context function will be
// called from C code.
//
// If the Context field is not 0, then it is a value returned by a
// previous call to the context function. This case is called when the
// context is no longer needed; that is, when the Go code is returning
// to its C code caller. This permits the context function to release
// any associated resources.
//
// While it would be correct for the context function to record a
// complete a stack trace whenever it is called, and simply copy that
// out in the traceback function, in a typical program the context
// function will be called many times without ever recording a
// traceback for that context. Recording a complete stack trace in a
// call to the context function is likely to be inefficient.
//
// The traceback function will be called with a single argument, a
// pointer to a struct:
//
//      struct {
//              Context    uintptr
//              SigContext uintptr
//              Buf        *uintptr
//              Max        uintptr
//      }
//
// In C syntax, this struct will be
//
//      struct {
//              uintptr_t  Context;
//              uintptr_t  SigContext;
//              uintptr_t* Buf;
//              uintptr_t  Max;
//      };
//
// The Context field will be zero to gather a traceback from the
// current program execution point. In this case, the traceback
// function will be called from C code.
//
// Otherwise Context will be a value previously returned by a call to
// the context function. The traceback function should gather a stack
// trace from that saved point in the program execution. The traceback
// function may be called from an execution thread other than the one
// that recorded the context, but only when the context is known to be
// valid and unchanging. The traceback function may also be called
// deeper in the call stack on the same thread that recorded the
// context. The traceback function may be called multiple times with
// the same Context value; it will usually be appropriate to cache the
// result, if possible, the first time this is called for a specific
// context value.
//
// If the traceback function is called from a signal handler on a Unix
// system, SigContext will be the signal context argument passed to
// the signal handler (a C ucontext_t* cast to uintptr_t). This may be
// used to start tracing at the point where the signal occurred. If
// the traceback function is not called from a signal handler,
// SigContext will be zero.
//
// Buf is where the traceback information should be stored. It should
// be PC values, such that Buf[0] is the PC of the caller, Buf[1] is
// the PC of that function's caller, and so on.  Max is the maximum
// number of entries to store.  The function should store a zero to
// indicate the top of the stack, or that the caller is on a different
// stack, presumably a Go stack.
//
// Unlike runtime.Callers, the PC values returned should, when passed
// to the symbolizer function, return the file/line of the call
// instruction.  No additional subtraction is required or appropriate.
//
// On all platforms, the traceback function is invoked when a call from
// Go to C to Go requests a stack trace. On linux/amd64, linux/ppc64le,
// and freebsd/amd64, the traceback function is also invoked when a
// signal is received by a thread that is executing a cgo call. The
// traceback function should not make assumptions about when it is
// called, as future versions of Go may make additional calls.
//
// The symbolizer function will be called with a single argument, a
// pointer to a struct:
//
//      struct {
//              PC      uintptr // program counter to fetch information for
//              File    *byte   // file name (NUL terminated)
//              Lineno  uintptr // line number
//              Func    *byte   // function name (NUL terminated)
//              Entry   uintptr // function entry point
//              More    uintptr // set non-zero if more info for this PC
//              Data    uintptr // unused by runtime, available for function
//      }
//
// In C syntax, this struct will be
//
//      struct {
//              uintptr_t PC;
//              char*     File;
//              uintptr_t Lineno;
//              char*     Func;
//              uintptr_t Entry;
//              uintptr_t More;
//              uintptr_t Data;
//      };
//
// The PC field will be a value returned by a call to the traceback
// function.
//
// The first time the function is called for a particular traceback,
// all the fields except PC will be 0. The function should fill in the
// other fields if possible, setting them to 0/nil if the information
// is not available. The Data field may be used to store any useful
// information across calls. The More field should be set to non-zero
// if there is more information for this PC, zero otherwise. If More
// is set non-zero, the function will be called again with the same
// PC, and may return different information (this is intended for use
// with inlined functions). If More is zero, the function will be
// called with the next PC value in the traceback. When the traceback
// is complete, the function will be called once more with PC set to
// zero; this may be used to free any information. Each call will
// leave the fields of the struct set to the same values they had upon
// return, except for the PC field when the More field is zero. The
// function must not keep a copy of the struct pointer between calls.
//
// When calling SetCgoTraceback, the version argument is the version
// number of the structs that the functions expect to receive.
// Currently this must be zero.
//
// The symbolizer function may be nil, in which case the results of
// the traceback function will be displayed as numbers. If the
// traceback function is nil, the symbolizer function will never be
// called. The context function may be nil, in which case the
// traceback function will only be called with the context field set
// to zero.  If the context function is nil, then calls from Go to C
// to Go will not show a traceback for the C portion of the call stack.
//
// SetCgoTraceback should be called only once, ideally from an init function.
func SetCgoTraceback(version int, traceback, context, symbolizer unsafe.Pointer) {
        if version != 0 {
                panic("unsupported version")
        }

