[gostls13.git] / src / cmd / go / internal / work / gc.go

// Copyright 2011 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 work

import (
        "bufio"
        "bytes"
        "fmt"
        "io"
        "log"
        "os"
        "path/filepath"
        "runtime"
        "strings"

        "cmd/go/internal/base"
        "cmd/go/internal/cfg"
        "cmd/go/internal/fsys"
        "cmd/go/internal/load"
        "cmd/go/internal/str"
        "cmd/internal/objabi"
        "cmd/internal/sys"
        "crypto/sha1"
)

// The 'path' used for GOROOT_FINAL when -trimpath is specified
const trimPathGoRootFinal = "go"

// The Go toolchain.

type gcToolchain struct{}

func (gcToolchain) compiler() string {
        return base.Tool("compile")
}

func (gcToolchain) linker() string {
        return base.Tool("link")
}

func pkgPath(a *Action) string {
        p := a.Package
        ppath := p.ImportPath
        if cfg.BuildBuildmode == "plugin" {
                ppath = pluginPath(a)
        } else if p.Name == "main" && !p.Internal.ForceLibrary {
                ppath = "main"
        }
        return ppath
}

func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
        p := a.Package
        objdir := a.Objdir
        if archive != "" {
                ofile = archive
        } else {
                out := "_go_.o"
                ofile = objdir + out
        }

        pkgpath := pkgPath(a)
        gcargs := []string{"-p", pkgpath}
        if p.Module != nil {
                v := p.Module.GoVersion
                if v == "" {
                        // We started adding a 'go' directive to the go.mod file unconditionally
                        // as of Go 1.12, so any module that still lacks such a directive must
                        // either have been authored before then, or have a hand-edited go.mod
                        // file that hasn't been updated by cmd/go since that edit.
                        //
                        // Unfortunately, through at least Go 1.16 we didn't add versions to
                        // vendor/modules.txt. So this could also be a vendored 1.16 dependency.
                        //
                        // Fortunately, there were no breaking changes to the language between Go
                        // 1.11 and 1.16, so if we assume Go 1.16 semantics we will not introduce
                        // any spurious errors — we will only mask errors, and not particularly
                        // important ones at that.
                        v = "1.16"
                }
                if allowedVersion(v) {
                        gcargs = append(gcargs, "-lang=go"+v)
                }
        }
        if p.Standard {
                gcargs = append(gcargs, "-std")
        }
        compilingRuntime := p.Standard && (p.ImportPath == "runtime" || strings.HasPrefix(p.ImportPath, "runtime/internal"))
        // The runtime package imports a couple of general internal packages.
        if p.Standard && (p.ImportPath == "internal/cpu" || p.ImportPath == "internal/bytealg" || p.ImportPath == "internal/abi") {
                compilingRuntime = true
        }
        if compilingRuntime {
                // runtime compiles with a special gc flag to check for
                // memory allocations that are invalid in the runtime package,
                // and to implement some special compiler pragmas.
                gcargs = append(gcargs, "-+")
        }

        // If we're giving the compiler the entire package (no C etc files), tell it that,
        // so that it can give good error messages about forward declarations.
        // Exceptions: a few standard packages have forward declarations for
        // pieces supplied behind-the-scenes by package runtime.
        extFiles := len(p.CgoFiles) + len(p.CFiles) + len(p.CXXFiles) + len(p.MFiles) + len(p.FFiles) + len(p.SFiles) + len(p.SysoFiles) + len(p.SwigFiles) + len(p.SwigCXXFiles)
        if p.Standard {
                switch p.ImportPath {
                case "bytes", "internal/poll", "net", "os":
                        fallthrough
                case "runtime/metrics", "runtime/pprof", "runtime/trace":
                        fallthrough
                case "sync", "syscall", "time":
                        extFiles++
                }
        }
        if extFiles == 0 {
                gcargs = append(gcargs, "-complete")
        }
        if cfg.BuildContext.InstallSuffix != "" {
                gcargs = append(gcargs, "-installsuffix", cfg.BuildContext.InstallSuffix)
        }
        if a.buildID != "" {
                gcargs = append(gcargs, "-buildid", a.buildID)
        }
        if p.Internal.OmitDebug || cfg.Goos == "plan9" || cfg.Goarch == "wasm" {
                gcargs = append(gcargs, "-dwarf=false")
        }
        if strings.HasPrefix(runtimeVersion, "go1") && !strings.Contains(os.Args[0], "go_bootstrap") {
                gcargs = append(gcargs, "-goversion", runtimeVersion)
        }
        if symabis != "" {
                gcargs = append(gcargs, "-symabis", symabis)
        }

