cmd/compile: when comparing 0-size types, make sure expr side-effects survive

Fixes #23837

Change-Id: I53f524d87946a0065f28a4ddbe47b40f2b43c459
Reviewed-on: https://go-review.googlesource.com/c/145757
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -3297,6 +3297,14 @@ func walkcompare(n *Node, init *Nodes) *Node {
 	}
 	if expr == nil {
 		expr = nodbool(n.Op == OEQ)
+		// We still need to use cmpl and cmpr, in case they contain
+		// an expression which might panic. See issue 23837.
+		t := temp(cmpl.Type)
+		a1 := nod(OAS, t, cmpl)
+		a1 = typecheck(a1, Etop)
+		a2 := nod(OAS, t, cmpr)
+		a2 = typecheck(a2, Etop)
+		init.Append(a1, a2)
 	}
 	n = finishcompare(n, expr, init)
 	return n

test/fixedbugs/issue23837.go

@@ -0,0 +1,70 @@
+// 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
+
+//go:noinline
+func f(p, q *struct{}) bool {
+	return *p == *q
+}
+
+type T struct {
+	x struct{}
+	y int
+}
+
+//go:noinline
+func g(p, q *T) bool {
+	return p.x == q.x
+}
+
+//go:noinline
+func h(p, q func() struct{}) bool {
+	return p() == q()
+}
+
+func fi(p, q *struct{}) bool {
+	return *p == *q
+}
+
+func gi(p, q *T) bool {
+	return p.x == q.x
+}
+
+func hi(p, q func() struct{}) bool {
+	return p() == q()
+}
+
+func main() {
+	shouldPanic(func() { f(nil, nil) })
+	shouldPanic(func() { g(nil, nil) })
+	shouldPanic(func() { h(nil, nil) })
+	shouldPanic(func() { fi(nil, nil) })
+	shouldPanic(func() { gi(nil, nil) })
+	shouldPanic(func() { hi(nil, nil) })
+	n := 0
+	inc := func() struct{} {
+		n++
+		return struct{}{}
+	}
+	h(inc, inc)
+	if n != 2 {
+		panic("inc not called")
+	}
+	hi(inc, inc)
+	if n != 4 {
+		panic("inc not called")
+	}
+}
+
+func shouldPanic(x func()) {
+	defer func() {
+		if recover() == nil {
+			panic("did not panic")
+		}
+	}()
+	x()
+}