cmd/asm, go/build: invoke cmd/asm only once per package

Prior to this CL, cmd/go invoked cmd/asm once
for every assembly file.
The exec and cmd/asm startup overhead dwarfed
the actual time spent assembling.
This CL adds support to cmd/asm to process
multiple input files and uses it in cmd/go.

This cuts 10% off the wall time for 'go build -a math'.

Fixes #15680

Change-Id: I12d2ee2c817207954961dc8f37b8f2b09f835550
Reviewed-on: https://go-review.googlesource.com/27636
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/asm/internal/flags/flags.go

@@ -15,7 +15,7 @@ import (
 
 var (
 	Debug      = flag.Bool("debug", false, "dump instructions as they are parsed")
-	OutputFile = flag.String("o", "", "output file; default foo.6 for /a/b/c/foo.s on amd64")
+	OutputFile = flag.String("o", "", "output file; default foo.o for /a/b/c/foo.s as first argument")
 	PrintOut   = flag.Bool("S", false, "print assembly and machine code")
 	TrimPath   = flag.String("trimpath", "", "remove prefix from recorded source file paths")
 	Shared     = flag.Bool("shared", false, "generate code that can be linked into a shared library")
@@ -49,7 +49,7 @@ func (m *MultiFlag) Set(val string) error {
 }
 
 func Usage() {
-	fmt.Fprintf(os.Stderr, "usage: asm [options] file.s\n")
+	fmt.Fprintf(os.Stderr, "usage: asm [options] file.s ...\n")
 	fmt.Fprintf(os.Stderr, "Flags:\n")
 	flag.PrintDefaults()
 	os.Exit(2)
@@ -58,12 +58,15 @@ func Usage() {
 func Parse() {
 	flag.Usage = Usage
 	flag.Parse()
-	if flag.NArg() != 1 {
+	if flag.NArg() == 0 {
 		flag.Usage()
 	}
 
 	// Flag refinement.
 	if *OutputFile == "" {
+		if flag.NArg() != 1 {
+			flag.Usage()
+		}
 		input := filepath.Base(flag.Arg(0))
 		if strings.HasSuffix(input, ".s") {
 			input = input[:len(input)-2]

src/cmd/asm/main.go

@@ -54,22 +54,32 @@ func main() {
 	fmt.Fprintf(buf, "go object %s %s %s\n", obj.Getgoos(), obj.Getgoarch(), obj.Getgoversion())
 	fmt.Fprintf(buf, "!\n")
 
-	lexer := lex.NewLexer(flag.Arg(0), ctxt)
+	var ok, diag bool
+	var failedFile string
+	for _, f := range flag.Args() {
+		lexer := lex.NewLexer(f, ctxt)
 		parser := asm.NewParser(ctxt, architecture, lexer)
-	diag := false
 		ctxt.DiagFunc = func(format string, args ...interface{}) {
 			diag = true
 			log.Printf(format, args...)
 		}
 		pList := obj.Linknewplist(ctxt)
-	var ok bool
 		pList.Firstpc, ok = parser.Parse()
+		if !ok {
+			failedFile = f
+			break
+		}
+	}
 	if ok {
 		// reports errors to parser.Errorf
 		obj.Writeobjdirect(ctxt, buf)
 	}
 	if !ok || diag {
-		log.Printf("assembly of %s failed", flag.Arg(0))
+		if failedFile != "" {
+			log.Printf("assembly of %s failed", failedFile)
+		} else {
+			log.Print("assembly failed")
+		}
 		os.Remove(*flags.OutputFile)
 		os.Exit(1)
 	}

src/cmd/go/build.go

@@ -1561,12 +1561,12 @@ func (b *builder) build(a *action) (err error) {
 	}
 
 	// Assemble .s files.
-	for _, file := range sfiles {
-		out := file[:len(file)-len(".s")] + ".o"
-		if err := buildToolchain.asm(b, a.p, obj, obj+out, file); err != nil {
+	if len(sfiles) > 0 {
+		ofiles, err := buildToolchain.asm(b, a.p, obj, sfiles)
+		if err != nil {
 			return err
 		}
-		objects = append(objects, out)
+		objects = append(objects, ofiles...)
 	}
 