        gcflags := str.StringList(forcedGcflags, p.Internal.Gcflags)
        if compilingRuntime {
                // Remove -N, if present.
                // It is not possible to build the runtime with no optimizations,
                // because the compiler cannot eliminate enough write barriers.
                for i := 0; i < len(gcflags); i++ {
                        if gcflags[i] == "-N" {
                                copy(gcflags[i:], gcflags[i+1:])
                                gcflags = gcflags[:len(gcflags)-1]
                                i--
                        }
                }
        }

        args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), gcflags, gcargs}
        if p.Internal.LocalPrefix != "" {
                // Workaround #43883.
                args = append(args, "-D", p.Internal.LocalPrefix)
        }
        if importcfg != nil {
                if err := b.writeFile(objdir+"importcfg", importcfg); err != nil {
                        return "", nil, err
                }
                args = append(args, "-importcfg", objdir+"importcfg")
        }
        if embedcfg != nil {
                if err := b.writeFile(objdir+"embedcfg", embedcfg); err != nil {
                        return "", nil, err
                }
                args = append(args, "-embedcfg", objdir+"embedcfg")
        }
        if ofile == archive {
                args = append(args, "-pack")
        }
        if asmhdr {
                args = append(args, "-asmhdr", objdir+"go_asm.h")
        }

        // Add -c=N to use concurrent backend compilation, if possible.
        if c := gcBackendConcurrency(gcflags); c > 1 {
                args = append(args, fmt.Sprintf("-c=%d", c))
        }

        for _, f := range gofiles {
                f := mkAbs(p.Dir, f)

                // Handle overlays. Convert path names using OverlayPath
                // so these paths can be handed directly to tools.
                // Deleted files won't show up in when scanning directories earlier,
                // so OverlayPath will never return "" (meaning a deleted file) here.
                // TODO(#39958): Handle cases where the package directory
                // doesn't exist on disk (this can happen when all the package's
                // files are in an overlay): the code expects the package directory
                // to exist and runs some tools in that directory.
                // TODO(#39958): Process the overlays when the
                // gofiles, cgofiles, cfiles, sfiles, and cxxfiles variables are
                // created in (*Builder).build. Doing that requires rewriting the
                // code that uses those values to expect absolute paths.
                f, _ = fsys.OverlayPath(f)

                args = append(args, f)
        }

        output, err = b.runOut(a, base.Cwd, nil, args...)
        return ofile, output, err
}

// gcBackendConcurrency returns the backend compiler concurrency level for a package compilation.
func gcBackendConcurrency(gcflags []string) int {
        // First, check whether we can use -c at all for this compilation.
        canDashC := concurrentGCBackendCompilationEnabledByDefault

        switch e := os.Getenv("GO19CONCURRENTCOMPILATION"); e {
        case "0":
                canDashC = false
        case "1":
                canDashC = true
        case "":
                // Not set. Use default.
        default:
                log.Fatalf("GO19CONCURRENTCOMPILATION must be 0, 1, or unset, got %q", e)
        }

CheckFlags:
        for _, flag := range gcflags {
                // Concurrent compilation is presumed incompatible with any gcflags,
                // except for known commonly used flags.
                // If the user knows better, they can manually add their own -c to the gcflags.
                switch flag {
                case "-N", "-l", "-S", "-B", "-C", "-I":
                        // OK
                default:
                        canDashC = false
                        break CheckFlags
                }
        }

