From 78200799a290da7d53ebbd50c04e432a4ab14eec Mon Sep 17 00:00:00 2001
From: Cherry Zhang <cherryyz@google.com>
Date: Tue, 14 Feb 2017 11:01:04 -0500
Subject: [PATCH] cmd/compile: undo special handling of zero-valued STRUCTLIT

CL 35261 introduces special handling of zero-valued STRUCTLIT for
efficient struct zeroing. But it didn't cover all use cases, for
example, CONVNOP STRUCTLIT is not handled.

On the other hand, CL 34566 handles zeroing earlier, so we don't
need the change in CL 35261 for efficient zeroing. Other uses of
zero-valued struct literals are very rare. So undo the change in
walk.go in CL 35261.

Add a test for efficient zeroing.

Fixes #19084.

Change-Id: I0807f7423fb44d47bf325b3c1ce9611a14953853
Reviewed-on: https://go-review.googlesource.com/36955
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
---
 src/cmd/compile/internal/gc/asm_test.go | 11 +++++++++++
 src/cmd/compile/internal/gc/walk.go     |  3 ---
 test/fixedbugs/issue19084.go            | 17 +++++++++++++++++
 3 files changed, 28 insertions(+), 3 deletions(-)
 create mode 100644 test/fixedbugs/issue19084.go

diff --git a/src/cmd/compile/internal/gc/asm_test.go b/src/cmd/compile/internal/gc/asm_test.go
index 7cffae0e5f..4373cfa6b6 100644
--- a/src/cmd/compile/internal/gc/asm_test.go
+++ b/src/cmd/compile/internal/gc/asm_test.go
@@ -314,6 +314,17 @@ func f(t *T) {
 		[]string{"\tMOVQ\t\\$0, \\(.*\\)", "\tMOVQ\t\\$0, 8\\(.*\\)", "\tMOVQ\t\\$0, 16\\(.*\\)"},
 	},
 	// TODO: add a test for *t = T{3,4,5} when we fix that.
+	// Also test struct containing pointers (this was special because of write barriers).
+	{"amd64", "linux", `
+type T struct {
+	a, b, c *int
+}
+func f(t *T) {
+	*t = T{}
+}
+`,
+		[]string{"\tMOVQ\t\\$0, \\(.*\\)", "\tMOVQ\t\\$0, 8\\(.*\\)", "\tMOVQ\t\\$0, 16\\(.*\\)", "\tCALL\truntime\\.writebarrierptr\\(SB\\)"},
+	},
 
 	// Rotate tests
 	{"amd64", "linux", `
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index bf7f253517..28b430f22d 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -1539,9 +1539,6 @@ opswitch:
 		n = r
 
 	case OARRAYLIT, OSLICELIT, OMAPLIT, OSTRUCTLIT, OPTRLIT:
-		if n.Op == OSTRUCTLIT && iszero(n) && !instrumenting { // TODO: SSA doesn't yet handle ARRAYLIT with length > 1
-			break
-		}
 		if isStaticCompositeLiteral(n) {
 			// n can be directly represented in the read-only data section.
 			// Make direct reference to the static data. See issue 12841.
diff --git a/test/fixedbugs/issue19084.go b/test/fixedbugs/issue19084.go
new file mode 100644
index 0000000000..ba5306320b
--- /dev/null
+++ b/test/fixedbugs/issue19084.go
@@ -0,0 +1,17 @@
+// compile
+
+// Copyright 2017 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.
+
+// Issue 19084: SSA doesn't handle CONVNOP STRUCTLIT
+
+package p
+
+type T struct {
+	a, b, c, d, e, f, g, h int // big, not SSA-able
+}
+
+func f() {
+	_ = T(T{})
+}