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"

	"github.com/go-asm/go/platform"

	"crypto/sha1"

	"github.com/go-asm/go/cmd/go/base"
	"github.com/go-asm/go/cmd/go/cfg"
	"github.com/go-asm/go/cmd/go/fsys"
	"github.com/go-asm/go/cmd/go/gover"
	"github.com/go-asm/go/cmd/go/load"
	"github.com/go-asm/go/cmd/go/str"
	"github.com/go-asm/go/cmd/objabi"
	"github.com/go-asm/go/cmd/quoted"
)

// Tests can override this by setting $TESTGO_TOOLCHAIN_VERSION.
var ToolchainVersion = runtime.Version()

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

var runtimePackages = map[string]struct{}{
	"github.com/go-asm/go/abi":             {},
	"github.com/go-asm/go/bytealg":         {},
	"github.com/go-asm/go/coverage/rtcov":  {},
	"github.com/go-asm/go/cpu":             {},
	"github.com/go-asm/go/goarch":          {},
	"github.com/go-asm/go/goos":            {},
	"runtime":                              {},
	"runtime/github.com/go-asm/go/atomic":  {},
	"runtime/github.com/go-asm/go/math":    {},
	"runtime/github.com/go-asm/go/sys":     {},
	"runtime/github.com/go-asm/go/syscall": {},
}

// 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)
	defaultGcFlags := []string{"-p", pkgpath}
	if p.Module != nil {
		v := p.Module.GoVersion
		if v == "" {
			v = gover.DefaultGoModVersion
		}
		if allowedVersion(v) {
			defaultGcFlags = append(defaultGcFlags, "-lang=go"+gover.Lang(v))
		}
	}
	if p.Standard {
		defaultGcFlags = append(defaultGcFlags, "-std")
	}
	_, compilingRuntime := runtimePackages[p.ImportPath]
	compilingRuntime = compilingRuntime && p.Standard
	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.
		defaultGcFlags = append(defaultGcFlags, "-+")
	}

	// 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", "github.com/go-asm/go/poll", "net", "os":
			fallthrough
		case "runtime/metrics", "runtime/pprof", "runtime/trace":
			fallthrough
		case "sync", "syscall", "time":
			extFiles++
		}
	}
	if extFiles == 0 {
		defaultGcFlags = append(defaultGcFlags, "-complete")
	}
	if cfg.BuildContext.InstallSuffix != "" {
		defaultGcFlags = append(defaultGcFlags, "-installsuffix", cfg.BuildContext.InstallSuffix)
	}
	if a.buildID != "" {
		defaultGcFlags = append(defaultGcFlags, "-buildid", a.buildID)
	}
	if p.Internal.OmitDebug || cfg.Goos == "plan9" || cfg.Goarch == "wasm" {
		defaultGcFlags = append(defaultGcFlags, "-dwarf=false")
	}
	if strings.HasPrefix(ToolchainVersion, "go1") && !strings.Contains(os.Args[0], "go_bootstrap") {
		defaultGcFlags = append(defaultGcFlags, "-goversion", ToolchainVersion)
	}
	if p.Internal.CoverageCfg != "" {
		defaultGcFlags = append(defaultGcFlags, "-coveragecfg="+p.Internal.CoverageCfg)
	}
	if p.Internal.PGOProfile != "" {
		defaultGcFlags = append(defaultGcFlags, "-pgoprofile="+p.Internal.PGOProfile)
	}
	if symabis != "" {
		defaultGcFlags = append(defaultGcFlags, "-symabis", symabis)
	}

	gcflags := str.StringList(forcedGcflags, p.Internal.Gcflags)
	if p.Internal.FuzzInstrument {
		gcflags = append(gcflags, fuzzInstrumentFlags()...)
	}
	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--
			}
		}
	}
	// Add -c=N to use concurrent backend compilation, if possible.
	if c := gcBackendConcurrency(gcflags); c > 1 {
		defaultGcFlags = append(defaultGcFlags, fmt.Sprintf("-c=%d", c))
	}

	args := []any{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), defaultGcFlags, gcflags}
	if p.Internal.LocalPrefix == "" {
		args = append(args, "-nolocalimports")
	} else {
		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")
	}

	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)
	}

	// TODO: Test and delete these conditions.
	if cfg.ExperimentErr != nil || cfg.Experiment.FieldTrack || cfg.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) []any {
	// Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
	inc := filepath.Join(cfg.GOROOT, "pkg", "include")
	pkgpath := pkgPath(a)
	args := []any{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 == "386" {
		// Define GO386_value from cfg.GO386.
		args = append(args, "-D", "GO386_"+cfg.GO386)
	}

	if cfg.Goarch == "amd64" {
		// Define GOAMD64_value from cfg.GOAMD64.
		args = append(args, "-D", "GOAMD64_"+cfg.GOAMD64)
	}

	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)
	}

	if cfg.Goarch == "ppc64" || cfg.Goarch == "ppc64le" {
		// Define GOPPC64_power8..N from cfg.PPC64.
		// We treat each powerpc version as a superset of functionality.
		switch cfg.GOPPC64 {
		case "power10":
			args = append(args, "-D", "GOPPC64_power10")
			fallthrough
		case "power9":
			args = append(args, "-D", "GOPPC64_power9")
			fallthrough
		default: // This should always be power8.
			args = append(args, "-D", "GOPPC64_power8")
		}
	}

	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 []any) error {
	newArgs := make([]any, 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 {
		return formatOutput(b.WorkDir, p.Dir, p.ImportPath, p.Desc(), err.Error()+"\n")
	}
	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, error) {
	for _, f := range ldflags {
		if f == "-extld" || strings.HasPrefix(f, "-extld=") {
			// don't override -extld if supplied
			return ldflags, nil
		}
	}
	joined, err := quoted.Join(compiler)
	if err != nil {
		return nil, err
	}
	return append(ldflags, "-extld="+joined), nil
}

// 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 !platform.MustLinkExternal(cfg.Goos, cfg.Goarch, false) {
			ldflags = append(ldflags, "-X=github.com/go-asm/go/cmd/objabi.buildID="+root.buildID)
		}
	}

	// Store default GODEBUG in binaries.
	if root.Package.DefaultGODEBUG != "" {
		ldflags = append(ldflags, "-X=runtime.godebugDefault="+root.Package.DefaultGODEBUG)
	}

	// 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, err := setextld(ldflags, compiler)
	if err != nil {
		return err
	}

	// 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.
	// On Linux, for a shared object, at least with the Gold linker,
	// the output file path is recorded in the .gnu.version_d section.
	dir := "."
	if cfg.BuildBuildmode == "c-shared" || cfg.BuildBuildmode == "plugin" {
		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, err := setextld(ldflags, compiler)
	if err != nil {
		return err
	}
	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))
}