[dev.ssa] cmd/compile: shrink stack guard

Our stack frame sizes look pretty good now.  Lower the stack
guard from 1024 to 720.
Tip is currently using 720.
We could go lower (to 640 at least) except PPC doesn't like that.

Change-Id: Ie5f96c0e822435638223f1e8a2bd1a1eed68e6aa
Reviewed-on: https://go-review.googlesource.com/19922
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/obj/stack.go

@@ -11,7 +11,7 @@ const (
 	STACKSYSTEM = 0
 	StackSystem = STACKSYSTEM
 	StackBig    = 4096
-	StackGuard  = 1024*stackGuardMultiplier + StackSystem
+	StackGuard  = 720*stackGuardMultiplier + StackSystem
 	StackSmall  = 128
 	StackLimit  = StackGuard - StackSystem - StackSmall
 )

src/runtime/stack.go

@@ -90,7 +90,7 @@ const (
 
 	// The stack guard is a pointer this many bytes above the
 	// bottom of the stack.
-	_StackGuard = 1024*sys.StackGuardMultiplier + _StackSystem
+	_StackGuard = 720*sys.StackGuardMultiplier + _StackSystem
 
 	// After a stack split check the SP is allowed to be this
 	// many bytes below the stack guard.  This saves an instruction

test/nosplit.go

@@ -302,13 +302,13 @@ TestCases:
 				// Instead of rewriting the test cases above, adjust
 				// the first stack frame to use up the extra bytes.
 				if i == 0 {
-					size += (1024 - 128) - 128
+					size += (720 - 128) - 128
 					// Noopt builds have a larger stackguard.
 					// See ../src/cmd/dist/buildruntime.go:stackGuardMultiplier
 					// This increase is included in obj.StackGuard
 					for _, s := range strings.Split(os.Getenv("GO_GCFLAGS"), " ") {
 						if s == "-N" {
-							size += 1024
+							size += 720
 						}
 					}
 				}