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/36955Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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", `

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.

test/fixedbugs/issue19084.go

+// 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{})
+}