        // TODO: Test and delete these conditions.
        if objabi.Experiment.FieldTrack || objabi.Experiment.PreemptibleLoops {
                canDashC = false
        }

        if !canDashC {
                return 1
        }

        // Decide how many concurrent backend compilations to allow.
        //
        // If we allow too many, in theory we might end up with p concurrent processes,
        // each with c concurrent backend compiles, all fighting over the same resources.
        // However, in practice, that seems not to happen too much.
        // Most build graphs are surprisingly serial, so p==1 for much of the build.
        // Furthermore, concurrent backend compilation is only enabled for a part
        // of the overall compiler execution, so c==1 for much of the build.
        // So don't worry too much about that interaction for now.
        //
        // However, in practice, setting c above 4 tends not to help very much.
        // See the analysis in CL 41192.
        //
        // TODO(josharian): attempt to detect whether this particular compilation
        // is likely to be a bottleneck, e.g. when:
        //   - it has no successor packages to compile (usually package main)
        //   - all paths through the build graph pass through it
        //   - critical path scheduling says it is high priority
        // and in such a case, set c to runtime.GOMAXPROCS(0).
        // By default this is the same as runtime.NumCPU.
        // We do this now when p==1.
        // To limit parallelism, set GOMAXPROCS below numCPU; this may be useful
        // on a low-memory builder, or if a deterministic build order is required.
        c := runtime.GOMAXPROCS(0)
        if cfg.BuildP == 1 {
                // No process parallelism, do not cap compiler parallelism.
                return c
        }
        // Some process parallelism. Set c to min(4, maxprocs).
        if c > 4 {
                c = 4
        }
        return c
}

// trimpath returns the -trimpath argument to use
// when compiling the action.
func (a *Action) trimpath() string {
        // Keep in sync with Builder.ccompile
        // The trimmed paths are a little different, but we need to trim in the
        // same situations.

        // Strip the object directory entirely.
        objdir := a.Objdir
        if len(objdir) > 1 && objdir[len(objdir)-1] == filepath.Separator {
                objdir = objdir[:len(objdir)-1]
        }
        rewrite := ""

        rewriteDir := a.Package.Dir
        if cfg.BuildTrimpath {
                importPath := a.Package.Internal.OrigImportPath
                if m := a.Package.Module; m != nil && m.Version != "" {
                        rewriteDir = m.Path + "@" + m.Version + strings.TrimPrefix(importPath, m.Path)
                } else {
                        rewriteDir = importPath
                }
                rewrite += a.Package.Dir + "=>" + rewriteDir + ";"
        }

        // Add rewrites for overlays. The 'from' and 'to' paths in overlays don't need to have
        // same basename, so go from the overlay contents file path (passed to the compiler)
        // to the path the disk path would be rewritten to.

        cgoFiles := make(map[string]bool)
        for _, f := range a.Package.CgoFiles {
                cgoFiles[f] = true
        }

        // TODO(matloob): Higher up in the stack, when the logic for deciding when to make copies
        // of c/c++/m/f/hfiles is consolidated, use the same logic that Build uses to determine
        // whether to create the copies in objdir to decide whether to rewrite objdir to the
        // package directory here.
        var overlayNonGoRewrites string // rewrites for non-go files
        hasCgoOverlay := false
        if fsys.OverlayFile != "" {
                for _, filename := range a.Package.AllFiles() {
                        path := filename
                        if !filepath.IsAbs(path) {
                                path = filepath.Join(a.Package.Dir, path)
                        }
                        base := filepath.Base(path)
                        isGo := strings.HasSuffix(filename, ".go") || strings.HasSuffix(filename, ".s")
                        isCgo := cgoFiles[filename] || !isGo
                        overlayPath, isOverlay := fsys.OverlayPath(path)
                        if isCgo && isOverlay {
                                hasCgoOverlay = true
                        }
                        if !isCgo && isOverlay {
                                rewrite += overlayPath + "=>" + filepath.Join(rewriteDir, base) + ";"
                        } else if isCgo {
                                // Generate rewrites for non-Go files copied to files in objdir.
                                if filepath.Dir(path) == a.Package.Dir {
                                        // This is a file copied to objdir.
                                        overlayNonGoRewrites += filepath.Join(objdir, base) + "=>" + filepath.Join(rewriteDir, base) + ";"
                                }
                        } else {
                                // Non-overlay Go files are covered by the a.Package.Dir rewrite rule above.
                        }
                }
        }
        if hasCgoOverlay {
                rewrite += overlayNonGoRewrites
        }
        rewrite += objdir + "=>"

