cmd/cgo: run gcc to get errors and debug info in parallel

This change kicks off the work to load the debug info when processing
each file, and then waits for all the files to be processed before
starting the single-goroutined part that processes them. The processing
is very order dependent so we won't try to make it concurrent. Though
in a later CL we can wait for only the relevant package to have been
processed concurrently before doing the single-goroutined processing for
it instead of waiting for all packages to be processed concurrently
before the single goroutine section.

We use a par.Queue to make sure we're not running too many gcc compiles
at the same time. The change to cmd/dist makes the par package available
to cgo.

Fixes #75167

Change-Id: I6a6a6964fb7f3a3684118b5ee66f1ad856b3ee59
Reviewed-on: https://go-review.googlesource.com/c/go/+/699020
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/cmd/cgo/gcc.go

@@ -183,16 +183,16 @@ func splitQuoted(s string) (r []string, err error) {
 	return args, err
 }
 
-// Translate rewrites f.AST, the original Go input, to remove
+// loadDebug runs gcc to load debug information for the File. The debug
-// references to the imported package C, replacing them with
+// information will be saved to the debugs field of the file, and be
-// references to the equivalent Go types, functions, and variables.
+// processed when Translate is called on the file later.
-func (p *Package) Translate(f *File) {
+// loadDebug is called concurrently with different files.
+func (f *File) loadDebug(p *Package) {
 	for _, cref := range f.Ref {
 		// Convert C.ulong to C.unsigned long, etc.
 		cref.Name.C = cname(cref.Name.Go)
 	}
 
-	var debugs []*debug // debug data from iterations of gccDebug
 	ft := fileTypedefs{typedefs: make(map[string]bool)}
 	numTypedefs := -1
 	for len(ft.typedefs) > numTypedefs {
@@ -211,7 +211,7 @@ func (p *Package) Translate(f *File) {
 		}
 		needType := p.guessKinds(f)
 		if len(needType) > 0 {
-			debugs = append(debugs, p.loadDWARF(f, &ft, needType))
+			f.debugs = append(f.debugs, p.loadDWARF(f, &ft, needType))
 		}
 
 		// In godefs mode we're OK with the typedefs, which
@@ -221,10 +221,16 @@ func (p *Package) Translate(f *File) {
 			break
 		}
 	}
+}
 
+// Translate rewrites f.AST, the original Go input, to remove
+// references to the imported package C, replacing them with
+// references to the equivalent Go types, functions, and variables.
+// Preconditions: File.loadDebug must be called prior to translate.
+func (p *Package) Translate(f *File) {
 	var conv typeConv
 	conv.Init(p.PtrSize, p.IntSize)
-	for _, d := range debugs {
+	for _, d := range f.debugs {
 		p.recordTypes(f, d, &conv)
 	}
 	p.prepareNames(f)
@@ -283,6 +289,7 @@ func (f *File) loadDefines(gccOptions []string) bool {
 // guessKinds tricks gcc into revealing the kind of each
 // name xxx for the references C.xxx in the Go input.
 // The kind is either a constant, type, or variable.
+// guessKinds is called concurrently with different files.
 func (p *Package) guessKinds(f *File) []*Name {
 	// Determine kinds for names we already know about,
 	// like #defines or 'struct foo', before bothering with gcc.
@@ -526,6 +533,7 @@ func (p *Package) guessKinds(f *File) []*Name {
 // loadDWARF parses the DWARF debug information generated
 // by gcc to learn the details of the constants, variables, and types
 // being referred to as C.xxx.
+// loadDwarf is called concurrently with different files.
 func (p *Package) loadDWARF(f *File, ft *fileTypedefs, names []*Name) *debug {
 	// Extract the types from the DWARF section of an object
 	// from a well-formed C program. Gcc only generates DWARF info
@@ -1789,6 +1797,7 @@ func gccTmp() string {
 
 // gccCmd returns the gcc command line to use for compiling
 // the input.
+// gccCommand is called concurrently for different files.
 func (p *Package) gccCmd(ofile string) []string {
 	c := append(gccBaseCmd,
 		"-w",         // no warnings
@@ -1832,6 +1841,7 @@ func (p *Package) gccCmd(ofile string) []string {
 
 // gccDebug runs gcc -gdwarf-2 over the C program stdin and
 // returns the corresponding DWARF data and, if present, debug data block.
+// gccDebug is called concurrently with different C programs.
 func (p *Package) gccDebug(stdin []byte, nnames int) (d *dwarf.Data, ints []int64, floats []float64, strs []string) {
 	ofile := gccTmp()
 	runGcc(stdin, p.gccCmd(ofile))
@@ -2222,6 +2232,7 @@ func gccDefines(stdin []byte, gccOptions []string) string {
 // gccErrors runs gcc over the C program stdin and returns
 // the errors that gcc prints. That is, this function expects
 // gcc to fail.
+// gccErrors is called concurrently with different C programs.
 func (p *Package) gccErrors(stdin []byte, extraArgs ...string) string {
 	// TODO(rsc): require failure
 	args := p.gccCmd(gccTmp())

src/cmd/cgo/main.go

@@ -30,6 +30,7 @@ import (
 	"cmd/internal/edit"
 	"cmd/internal/hash"
 	"cmd/internal/objabi"
+	"cmd/internal/par"
 	"cmd/internal/telemetry/counter"
 )
 
@@ -74,6 +75,8 @@ type File struct {
 	NoCallbacks map[string]bool     // C function names that with #cgo nocallback directive
 	NoEscapes   map[string]bool     // C function names that with #cgo noescape directive
 	Edit        *edit.Buffer
+
+	debugs []*debug // debug data from iterations of gccDebug. Initialized by File.loadDebug.
 }
 
 func (f *File) offset(p token.Pos) int {
@@ -391,7 +394,7 @@ func main() {
 	h := hash.New32()
 	io.WriteString(h, *importPath)
 	var once sync.Once
-	var wg sync.WaitGroup
+	q := par.NewQueue(runtime.GOMAXPROCS(0))
 	fs := make([]*File, len(goFiles))
 	for i, input := range goFiles {
 		if *srcDir != "" {
@@ -413,9 +416,7 @@ func main() {
 			fatalf("%s", err)
 		}
 
-		wg.Add(1)
+		q.Add(func() {
-		go func() {
-			defer wg.Done()
 			// Apply trimpath to the file path. The path won't be read from after this point.
 			input, _ = objabi.ApplyRewrites(input, *trimpath)
 			if strings.ContainsAny(input, "\r\n") {
@@ -436,10 +437,12 @@ func main() {
 			})
 
 			fs[i] = f
-		}()
+
+			f.loadDebug(p)
+		})
 	}
 
-	wg.Wait()
+	<-q.Idle()
 
 	cPrefix = fmt.Sprintf("_%x", h.Sum(nil)[0:6])
 

src/cmd/dist/buildtool.go

@@ -49,6 +49,7 @@ var bootstrapDirs = []string{
 	"cmd/internal/macho",
 	"cmd/internal/obj/...",
 	"cmd/internal/objabi",
+	"cmd/internal/par",
 	"cmd/internal/pgo",
 	"cmd/internal/pkgpath",
 	"cmd/internal/quoted",