cmd/compile: handle infinite loops in shortcircuit pass

The newly upgraded shortcircuit pass attempted to remove infinite loops.
Stop doing that.

Fixes #33903

Change-Id: I0fc9c1b5f2427e54ce650806602ef5e3ad65aca5
Reviewed-on: https://go-review.googlesource.com/c/go/+/192144
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

diff --git a/src/cmd/compile/internal/ssa/shortcircuit.go b/src/cmd/compile/internal/ssa/shortcircuit.go
index e881a4cf1eface33f331ac85125f2dfef6c55261..5bf08880438d105e2e7a9550ae818451cad527f5 100644 (file)
--- a/src/cmd/compile/internal/ssa/shortcircuit.go
+++ b/src/cmd/compile/internal/ssa/shortcircuit.go
@@ -50,7 +50,7 @@ func shortcircuit(f *Func) {
                }
        }
 
-       // Step 3: Redirect control flow around known branches.
+       // Step 2: Redirect control flow around known branches.
        // p:
        //   ... goto b ...
        // b: <- p ...
@@ -124,7 +124,6 @@ func shortcircuitBlock(b *Block) bool {
                if a.Op != OpConstBool {
                        continue
                }
-               changed = true
                // The predecessor we come in from.
                e1 := b.Preds[i]
                p := e1.b
@@ -138,8 +137,15 @@ func shortcircuitBlock(b *Block) bool {
                }
                e2 := b.Succs[si]
                t := e2.b
+               if p == b || t == b {
+                       // This is an infinite loop; we can't remove it. See issue 33903.
+                       continue
+               }
                ti := e2.i
 
+               // Update CFG and Phis.
+               changed = true
+
                // Remove b's incoming edge from p.
                b.removePred(i)
                n := len(b.Preds)

diff --git a/test/fixedbugs/issue33903.go b/test/fixedbugs/issue33903.go
new file mode 100644 (file)
index 0000000..de03282
--- /dev/null
+++ b/test/fixedbugs/issue33903.go
@@ -0,0 +1,16 @@
+// compile
+
+// Copyright 2019 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.
+
+// Check that the shortcircuit pass correctly handles infinite loops.
+
+package p
+
+func f() {
+       var p, q bool
+       for {
+               p = p && q
+       }
+}