cmd/gc: don't believe that variables mentioned 256 times are unused.

Such variables would be put at 0(SP), leading to serious
corruptions at zero initialization.
Fixes #3084.

R=golang-dev, r
CC=golang-dev, remy
https://golang.org/cl/5683052

src/cmd/5g/ggen.c

@@ -29,10 +29,10 @@ markautoused(Prog* p)
 {
 	for (; p; p = p->link) {
 		if (p->from.name == D_AUTO && p->from.node)
-			p->from.node->used++;
+			p->from.node->used = 1;
 
 		if (p->to.name == D_AUTO && p->to.node)
-			p->to.node->used++;
+			p->to.node->used = 1;
 	}
 }
 

src/cmd/6g/ggen.c

@@ -26,10 +26,10 @@ markautoused(Prog* p)
 {
 	for (; p; p = p->link) {
 		if (p->from.type == D_AUTO && p->from.node)
-			p->from.node->used++;
+			p->from.node->used = 1;
 
 		if (p->to.type == D_AUTO && p->to.node)
-			p->to.node->used++;
+			p->to.node->used = 1;
 	}
 }
 

src/cmd/8g/ggen.c

@@ -28,10 +28,10 @@ markautoused(Prog* p)
 {
 	for (; p; p = p->link) {
 		if (p->from.type == D_AUTO && p->from.node)
-			p->from.node->used++;
+			p->from.node->used = 1;
 
 		if (p->to.type == D_AUTO && p->to.node)
-			p->to.node->used++;
+			p->to.node->used = 1;
 	}
 }
 

test/fixedbugs/bug423.go

+// run
+
+// Copyright 2012 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.
+
+// gc used to overflow a counter when a variable was
+// mentioned 256 times, and generate stack corruption.
+
+package main
+
+func main() {
+	F(1)
+}
+
+func F(arg int) {
+	var X int64
+	_ = X // used once
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0 // used 32 times
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0 // used 64 times
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0 // used 96 times
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0 // used 128 times
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0 // used 200 times
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0
+	X = 0 // used 256 times
+	if arg != 0 {
+		panic("argument was changed")
+	}
+}