        if cgoTraceback != nil && cgoTraceback != traceback ||
                cgoContext != nil && cgoContext != context ||
                cgoSymbolizer != nil && cgoSymbolizer != symbolizer {
                panic("call SetCgoTraceback only once")
        }

        cgoTraceback = traceback
        cgoContext = context
        cgoSymbolizer = symbolizer

        // The context function is called when a C function calls a Go
        // function. As such it is only called by C code in runtime/cgo.
        if _cgo_set_context_function != nil {
                cgocall(_cgo_set_context_function, context)
        }
}

var cgoTraceback unsafe.Pointer
var cgoContext unsafe.Pointer
var cgoSymbolizer unsafe.Pointer

// cgoTracebackArg is the type passed to cgoTraceback.
type cgoTracebackArg struct {
        context    uintptr
        sigContext uintptr
        buf        *uintptr
        max        uintptr
}

// cgoContextArg is the type passed to the context function.
type cgoContextArg struct {
        context uintptr
}

// cgoSymbolizerArg is the type passed to cgoSymbolizer.
type cgoSymbolizerArg struct {
        pc       uintptr
        file     *byte
        lineno   uintptr
        funcName *byte
        entry    uintptr
        more     uintptr
        data     uintptr
}

// cgoTraceback prints a traceback of callers.
func printCgoTraceback(callers *cgoCallers) {
        if cgoSymbolizer == nil {
                for _, c := range callers {
                        if c == 0 {
                                break
                        }
                        print("non-Go function at pc=", hex(c), "\n")
                }
                return
        }

        var arg cgoSymbolizerArg
        for _, c := range callers {
                if c == 0 {
                        break
                }
                printOneCgoTraceback(c, 0x7fffffff, &arg)
        }
        arg.pc = 0
        callCgoSymbolizer(&arg)
}

// printOneCgoTraceback prints the traceback of a single cgo caller.
// This can print more than one line because of inlining.
// Returns the number of frames printed.
func printOneCgoTraceback(pc uintptr, max int, arg *cgoSymbolizerArg) int {
        c := 0
        arg.pc = pc
        for c <= max {
                callCgoSymbolizer(arg)
                if arg.funcName != nil {
                        // Note that we don't print any argument
                        // information here, not even parentheses.
                        // The symbolizer must add that if appropriate.
                        println(gostringnocopy(arg.funcName))
                } else {
                        println("non-Go function")
                }
                print("\t")
                if arg.file != nil {
                        print(gostringnocopy(arg.file), ":", arg.lineno, " ")
                }
                print("pc=", hex(pc), "\n")
                c++
                if arg.more == 0 {
                        break
                }
        }
        return c
}

// callCgoSymbolizer calls the cgoSymbolizer function.
func callCgoSymbolizer(arg *cgoSymbolizerArg) {
        call := cgocall
        if panicking > 0 || getg().m.curg != getg() {
                // We do not want to call into the scheduler when panicking
                // or when on the system stack.
                call = asmcgocall
        }
        if msanenabled {
                msanwrite(unsafe.Pointer(arg), unsafe.Sizeof(cgoSymbolizerArg{}))
        }
        call(cgoSymbolizer, noescape(unsafe.Pointer(arg)))
}

// cgoContextPCs gets the PC values from a cgo traceback.
func cgoContextPCs(ctxt uintptr, buf []uintptr) {
        if cgoTraceback == nil {
                return
        }
        call := cgocall
        if panicking > 0 || getg().m.curg != getg() {
                // We do not want to call into the scheduler when panicking
                // or when on the system stack.
                call = asmcgocall
        }
        arg := cgoTracebackArg{
                context: ctxt,
                buf:     (*uintptr)(noescape(unsafe.Pointer(&buf[0]))),
                max:     uintptr(len(buf)),
        }
        if msanenabled {
                msanwrite(unsafe.Pointer(&arg), unsafe.Sizeof(arg))
        }
        call(cgoTraceback, noescape(unsafe.Pointer(&arg)))
}