 	// NOTE(rsc): On Windows, it is critically important that the
@@ -2203,9 +2203,9 @@ type toolchain interface {
 	// cc runs the toolchain's C compiler in a directory on a C file
 	// to produce an output file.
 	cc(b *builder, p *Package, objdir, ofile, cfile string) error
-	// asm runs the assembler in a specific directory on a specific file
-	// to generate the named output file.
-	asm(b *builder, p *Package, obj, ofile, sfile string) error
+	// asm runs the assembler in a specific directory on specific files
+	// and returns a list of named output files.
+	asm(b *builder, p *Package, obj string, sfiles []string) ([]string, error)
 	// pkgpath builds an appropriate path for a temporary package file.
 	pkgpath(basedir string, p *Package) string
 	// pack runs the archive packer in a specific directory to create
@@ -2242,8 +2242,8 @@ func (noToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr bool,
 	return "", nil, noCompiler()
 }
 
-func (noToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
-	return noCompiler()
+func (noToolchain) asm(b *builder, p *Package, obj string, sfiles []string) ([]string, error) {
+	return nil, noCompiler()
 }
 
 func (noToolchain) pkgpath(basedir string, p *Package) string {
@@ -2340,10 +2340,10 @@ func (gcToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr bool,
 	return ofile, output, err
 }
 
-func (gcToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
+func (gcToolchain) asm(b *builder, p *Package, obj string, sfiles []string) ([]string, error) {
 	// Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
 	inc := filepath.Join(goroot, "pkg", "include")
-	sfile = mkAbs(p.Dir, sfile)
+	ofile := obj + "asm.o"
 	args := []interface{}{buildToolExec, tool("asm"), "-o", ofile, "-trimpath", b.work, "-I", obj, "-I", inc, "-D", "GOOS_" + goos, "-D", "GOARCH_" + goarch, buildAsmflags}
 	if p.ImportPath == "runtime" && goarch == "386" {
 		for _, arg := range buildAsmflags {
@@ -2352,11 +2352,13 @@ func (gcToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
 			}
 		}
 	}
-	args = append(args, sfile)
+	for _, sfile := range sfiles {
+		args = append(args, mkAbs(p.Dir, sfile))
+	}
 	if err := b.run(p.Dir, p.ImportPath, nil, args...); err != nil {
-		return err
+		return nil, err
 	}
-	return nil
+	return []string{ofile}, nil
 }
 
 // toolVerify checks that the command line args writes the same output file
@@ -2623,7 +2625,11 @@ func (tools gccgoToolchain) gc(b *builder, p *Package, archive, obj string, asmh
 	return ofile, output, err
 }
 
-func (tools gccgoToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
+func (tools gccgoToolchain) asm(b *builder, p *Package, obj string, sfiles []string) ([]string, error) {
+	var ofiles []string
+	for _, sfile := range sfiles {
+		ofile := obj + sfile[:len(sfile)-len(".s")] + ".o"
+		ofiles = append(ofiles, ofile)
 		sfile = mkAbs(p.Dir, sfile)
 		defs := []string{"-D", "GOOS_" + goos, "-D", "GOARCH_" + goarch}
 		if pkgpath := gccgoCleanPkgpath(p); pkgpath != "" {
@@ -2631,7 +2637,12 @@ func (tools gccgoToolchain) asm(b *builder, p *Package, obj, ofile, sfile string
 		}
 		defs = tools.maybePIC(defs)
 		defs = append(defs, b.gccArchArgs()...)
-	return b.run(p.Dir, p.ImportPath, nil, tools.compiler(), "-xassembler-with-cpp", "-I", obj, "-c", "-o", ofile, defs, sfile)
+		err := b.run(p.Dir, p.ImportPath, nil, tools.compiler(), "-xassembler-with-cpp", "-I", obj, "-c", "-o", ofile, defs, sfile)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return ofiles, nil
 }
 
 func (gccgoToolchain) pkgpath(basedir string, p *Package) string {