cmd/compile: ensure that operand of ORETURN is not double-walked

Inlining of switch statements into a RETURNed expression
can sometimes lead to the switch being walked twice, which
results in a miscompiled switch statement. The bug depends
on:

1) multiple results
2) named results
3) a return statement whose expression includes a call to a
function containing a switch statement that is inlined.

It may also be significant that the default case of that
switch is a panic(), though that's not proven.

Rearranged the walk case for ORETURN so that double walks are
not possible.  Added a test, because this is so fiddly.
Added a check against double walks, verified that it fires
w/o other fix.

Fixes #25776.

Change-Id: I2d594351fa082632512ef989af67eb887059729b
Reviewed-on: https://go-review.googlesource.com/118318
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/gc/swt.go

@@ -241,6 +241,11 @@ func walkswitch(sw *Node) {
 // search using if..goto, although binary search
 // is used with long runs of constants.
 func (s *exprSwitch) walk(sw *Node) {
+	// Guard against double walk, see #25776.
+	if sw.List.Len() == 0 && sw.Nbody.Len() > 0 {
+		Fatalf("second walk of switch")
+	}
+
 	casebody(sw, nil)
 
 	cond := sw.Left

src/cmd/compile/internal/gc/walk.go

@@ -287,7 +287,6 @@ func walkstmt(n *Node) *Node {
 		walkstmtlist(n.Rlist.Slice())
 
 	case ORETURN:
-		walkexprlist(n.List.Slice(), &n.Ninit)
 		if n.List.Len() == 0 {
 			break
 		}
@@ -317,6 +316,9 @@ func walkstmt(n *Node) *Node {
 
 			if samelist(rl, n.List.Slice()) {
 				// special return in disguise
+				// TODO(josharian, 1.12): is "special return" still relevant?
+				// Tests still pass w/o this. See comments on https://go-review.googlesource.com/c/go/+/118318
+				walkexprlist(n.List.Slice(), &n.Ninit)
 				n.List.Set(nil)
 
 				break
@@ -329,6 +331,7 @@ func walkstmt(n *Node) *Node {
 			n.List.Set(reorder3(ll))
 			break
 		}
+		walkexprlist(n.List.Slice(), &n.Ninit)
 
 		ll := ascompatte(nil, false, Curfn.Type.Results(), n.List.Slice(), 1, &n.Ninit)
 		n.List.Set(ll)

test/fixedbugs/issue25776.go

@@ -0,0 +1,99 @@
+// run
+
+// Copyright 2018 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
+
+const (
+	Upper       = true
+	blas_Upper  = 121
+	badTriangle = "bad triangle"
+)
+
+// Triangular represents a triangular matrix. Triangular matrices are always square.
+type Triangular interface {
+	// Triangular returns the number of rows/columns in the matrix and its
+	// orientation.
+	Tryangle() (mmmm int, kynd bool)
+	Triangle() (mmmm int, kynd bool)
+}
+
+// blas64_Triangular represents a triangular matrix using the conventional storage scheme.
+type blas64_Triangular struct {
+	Stride int
+	Uplo   int
+}
+
+// TriDense represents an upper or lower triangular matrix in dense storage
+// format.
+type TriDense struct {
+	mat blas64_Triangular
+}
+
+func NewTriDense() *TriDense {
+	return &TriDense{
+		mat: blas64_Triangular{
+			Stride: 3,
+			Uplo:   blas_Upper,
+		},
+	}
+}
+
+func (t *TriDense) isUpper() bool {
+	return isUpperUplo(t.mat.Uplo)
+}
+
+func (t *TriDense) triKind() bool {
+	return isUpperUplo(t.mat.Uplo)
+}
+
+func isUpperUplo(u int) bool {
+	switch u {
+	case blas_Upper:
+		return true
+	default:
+		panic(badTriangle)
+	}
+}
+
+func (t *TriDense) IsZero() bool {
+	return t.mat.Stride == 0
+}
+
+//go:noinline
+func (t *TriDense) ScaleTri(f float64, a Triangular) {
+	n, kind := a.Triangle()
+	if kind == false {
+		println("ScaleTri n, kind=", n, ", ", kind, " (FAIL, expected true)")
+	}
+}
+
+//go:noinline
+func (t *TriDense) ScaleTry(f float64, a Triangular) {
+	n, kind := a.Tryangle()
+	if kind == false {
+		println("ScaleTry n, kind=", n, ", ", kind, " (FAIL, expected true)")
+	}
+}
+
+// Triangle failed (before fix)
+func (t *TriDense) Triangle() (nnnn int, kind bool) {
+	return 3, !t.IsZero() && t.triKind()
+}
+
+// Tryangle works -- difference is not-named output parameters.
+func (t *TriDense) Tryangle() (int, bool) {
+	return 3, !t.IsZero() && t.triKind()
+}
+
+func main() {
+	ta := NewTriDense()
+	n, kind := ta.Triangle()
+	if kind == false {
+		println("    main n, kind=", n, ", ", kind, " (FAIL, expected true)")
+	}
+	ta.ScaleTri(1, ta)
+	ta.ScaleTry(1, ta)
+}