cmd/compile: fixed which-result confusion in presence of 0-width types

A function returning multiple results, some of them zero-width,
will have more than one result present at an offset.  Be sure
that offset AND type match.

Includes test.

Change-Id: I3eb1f56116d989b4e73f533fefabb1bf554c901b
Reviewed-on: https://go-review.googlesource.com/c/go/+/297169
Trust: David Chase <[email protected]>
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Jeremy Faller <[email protected]>

Author: dr2chase

src/cmd/compile/internal/ssa/op.go

@@ -86,12 +86,15 @@ type AuxCall struct {
 	abiInfo *abi.ABIParamResultInfo // TODO remove fields above redundant with this information.
 }
 
-// ResultForOffset returns the index of the result at a particular offset among the results
+// ResultForOffsetAndType returns the index of a t-typed result at *A* particular offset among the results.
+// An arbitrary number of zero-width-typed results may reside at the same offset with a single not-zero-width
+// typed result, but the ones with the same type are all indistinguishable so it doesn't matter "which one"
+// is obtained.
 // This does not include the mem result for the call opcode.
-func (a *AuxCall) ResultForOffset(offset int64) int64 {
+func (a *AuxCall) ResultForOffsetAndType(offset int64, t *types.Type) int64 {
 	which := int64(-1)
 	for i := int64(0); i < a.NResults(); i++ { // note aux NResults does not include mem result.
-		if a.OffsetOfResult(i) == offset {
+		if a.OffsetOfResult(i) == offset && a.TypeOfResult(i) == t {
 			which = i
 			break
 		}

src/cmd/compile/internal/ssagen/ssa.go

@@ -2909,7 +2909,7 @@ func (s *state) expr(n ir.Node) *ssa.Value {
 			addr := s.constOffPtrSP(types.NewPtr(n.Type()), n.Offset)
 			return s.rawLoad(n.Type(), addr)
 		}
-		which := s.prevCall.Aux.(*ssa.AuxCall).ResultForOffset(n.Offset)
+		which := s.prevCall.Aux.(*ssa.AuxCall).ResultForOffsetAndType(n.Offset, n.Type())
 		if which == -1 {
 			// Do the old thing // TODO: Panic instead.
 			addr := s.constOffPtrSP(types.NewPtr(n.Type()), n.Offset)
@@ -5119,7 +5119,7 @@ func (s *state) addr(n ir.Node) *ssa.Value {
 		if s.prevCall == nil || s.prevCall.Op != ssa.OpStaticLECall && s.prevCall.Op != ssa.OpInterLECall && s.prevCall.Op != ssa.OpClosureLECall {
 			return s.constOffPtrSP(t, n.Offset)
 		}
-		which := s.prevCall.Aux.(*ssa.AuxCall).ResultForOffset(n.Offset)
+		which := s.prevCall.Aux.(*ssa.AuxCall).ResultForOffsetAndType(n.Offset, n.Type())
 		if which == -1 {
 			// Do the old thing // TODO: Panic instead.
 			return s.constOffPtrSP(t, n.Offset)

test/abi/f_ret_z_not.go

@@ -0,0 +1,33 @@
+// run
+
+// Copyright 2021 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 main
+
+import "fmt"
+
+type Z struct {
+}
+
+type NZ struct {
+	x, y int
+}
+
+//go:noinline
+func f(x,y int) (Z,NZ,Z) {
+	var z Z
+	return z,NZ{x,y},z
+}
+
+//go:noinline
+func g() (Z,NZ,Z) {
+	a,b,c := f(3,4)
+	return c,b,a
+}
+
+func main() {
+	_,b,_ := g()
+	fmt.Println(b.x+b.y)
+}

test/abi/f_ret_z_not.out

@@ -0,0 +1 @@
+7