cmd/compile: add mutex profiling support

Updates #15756
Updates #19822

Change-Id: I98b17dcbbfd80e7e164b0523185382175fe2d89b
Reviewed-on: https://go-review.googlesource.com/39554
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

@@ -77,6 +77,8 @@ Flags:
 		Set runtime.MemProfileRate for the compilation to rate.
 	-msan
 		Insert calls to C/C++ memory sanitizer.
+	-mutexprofile file
+		Write mutex profile for the compilation to file.
 	-nolocalimports
 		Disallow local (relative) imports.
 	-o file

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

+// Copyright 2017 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.
+
+// +build !go1.8
+
+package gc
+
+import "runtime"
+
+func startMutexProfiling() {
+	Fatalf("mutex profiling unavailable in version %v", runtime.Version())
+}

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

@@ -222,6 +222,7 @@ func Main(archInit func(*Arch)) {
 	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(&mutexprofile, "mutexprofile", "", "write mutex profile to `file`")
 	flag.StringVar(&benchfile, "bench", "", "append benchmark times to `file`")
 	obj.Flagparse(usage)
 

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

+// Copyright 2017 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.
+
+// +build go1.8
+
+package gc
+
+import "runtime"
+
+func startMutexProfiling() {
+	runtime.SetMutexProfileFraction(1)
+}

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

@@ -38,6 +38,7 @@ var (
 	memprofilerate int64
 	traceprofile   string
 	traceHandler   func(string)
+	mutexprofile   string
 )
 
 func startProfile() {
@@ -85,6 +86,17 @@ func startProfile() {
 			f.Close()
 		})
 	}
+	if mutexprofile != "" {
+		f, err := os.Create(mutexprofile)
+		if err != nil {
+			Fatalf("%v", err)
+		}
+		startMutexProfiling()
+		atExit(func() {
+			pprof.Lookup("mutex").WriteTo(f, 0)
+			f.Close()
+		})
+	}
 	if traceprofile != "" && traceHandler != nil {
 		traceHandler(traceprofile)
 	}