Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 693 insertions, 0 deletions

diff --git a/src/cmd/go/internal/work/gc.go b/src/cmd/go/internal/work/gc.go
new file mode 100644
index 0000000..cc4e2b2
--- /dev/null
+++ b/src/cmd/go/internal/work/gc.go
@@ -0,0 +1,693 @@
+// 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 && p.Module.GoVersion != "" && allowedVersion(p.Module.GoVersion) {
+		gcargs = append(gcargs, "-lang=go"+p.Module.GoVersion)
+	}
+	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") {
+		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, "-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.Fieldtrack_enabled != 0 || objabi.Preemptibleloops_enabled != 0 {
+		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.NumCPU.
+	// We do this now when p==1.
+	if cfg.BuildP == 1 {
+		// No process parallelism. Max out c.
+		return runtime.NumCPU()
+	}
+	// Some process parallelism. Set c to min(4, numcpu).
+	c := 4
+	if ncpu := runtime.NumCPU(); ncpu < c {
+		c = ncpu
+	}
+	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 {
+		if m := a.Package.Module; m != nil && m.Version != "" {
+			rewriteDir = m.Path + "@" + m.Version + strings.TrimPrefix(a.Package.ImportPath, m.Path)
+		} else {
+			rewriteDir = a.Package.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 objabi.Regabi_enabled != 0 {
+			// In order to make it easier to port runtime assembly
+			// to the register ABI, we introduce a macro
+			// indicating the experiment is enabled.
+			//
+			// Note: a similar change also appears in
+			// cmd/dist/build.go.
+			//
+			// TODO(austin): Remove this once we commit to the
+			// register ABI (#40724).
+			args = append(args, "-D=GOEXPERIMENT_REGABI=1")
+		}
+	}
+
+	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))
+}