runtime: fix 1-byte return during x.(T) for 0-byte T

The 1-byte write was silently clearing a byte on the stack.
If there was another function call with more arguments
in the same stack frame, no harm done.
Otherwise, if the variable at that location was already zero,
no harm done.
Otherwise, problems.

Fixes #8139.

LGTM=dsymonds
R=golang-codereviews, dsymonds
CC=golang-codereviews, iant, r
https://golang.org/cl/100940043

src/pkg/runtime/iface.goc

@@ -209,9 +209,19 @@ func convT2E(t *Type, elem *byte) (ret Eface) {
 
 static void assertI2Tret(Type *t, Iface i, byte *ret);
 
+/*
+ * NOTE: Cannot use 'func' here, because we have to declare
+ * a return value, the only types we have are at least 1 byte large,
+ * goc2c will zero the return value, and the actual return value
+ * might have size 0 bytes, in which case the zeroing of the
+ * 1 or more bytes would be wrong.
+ * Using C lets us control (avoid) the initial zeroing.
+ */
 #pragma textflag NOSPLIT
-func assertI2T(t *Type, i Iface) (ret byte, ...) {
-	assertI2Tret(t, i, &ret);
+void
+runtime·assertI2T(Type *t, Iface i, GoOutput retbase)
+{
+	assertI2Tret(t, i, (byte*)&retbase);
 }
 
 static void
@@ -260,9 +270,14 @@ func assertI2TOK(t *Type, i Iface) (ok bool) {
 
 static void assertE2Tret(Type *t, Eface e, byte *ret);
 
+/*
+ * NOTE: Cannot use 'func' here. See assertI2T above.
+ */
 #pragma textflag NOSPLIT
-func assertE2T(t *Type, e Eface) (ret byte, ...) {
-	assertE2Tret(t, e, &ret);
+void
+runtime·assertE2T(Type *t, Eface e, GoOutput retbase)
+{
+	assertE2Tret(t, e, (byte*)&retbase);
 }
 
 static void

test/fixedbugs/issue8139.go

+// run
+
+// Copyright 2014 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.
+
+// Issue 8139. The x.(T) assertions used to write 1 (unexpected)
+// return byte for the 0-byte return value T.
+
+package main
+
+import "fmt"
+
+type T struct{}
+
+func (T) M() {}
+
+type M interface {
+	M()
+}
+
+var e interface{} = T{}
+var i M = T{}
+var b bool
+
+func f1() int {
+	if b {
+		return f1() // convince inliner not to inline
+	}
+	z := 0x11223344
+	_ = e.(T)
+	return z
+}
+
+func f2() int {
+	if b {
+		return f1() // convince inliner not to inline
+	}
+	z := 0x11223344
+	_ = i.(T)
+	return z
+}
+
+func main() {
+	x := f1()
+	y := f2()
+	if x != 0x11223344 || y != 0x11223344 {
+		fmt.Printf("BUG: x=%#x y=%#x, want 0x11223344 for both\n", x, y)
+	}
+}