        return rewrite
}

func asmArgs(a *Action, p *load.Package) []interface{} {
        // Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
        inc := filepath.Join(cfg.GOROOT, "pkg", "include")
        pkgpath := pkgPath(a)
        args := []interface{}{cfg.BuildToolexec, base.Tool("asm"), "-p", pkgpath, "-trimpath", a.trimpath(), "-I", a.Objdir, "-I", inc, "-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch, forcedAsmflags, p.Internal.Asmflags}
        if p.ImportPath == "runtime" && cfg.Goarch == "386" {
                for _, arg := range forcedAsmflags {
                        if arg == "-dynlink" {
                                args = append(args, "-D=GOBUILDMODE_shared=1")
                        }
                }
        }
        if objabi.IsRuntimePackagePath(pkgpath) {
                args = append(args, "-compiling-runtime")
        }

        if cfg.Goarch == "mips" || cfg.Goarch == "mipsle" {
                // Define GOMIPS_value from cfg.GOMIPS.
                args = append(args, "-D", "GOMIPS_"+cfg.GOMIPS)
        }

        if cfg.Goarch == "mips64" || cfg.Goarch == "mips64le" {
                // Define GOMIPS64_value from cfg.GOMIPS64.
                args = append(args, "-D", "GOMIPS64_"+cfg.GOMIPS64)
        }

        return args
}

func (gcToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
        p := a.Package
        args := asmArgs(a, p)

        var ofiles []string
        for _, sfile := range sfiles {
                overlayPath, _ := fsys.OverlayPath(mkAbs(p.Dir, sfile))
                ofile := a.Objdir + sfile[:len(sfile)-len(".s")] + ".o"
                ofiles = append(ofiles, ofile)
                args1 := append(args, "-o", ofile, overlayPath)
                if err := b.run(a, p.Dir, p.ImportPath, nil, args1...); err != nil {
                        return nil, err
                }
        }
        return ofiles, nil
}

func (gcToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
        mkSymabis := func(p *load.Package, sfiles []string, path string) error {
                args := asmArgs(a, p)
                args = append(args, "-gensymabis", "-o", path)
                for _, sfile := range sfiles {
                        if p.ImportPath == "runtime/cgo" && strings.HasPrefix(sfile, "gcc_") {
                                continue
                        }
                        op, _ := fsys.OverlayPath(mkAbs(p.Dir, sfile))
                        args = append(args, op)
                }

                // Supply an empty go_asm.h as if the compiler had been run.
                // -gensymabis parsing is lax enough that we don't need the
                // actual definitions that would appear in go_asm.h.
                if err := b.writeFile(a.Objdir+"go_asm.h", nil); err != nil {
                        return err
                }

                return b.run(a, p.Dir, p.ImportPath, nil, args...)
        }

        var symabis string // Only set if we actually create the file
        p := a.Package
        if len(sfiles) != 0 {
                symabis = a.Objdir + "symabis"
                if err := mkSymabis(p, sfiles, symabis); err != nil {
                        return "", err
                }
        }

        return symabis, nil
}

// toolVerify checks that the command line args writes the same output file
// if run using newTool instead.
// Unused now but kept around for future use.
func toolVerify(a *Action, b *Builder, p *load.Package, newTool string, ofile string, args []interface{}) error {
        newArgs := make([]interface{}, len(args))
        copy(newArgs, args)
        newArgs[1] = base.Tool(newTool)
        newArgs[3] = ofile + ".new" // x.6 becomes x.6.new
        if err := b.run(a, p.Dir, p.ImportPath, nil, newArgs...); err != nil {
                return err
        }
        data1, err := os.ReadFile(ofile)
        if err != nil {
                return err
        }
        data2, err := os.ReadFile(ofile + ".new")
        if err != nil {
                return err
        }
        if !bytes.Equal(data1, data2) {
                return fmt.Errorf("%s and %s produced different output files:\n%s\n%s", filepath.Base(args[1].(string)), newTool, strings.Join(str.StringList(args...), " "), strings.Join(str.StringList(newArgs...), " "))
        }
        os.Remove(ofile + ".new")
        return nil
}

func (gcToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
        var absOfiles []string
        for _, f := range ofiles {
                absOfiles = append(absOfiles, mkAbs(a.Objdir, f))
        }
        absAfile := mkAbs(a.Objdir, afile)

