cmd/compile: make -memprofilerate work

There are multiple things preventing the -memprofilerate flag from
working right now:

- CmdFlags.MemProfileRate has type int64, which is currently not
  supported by the compiler's reflection-based registerFlags.
  Unfortunately, rather than letting you know this, registerFlags
  simply ignores this field.

- Nothing consumes CmdFlags.MemProfileRate anyway. startProfile
  instead uses a package-local memprofilerate variable that is never
  set to anything.

Fix this by making CmdFlags.MemProfileRate an int (that's what
runtime.MemProfileRate is anyway) and using it in startProfile. While
we're here, prevent similar flag parsing bugs in the future by making
registerFlags panic if it encounters a flag field of unsupported type.

Change-Id: Ib9a1fcd8f4c5e9d7175a4fabc375f31e79774f9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/359955
Trust: Austin Clements <austin@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/base/flag.go

@@ -109,7 +109,7 @@ type CmdFlags struct {
 	Live               CountFlag    "help:\"debug liveness analysis\""
 	MSan               bool         "help:\"build code compatible with C/C++ memory sanitizer\""
 	MemProfile         string       "help:\"write memory profile to `file`\""
-	MemProfileRate     int64        "help:\"set runtime.MemProfileRate to `rate`\""
+	MemProfileRate     int          "help:\"set runtime.MemProfileRate to `rate`\""
 	MutexProfile       string       "help:\"write mutex profile to `file`\""
 	NoLocalImports     bool         "help:\"reject local (relative) imports\""
 	Pack               bool         "help:\"write to file.a instead of file.o\""
@@ -330,6 +330,8 @@ func registerFlags() {
 		case funcType:
 			f := v.Field(i).Interface().(func(string))
 			objabi.Flagfn1(name, help, f)
+		default:
+			panic(fmt.Sprintf("base.Flag.%s has unexpected type %s", f.Name, f.Type))
 		}
 	}
 }

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

@@ -12,10 +12,7 @@ import (
 	"cmd/compile/internal/base"
 )
 
-var (
-	memprofilerate int64
-	traceHandler   func(string)
-)
+var traceHandler func(string)
 
 func startProfile() {
 	if base.Flag.CPUProfile != "" {
@@ -29,8 +26,8 @@ func startProfile() {
 		base.AtExit(pprof.StopCPUProfile)
 	}
 	if base.Flag.MemProfile != "" {
-		if memprofilerate != 0 {
-			runtime.MemProfileRate = int(memprofilerate)
+		if base.Flag.MemProfileRate != 0 {
+			runtime.MemProfileRate = base.Flag.MemProfileRate
 		}
 		f, err := os.Create(base.Flag.MemProfile)
 		if err != nil {