cmd/cgo, cmd/dist, cmd/go: cgo with clang fixes

1. Workaround the smart clang diagnostics with -Qunused-arguments:
clang: error: argument unused during compilation: '-XXX'
2. if "clang -print-libgcc-file-name" returns non-absolute path, don't
provide that on linker command line.
3. Fix dwarf.PtrType.Size() in cmd/cgo as clang doesn't generate
DW_AT_byte_size for pointer types.
4. Workaround warnings for -Wno-unneeded-internal-declaration with
-Wno-unknown-warning-option.
5. Add -Wno-unused-function.
6. enable race detector test on darwin with clang
(at least Apple clang version 1.7 (tags/Apple/clang-77) works).

Requires CL 7354043.

Update #4829
This should fix most parts of the problem, but one glitch still remains.
DWARF generated by newer clang doesn't differentiate these
two function types:
    void *malloc(size_t);
    void *malloc(unsigned long int);
so you might need to do this to make make.bash pass:
sed -i -e 's/C.malloc(C.size_t/C.malloc(C.ulong/' pkg/os/user/lookup_unix.go

R=golang-dev, dave, iant, rsc
CC=golang-dev
https://golang.org/cl/7351044

src/cmd/cgo/gcc.go

@@ -783,7 +783,13 @@ func (p *Package) gccCmd() []string {
 	if strings.Contains(p.gccName(), "clang") {
 		c = append(c,
 			"-ferror-limit=0",
+			// Apple clang version 1.7 (tags/Apple/clang-77) (based on LLVM 2.9svn)
+			// doesn't have -Wno-unneeded-internal-declaration, so we need yet another
+			// flag to disable the warning. Yes, really good diagnostics, clang.
+			"-Wno-unknown-warning-option",
 			"-Wno-unneeded-internal-declaration",
+			"-Wno-unused-function",
+			"-Qunused-arguments",
 		)
 	}
 
@@ -1049,6 +1055,12 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		return t
 	}
 
+	// clang won't generate DW_AT_byte_size for pointer types,
+	// so we have to fix it here.
+	if dt, ok := base(dtype).(*dwarf.PtrType); ok && dt.ByteSize == -1 {
+		dt.ByteSize = c.ptrSize
+	}
+
 	t := new(Type)
 	t.Size = dtype.Size()
 	t.Align = -1

src/cmd/dist/build.c

@@ -409,6 +409,7 @@ static char *proto_gccargs[] = {
 	"-Wno-comment",
 	"-Werror",
 	"-fno-common",
+	"-ggdb",
 	"-pipe",
 	"-O2",
 };
@@ -604,10 +605,10 @@ install(char *dir)
 		splitfields(&gccargs, bstr(&b));
 		for(i=0; i<nelem(proto_gccargs); i++)
 			vadd(&gccargs, proto_gccargs[i]);
-		if(clang)
-			vadd(&gccargs, "-g");
-		else
-			vadd(&gccargs, "-ggdb");
+		if(clang) {
+			// clang is too smart about unused command-line arguments
+			vadd(&gccargs, "-Qunused-arguments");
+		}
 	}
 
 	islib = hasprefix(dir, "lib") || streq(dir, "cmd/cc") || streq(dir, "cmd/gc");

src/cmd/go/build.go

@@ -1609,6 +1609,8 @@ func gccgoCleanPkgpath(p *Package) string {
 func (b *builder) libgcc(p *Package) (string, error) {
 	var buf bytes.Buffer
 
