runtime: use 2*regSize for saved frame pointer check

Previously, we checked for a saved frame pointer by looking for a
2*ptrSize gap between the argument pointer and the locals pointer.
The intent of this check was to look for a two stack slot gap (caller
IP and saved frame pointer), but stack slots are regSize, not ptrSize.

Correct this by checking instead for a 2*regSize gap.

On most platforms, this made no difference because ptrSize==regSize.
However, on amd64p32 (nacl), the saved frame pointer check incorrectly
fired when there was no saved frame pointer because the one stack slot
for the caller IP left an 8 byte gap, which is 2*ptrSize (but not
2*regSize) on amd64p32.

Fixes #9760.

Change-Id: I6eedcf681fe5bf2bf924dde8a8f2d9860a4d758e
Reviewed-on: https://go-review.googlesource.com/3781Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/stack1.go

@@ -466,9 +466,10 @@ func adjustframe(frame *stkframe, arg unsafe.Pointer) bool {
 	}
 
 	// Adjust saved base pointer if there is one.
-	if thechar == '6' && frame.argp-frame.varp == 2*ptrSize {
+	if thechar == '6' && frame.argp-frame.varp == 2*regSize {
 		if !framepointer_enabled {
 			print("runtime: found space for saved base pointer, but no framepointer experiment\n")
+			print("argp=", hex(frame.argp), " varp=", hex(frame.varp), "\n")
 			throw("bad frame layout")
 		}
 		if stackDebug >= 3 {

src/runtime/traceback.go

@@ -235,7 +235,7 @@ func gentraceback(pc0 uintptr, sp0 uintptr, lr0 uintptr, gp *g, skip int, pcbuf
 		// If framepointer_enabled and there's a frame, then
 		// there's a saved bp here.
 		if GOARCH == "amd64" && frame.varp > frame.sp && framepointer_enabled {
-			frame.varp -= ptrSize
+			frame.varp -= regSize
 		}
 
 		// Derive size of arguments.