        // The archive file should have been created by the compiler.
        // Since it used to not work that way, verify.
        if !cfg.BuildN {
                if _, err := os.Stat(absAfile); err != nil {
                        base.Fatalf("os.Stat of archive file failed: %v", err)
                }
        }

        p := a.Package
        if cfg.BuildN || cfg.BuildX {
                cmdline := str.StringList(base.Tool("pack"), "r", absAfile, absOfiles)
                b.Showcmd(p.Dir, "%s # internal", joinUnambiguously(cmdline))
        }
        if cfg.BuildN {
                return nil
        }
        if err := packInternal(absAfile, absOfiles); err != nil {
                b.showOutput(a, p.Dir, p.Desc(), err.Error()+"\n")
                return errPrintedOutput
        }
        return nil
}

func packInternal(afile string, ofiles []string) error {
        dst, err := os.OpenFile(afile, os.O_WRONLY|os.O_APPEND, 0)
        if err != nil {
                return err
        }
        defer dst.Close() // only for error returns or panics
        w := bufio.NewWriter(dst)

        for _, ofile := range ofiles {
                src, err := os.Open(ofile)
                if err != nil {
                        return err
                }
                fi, err := src.Stat()
                if err != nil {
                        src.Close()
                        return err
                }
                // Note: Not using %-16.16s format because we care
                // about bytes, not runes.
                name := fi.Name()
                if len(name) > 16 {
                        name = name[:16]
                } else {
                        name += strings.Repeat(" ", 16-len(name))
                }
                size := fi.Size()
                fmt.Fprintf(w, "%s%-12d%-6d%-6d%-8o%-10d`\n",
                        name, 0, 0, 0, 0644, size)
                n, err := io.Copy(w, src)
                src.Close()
                if err == nil && n < size {
                        err = io.ErrUnexpectedEOF
                } else if err == nil && n > size {
                        err = fmt.Errorf("file larger than size reported by stat")
                }
                if err != nil {
                        return fmt.Errorf("copying %s to %s: %v", ofile, afile, err)
                }
                if size&1 != 0 {
                        w.WriteByte(0)
                }
        }

        if err := w.Flush(); err != nil {
                return err
        }
        return dst.Close()
}

// setextld sets the appropriate linker flags for the specified compiler.
func setextld(ldflags []string, compiler []string) []string {
        for _, f := range ldflags {
                if f == "-extld" || strings.HasPrefix(f, "-extld=") {
                        // don't override -extld if supplied
                        return ldflags
                }
        }
        ldflags = append(ldflags, "-extld="+compiler[0])
        if len(compiler) > 1 {
                extldflags := false
                add := strings.Join(compiler[1:], " ")
                for i, f := range ldflags {
                        if f == "-extldflags" && i+1 < len(ldflags) {
                                ldflags[i+1] = add + " " + ldflags[i+1]
                                extldflags = true
                                break
                        } else if strings.HasPrefix(f, "-extldflags=") {
                                ldflags[i] = "-extldflags=" + add + " " + ldflags[i][len("-extldflags="):]
                                extldflags = true
                                break
                        }
                }
                if !extldflags {
                        ldflags = append(ldflags, "-extldflags="+add)
                }
        }
        return ldflags
}

// pluginPath computes the package path for a plugin main package.
//
// This is typically the import path of the main package p, unless the
// plugin is being built directly from source files. In that case we
// combine the package build ID with the contents of the main package
// source files. This allows us to identify two different plugins
// built from two source files with the same name.
func pluginPath(a *Action) string {
        p := a.Package
        if p.ImportPath != "command-line-arguments" {
                return p.ImportPath
        }
        h := sha1.New()
        buildID := a.buildID
        if a.Mode == "link" {
                // For linking, use the main package's build ID instead of
                // the binary's build ID, so it is the same hash used in
                // compiling and linking.
                // When compiling, we use actionID/actionID (instead of
                // actionID/contentID) as a temporary build ID to compute
                // the hash. Do the same here. (See buildid.go:useCache)
                // The build ID matters because it affects the overall hash
                // in the plugin's pseudo-import path returned below.
                // We need to use the same import path when compiling and linking.
                id := strings.Split(buildID, buildIDSeparator)
                buildID = id[1] + buildIDSeparator + id[1]
        }
        fmt.Fprintf(h, "build ID: %s\n", buildID)
        for _, file := range str.StringList(p.GoFiles, p.CgoFiles, p.SFiles) {
                data, err := os.ReadFile(filepath.Join(p.Dir, file))
                if err != nil {
                        base.Fatalf("go: %s", err)
                }
                h.Write(data)
        }
        return fmt.Sprintf("plugin/unnamed-%x", h.Sum(nil))
}

