From 85cfa4d55e177a70913ce8977ce847af7c4298e2 Mon Sep 17 00:00:00 2001 From: Keith Randall Date: Thu, 12 Jul 2018 11:44:05 -0700 Subject: [PATCH] cmd/compile: handle degenerate write barrier case If both branches of a write barrier test go to the same block, then there's no unsafe points. This can only happen if the resulting memory state is somehow dead, which can only occur in degenerate cases, like infinite loops. No point in cleaning up the useless branch in these situations. Fixes #26024. Change-Id: I93a7df9fdf2fc94c6c4b1fe61180dc4fd4a0871f Reviewed-on: https://go-review.googlesource.com/123655 Reviewed-by: David Chase Run-TryBot: Keith Randall TryBot-Result: Gobot Gobot --- src/cmd/compile/internal/gc/plive.go | 5 +++++ test/fixedbugs/issue26024.go | 28 ++++++++++++++++++++++++++++ 2 files changed, 33 insertions(+) create mode 100644 test/fixedbugs/issue26024.go diff --git a/src/cmd/compile/internal/gc/plive.go b/src/cmd/compile/internal/gc/plive.go index 88b4380637..f78d051b06 100644 --- a/src/cmd/compile/internal/gc/plive.go +++ b/src/cmd/compile/internal/gc/plive.go @@ -692,6 +692,11 @@ func (lv *Liveness) markUnsafePoints() { lv.f.Fatalf("expected branch at write barrier block %v", wbBlock) } s0, s1 := wbBlock.Succs[0].Block(), wbBlock.Succs[1].Block() + if s0 == s1 { + // There's no difference between write barrier on and off. + // Thus there's no unsafe locations. See issue 26024. + continue + } if s0.Kind != ssa.BlockPlain || s1.Kind != ssa.BlockPlain { lv.f.Fatalf("expected successors of write barrier block %v to be plain", wbBlock) } diff --git a/test/fixedbugs/issue26024.go b/test/fixedbugs/issue26024.go new file mode 100644 index 0000000000..78877b21fe --- /dev/null +++ b/test/fixedbugs/issue26024.go @@ -0,0 +1,28 @@ +// compile +// 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 r + +// f compiles into code where no stores remain in the two successors +// of a write barrier block; i.e., they are empty. Pre-fix, this +// results in an unexpected input to markUnsafePoints, that expects to +// see a pair of non-empty plain blocks. +func f() { + var i int + var s string + for len(s) < len(s) { + i++ + s = "a" + } + var b bool + var sa []string + for true { + sa = []string{""} + for b || i == 0 { + } + b = !b + _ = sa + } +}