cmd/compile,runtime: skip zero'ing order array for select statements

The order array was zero initialized by the compiler, but ends up being
overwritten by the runtime anyway.

So let the runtime takes full responsibility for initializing, save us
one instruction per select.

Fixes #40399

Change-Id: Iec1eca27ad7180d4fcb3cc9ef97348206b7fe6b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/251517
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -251,10 +251,8 @@ func walkselectcases(cases *Nodes) []*Node {
 	r = typecheck(r, ctxStmt)
 	init = append(init, r)
 
+	// No initialization for order; runtime.selectgo is responsible for that.
 	order := temp(types.NewArray(types.Types[TUINT16], 2*int64(ncas)))
-	r = nod(OAS, order, nil)
-	r = typecheck(r, ctxStmt)
-	init = append(init, r)
 
 	var pc0, pcs *Node
 	if flag_race {

src/runtime/select.go

@@ -118,6 +118,7 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, blo
 	scases := cas1[:ncases:ncases]
 	pollorder := order1[:ncases:ncases]
 	lockorder := order1[ncases:][:ncases:ncases]
+	// NOTE: pollorder/lockorder's underlying array was not zero-initialized by compiler.
 
 	// Even when raceenabled is true, there might be select
 	// statements in packages compiled without -race (e.g.,

test/codegen/select.go

@@ -0,0 +1,20 @@
+// asmcheck
+
+// Copyright 2020 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
+
+func f() {
+	ch1 := make(chan int)
+	ch2 := make(chan int)
+	for {
+		// amd64:-`MOVQ\t[$]0, ""..autotmp_3`
+		select {
+		case <-ch1:
+		case <-ch2:
+		default:
+		}
+	}
+}