windows/386: clean stack after syscall (it is necessary after call cdecl functions and does not have an effect after stdcall)

Result of discussion here: http://groups.google.com/group/golang-nuts/browse_thread/thread/357c806cbb57ca62

R=golang-dev, bradfitz, alex.brainman, hectorchu, rsc
CC=golang-dev
https://golang.org/cl/4961045

src/pkg/runtime/syscall_windows_test.go

@@ -0,0 +1,61 @@
+// Copyright 2010 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.
+
+package runtime_test
+
+import (
+	"syscall"
+	"unsafe"
+	"testing"
+)
+
+func TestStdCall(t *testing.T) {
+	type Rect struct {
+		left, top, right, bottom int32
+	}
+
+	h, e := syscall.LoadLibrary("user32.dll")
+	if e != 0 {
+		t.Fatal("LoadLibrary(USER32)")
+	}
+	p, e := syscall.GetProcAddress(h, "UnionRect")
+	if e != 0 {
+		t.Fatal("GetProcAddress(USER32.UnionRect)")
+	}
+
+	res := Rect{}
+	expected := Rect{1, 1, 40, 60}
+	a, _, _ := syscall.Syscall(uintptr(p),
+		3,
+		uintptr(unsafe.Pointer(&res)),
+		uintptr(unsafe.Pointer(&Rect{10, 1, 14, 60})),
+		uintptr(unsafe.Pointer(&Rect{1, 2, 40, 50})))
+	if a != 1 || res.left != expected.left ||
+		res.top != expected.top ||
+		res.right != expected.right ||
+		res.bottom != expected.bottom {
+		t.Error("stdcall USER32.UnionRect returns", a, "res=", res)
+	}
+}
+
+func TestCDecl(t *testing.T) {
+	h, e := syscall.LoadLibrary("user32.dll")
+	if e != 0 {
+		t.Fatal("LoadLibrary(USER32)")
+	}
+	p, e := syscall.GetProcAddress(h, "wsprintfA")
+	if e != 0 {
+		t.Fatal("GetProcAddress(USER32.wsprintfA)")
+	}
+
+	var buf [50]byte
+	a, _, _ := syscall.Syscall6(uintptr(p),
+		5,
+		uintptr(unsafe.Pointer(&buf[0])),
+		uintptr(unsafe.Pointer(syscall.StringBytePtr("%d %d %d"))),
+		1000, 2000, 3000, 0)
+	if string(buf[:a]) != "1000 2000 3000" {
+		t.Error("cdecl USER32.wsprintfA returns", a, "buf=", buf[:a])
+	}
+}

src/pkg/runtime/windows/386/sys.s

@@ -12,6 +12,7 @@ TEXT runtime·asmstdcall(SB),7,$0
 	MOVL	$0, 0x34(FS)
 
 	// Copy args to the stack.
+	MOVL	SP, BP
 	MOVL	wincall_n(DX), CX	// words
 	MOVL	CX, BX
 	SALL	$2, BX
@@ -21,9 +22,11 @@ TEXT runtime·asmstdcall(SB),7,$0
 	CLD
 	REP; MOVSL
 
-	// Call stdcall function.
+	// Call stdcall or cdecl function.
+	// DI SI BP BX are preserved, SP is not
 	MOVL	wincall_fn(DX), AX
 	CALL	AX
+	MOVL	BP, SP
 
 	// Return result.
 	MOVL	c+0(FP), DX