From 27a30186abc18a8fc22b8ab40e3ee9f29d81c1d4 Mon Sep 17 00:00:00 2001 From: Cuong Manh Le Date: Sat, 29 Aug 2020 03:25:21 +0700 Subject: [PATCH] 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 TryBot-Result: Gobot Gobot Reviewed-by: Matthew Dempsky --- src/cmd/compile/internal/gc/select.go | 4 +--- src/runtime/select.go | 1 + test/codegen/select.go | 20 ++++++++++++++++++++ 3 files changed, 22 insertions(+), 3 deletions(-) create mode 100644 test/codegen/select.go diff --git a/src/cmd/compile/internal/gc/select.go b/src/cmd/compile/internal/gc/select.go index 3812a0e1fa..97e0424ce0 100644 --- a/src/cmd/compile/internal/gc/select.go +++ b/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 { diff --git a/src/runtime/select.go b/src/runtime/select.go index 80768b285b..a506747910 100644 --- a/src/runtime/select.go +++ b/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., diff --git a/test/codegen/select.go b/test/codegen/select.go new file mode 100644 index 0000000000..4426924b36 --- /dev/null +++ b/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: + } + } +} -- 2.48.1