func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
        cxx := len(root.Package.CXXFiles) > 0 || len(root.Package.SwigCXXFiles) > 0
        for _, a := range root.Deps {
                if a.Package != nil && (len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0) {
                        cxx = true
                }
        }
        var ldflags []string
        if cfg.BuildContext.InstallSuffix != "" {
                ldflags = append(ldflags, "-installsuffix", cfg.BuildContext.InstallSuffix)
        }
        if root.Package.Internal.OmitDebug {
                ldflags = append(ldflags, "-s", "-w")
        }
        if cfg.BuildBuildmode == "plugin" {
                ldflags = append(ldflags, "-pluginpath", pluginPath(root))
        }

        // Store BuildID inside toolchain binaries as a unique identifier of the
        // tool being run, for use by content-based staleness determination.
        if root.Package.Goroot && strings.HasPrefix(root.Package.ImportPath, "cmd/") {
                // External linking will include our build id in the external
                // linker's build id, which will cause our build id to not
                // match the next time the tool is built.
                // Rely on the external build id instead.
                if !sys.MustLinkExternal(cfg.Goos, cfg.Goarch) {
                        ldflags = append(ldflags, "-X=cmd/internal/objabi.buildID="+root.buildID)
                }
        }

        // If the user has not specified the -extld option, then specify the
        // appropriate linker. In case of C++ code, use the compiler named
        // by the CXX environment variable or defaultCXX if CXX is not set.
        // Else, use the CC environment variable and defaultCC as fallback.
        var compiler []string
        if cxx {
                compiler = envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
        } else {
                compiler = envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
        }
        ldflags = append(ldflags, "-buildmode="+ldBuildmode)
        if root.buildID != "" {
                ldflags = append(ldflags, "-buildid="+root.buildID)
        }
        ldflags = append(ldflags, forcedLdflags...)
        ldflags = append(ldflags, root.Package.Internal.Ldflags...)
        ldflags = setextld(ldflags, compiler)

        // On OS X when using external linking to build a shared library,
        // the argument passed here to -o ends up recorded in the final
        // shared library in the LC_ID_DYLIB load command.
        // To avoid putting the temporary output directory name there
        // (and making the resulting shared library useless),
        // run the link in the output directory so that -o can name
        // just the final path element.
        // On Windows, DLL file name is recorded in PE file
        // export section, so do like on OS X.
        dir := "."
        if (cfg.Goos == "darwin" || cfg.Goos == "windows") && cfg.BuildBuildmode == "c-shared" {
                dir, out = filepath.Split(out)
        }

        env := []string{}
        if cfg.BuildTrimpath {
                env = append(env, "GOROOT_FINAL="+trimPathGoRootFinal)
        }
        return b.run(root, dir, root.Package.ImportPath, env, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags, mainpkg)
}

func (gcToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
        ldflags := []string{"-installsuffix", cfg.BuildContext.InstallSuffix}
        ldflags = append(ldflags, "-buildmode=shared")
        ldflags = append(ldflags, forcedLdflags...)
        ldflags = append(ldflags, root.Package.Internal.Ldflags...)
        cxx := false
        for _, a := range allactions {
                if a.Package != nil && (len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0) {
                        cxx = true
                }
        }
        // If the user has not specified the -extld option, then specify the
        // appropriate linker. In case of C++ code, use the compiler named
        // by the CXX environment variable or defaultCXX if CXX is not set.
        // Else, use the CC environment variable and defaultCC as fallback.
        var compiler []string
        if cxx {
                compiler = envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
        } else {
                compiler = envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
        }
        ldflags = setextld(ldflags, compiler)
        for _, d := range toplevelactions {
                if !strings.HasSuffix(d.Target, ".a") { // omit unsafe etc and actions for other shared libraries
                        continue
                }
                ldflags = append(ldflags, d.Package.ImportPath+"="+d.Target)
        }
        return b.run(root, ".", out, nil, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags)
}

func (gcToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
        return fmt.Errorf("%s: C source files not supported without cgo", mkAbs(a.Package.Dir, cfile))
}