cmd/compile: add block profiling support

Updates #15756

Change-Id: Ic635812b324af926333122c02908cebfb24d7bce
Reviewed-on: https://go-review.googlesource.com/39208
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/doc.go

@@ -44,6 +44,8 @@ Flags:
 		Print compiler version and exit.
 	-asmhdr file
 		Write assembly header to file.
+	-blockprofile file
+		Write block profile for the compilation to file.
 	-complete
 		Assume package has no non-Go components.
 	-cpuprofile file

src/cmd/compile/internal/gc/main.go

@@ -221,6 +221,7 @@ func Main(archInit func(*Arch)) {
 	flag.StringVar(&memprofile, "memprofile", "", "write memory profile to `file`")
 	flag.Int64Var(&memprofilerate, "memprofilerate", 0, "set runtime.MemProfileRate to `rate`")
 	flag.StringVar(&traceprofile, "traceprofile", "", "write an execution trace to `file`")
+	flag.StringVar(&blockprofile, "blockprofile", "", "write block profile to `file`")
 	flag.StringVar(&benchfile, "bench", "", "append benchmark times to `file`")
 	obj.Flagparse(usage)
 

src/cmd/compile/internal/gc/util.go

@@ -32,6 +32,7 @@ func Exit(code int) {
 }
 
 var (
+	blockprofile   string
 	cpuprofile     string
 	memprofile     string
 	memprofilerate int64
@@ -73,6 +74,17 @@ func startProfile() {
 		// Not doing memory profiling; disable it entirely.
 		runtime.MemProfileRate = 0
 	}
+	if blockprofile != "" {
+		f, err := os.Create(blockprofile)
+		if err != nil {
+			Fatalf("%v", err)
+		}
+		runtime.SetBlockProfileRate(1)
+		atExit(func() {
+			pprof.Lookup("block").WriteTo(f, 0)
+			f.Close()
+		})
+	}
 	if traceprofile != "" && traceHandler != nil {
 		traceHandler(traceprofile)
 	}