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 issue 8139.

LGTM=dsymonds
R=golang-codereviews, dsymonds
CC=golang-codereviews, iant, r
https://codereview.appspot.com/100940043

1 files changed, 19 insertions, 4 deletions

diff --git a/src/pkg/runtime/iface.goc b/src/pkg/runtime/iface.goc
index 96bb8b8aa..c0a17e303 100644
--- a/src/pkg/runtime/iface.goc
+++ b/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