+	gccCmd := b.gccCmd(p.Dir)
+
 	prev := b.print
 	if buildN {
 		// In -n mode we temporarily swap out the builder's
@@ -1619,7 +1621,7 @@ func (b *builder) libgcc(p *Package) (string, error) {
 			return fmt.Fprint(&buf, a...)
 		}
 	}
-	f, err := b.runOut(p.Dir, p.ImportPath, b.gccCmd(p.Dir), "-print-libgcc-file-name")
+	f, err := b.runOut(p.Dir, p.ImportPath, gccCmd, "-print-libgcc-file-name")
 	if err != nil {
 		return "", fmt.Errorf("gcc -print-libgcc-file-name: %v (%s)", err, f)
 	}
@@ -1629,6 +1631,13 @@ func (b *builder) libgcc(p *Package) (string, error) {
 		b.print(s)
 		return "$LIBGCC", nil
 	}
+
+	// clang might not be able to find libgcc, and in that case,
+	// it will simply return "libgcc.a", which is of no use to us.
+	if strings.Contains(gccCmd[0], "clang") && !filepath.IsAbs(string(f)) {
+		return "", nil
+	}
+
 	return strings.Trim(string(f), "\r\n"), nil
 }
 
@@ -1662,19 +1671,21 @@ func (b *builder) gccCmd(objdir string) []string {
 	}
 	a = append(a, b.gccArchArgs()...)
 	// gcc-4.5 and beyond require explicit "-pthread" flag
-	// for multithreading with pthread library, but clang whines
-	// about unused arguments if we pass it.
+	// for multithreading with pthread library.
 	if buildContext.CgoEnabled {
 		switch goos {
 		case "windows":
 			a = append(a, "-mthreads")
 		default:
-			if !strings.Contains(a[0], "clang") {
-				a = append(a, "-pthread")
-			}
+			a = append(a, "-pthread")
 		}
 	}
 
+	// clang is too smart about command-line arguments
+	if strings.Contains(a[0], "clang") {
+		a = append(a, "-Qunused-arguments")
+	}
+
 	// On OS X, some of the compilers behave as if -fno-common
 	// is always set, and the Mach-O linker in 6l/8l assumes this.
 	// See http://golang.org/issue/3253.
@@ -1706,6 +1717,7 @@ var cgoRe = regexp.MustCompile(`[/\\:]`)
 
 var (
 	cgoLibGccFile     string
+	cgoLibGccErr      error
 	cgoLibGccFileOnce sync.Once
 )
 
@@ -1800,21 +1812,19 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 	}
 
 	cgoLibGccFileOnce.Do(func() {
-		cgoLibGccFile, err = b.libgcc(p)
+		cgoLibGccFile, cgoLibGccErr = b.libgcc(p)
 	})
-	if cgoLibGccFile == "" {
-		if err == nil {
-			err = errors.New("failed to get libgcc filename")
-		}
+	if cgoLibGccFile == "" && cgoLibGccErr != nil {
 		return nil, nil, err
 	}
 
 	var staticLibs []string
 	if goos == "windows" {
 		// libmingw32 and libmingwex might also use libgcc, so libgcc must come last
-		staticLibs = []string{"-lmingwex", "-lmingw32", cgoLibGccFile}
-	} else {
-		staticLibs = []string{cgoLibGccFile}
+		staticLibs = []string{"-lmingwex", "-lmingw32"}
+	}
+	if cgoLibGccFile != "" {
+		staticLibs = append(staticLibs, cgoLibGccFile)
 	}
 
 	for _, cfile := range cfiles {

src/run.bash

@@ -48,10 +48,8 @@ go test sync -short -timeout=120s -cpu=10
 
 # Race detector only supported on Linux and OS X,
 # and only on amd64, and only when cgo is enabled.
-# Also, clang can't seem to link the .syso files, so only
-# run if we're using gcc.
-case "$GOHOSTOS-$GOOS-$GOARCH-$CGO_ENABLED-${CC:-gcc}" in
-linux-linux-amd64-1-*gcc* | darwin-darwin-amd64-1-*gcc*)
+case "$GOHOSTOS-$GOOS-$GOARCH-$CGO_ENABLED" in
+linux-linux-amd64-1 | darwin-darwin-amd64-1)
 	echo
 	echo '# Testing race detector.'
 	go test -race -i flag