cmd/compile: refactor memclrrange for arrays and slices

Rename memclrrange to signify that it does not handle
all types of range clears.

Simplify checks to detect the range clear idiom for
arrays and slices.

Add tests to verify the optimization for the slice
range clear idiom is being applied by the compiler.

Change-Id: I5c3b7c9a479699ebdb4c407fde692f30f377860c
Reviewed-on: https://go-review.googlesource.com/110477
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -204,7 +204,7 @@ func walkrange(n *Node) *Node {
 		Fatalf("walkrange")
 
 	case TARRAY, TSLICE:
-		if memclrrange(n, v1, v2, a) {
+		if arrayClear(n, v1, v2, a) {
 			lineno = lno
 			return n
 		}
@@ -460,23 +460,28 @@ func walkrange(n *Node) *Node {
 // in which the evaluation of a is side-effect-free.
 //
 // Parameters are as in walkrange: "for v1, v2 = range a".
-func memclrrange(n, v1, v2, a *Node) bool {
+func arrayClear(n, v1, v2, a *Node) bool {
 	if Debug['N'] != 0 || instrumenting {
 		return false
 	}
+
 	if v1 == nil || v2 != nil {
 		return false
 	}
-	if n.Nbody.Len() == 0 || n.Nbody.First() == nil || n.Nbody.Len() > 1 {
+
+	if n.Nbody.Len() != 1 || n.Nbody.First() == nil {
 		return false
 	}
+
 	stmt := n.Nbody.First() // only stmt in body
 	if stmt.Op != OAS || stmt.Left.Op != OINDEX {
 		return false
 	}
+
 	if !samesafeexpr(stmt.Left.Left, a) || !samesafeexpr(stmt.Left.Right, v1) {
 		return false
 	}
+
 	elemsize := n.Type.Elem().Width
 	if elemsize <= 0 || !isZero(stmt.Right) {
 		return false

test/codegen/slices.go

@@ -0,0 +1,32 @@
+// asmcheck
+
+// 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 codegen
+
+// This file contains code generation tests related to the handling of
+// slice types.
+
+// ------------------ //
+//      Clear         //
+// ------------------ //
+
+// Issue #5373 optimize memset idiom
+
+func SliceClear(s []int) []int {
+	// amd64:`.*memclrNoHeapPointers`
+	for i := range s {
+		s[i] = 0
+	}
+	return s
+}
+
+func SliceClearPointers(s []*int) []*int {
+	// amd64:`.*memclrHasPointers`
+	for i := range s {
+		s[i] = nil
+	}
+	return s
+}