gc: fix complex move bug

R=ken2
CC=golang-dev
https://golang.org/cl/4439044

src/cmd/gc/cplx.c

@@ -21,7 +21,7 @@ void
 complexmove(Node *f, Node *t)
 {
 	int ft, tt;
-	Node n1, n2, n3, n4;
+	Node n1, n2, n3, n4, t3, t4;
 
 	if(debug['g']) {
 		dump("\ncomplexmove-f", f);
@@ -54,8 +54,21 @@ complexmove(Node *f, Node *t)
 		subnode(&n1, &n2, f);
 		subnode(&n3, &n4, t);
 
-		cgen(&n1, &n3);
-		cgen(&n2, &n4);
+		// Copy fully into registers before doing stores,
+		// in case the source and destination overlap.
+		// Might be picking up a complex128 from one
+		// location on the stack and writing it 8 bytes
+		// (half a complex128) later, in which case the
+		// first write would smash the source for the second read.
+		regalloc(&t3, types[tt+TFLOAT64-TCOMPLEX128], N);
+		regalloc(&t4, types[tt+TFLOAT64-TCOMPLEX128], N);
+		cgen(&n1, &t3);
+		cgen(&n2, &t4);
+
+		cgen(&t3, &n3);
+		cgen(&t4, &n4);
+		regfree(&t3);
+		regfree(&t4);
 		break;
 	}
 }

test/fixedbugs/bug329.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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.
+
+// Test that when moving a complex128 returned by one function
+// into the argument position for another function, the right thing
+// happens, even when the two positions half-overlap.
+
+package main
+
+type Value struct {
+	X interface{}
+	Y int
+}
+
+type Struct struct {
+	X complex128
+}
+
+const magic = 1+2i
+
+func (Value) Complex(x complex128) {
+	if x != magic {
+		println(x)
+		panic("bad complex magic")
+	}
+}
+
+func f(x *byte, y, z int) complex128 {
+	return magic
+}
+
+func (Value) Struct(x Struct) {
+	if x.X != magic {
+		println(x.X)
+		panic("bad struct magic")
+	}
+}
+
+func f1(x *byte, y, z int) Struct {
+	return Struct{magic}
+}
+
+func main() {
+	var v Value
+	v.Struct(f1(nil, 0, 0))  // ok
+	v.Complex(f(nil, 0, 0))  // used to fail
+}