cmd/compile: disable shortcircuit optimization for intertwined phi values

randall77 · randall77 · commit 771c57e68ed5 · 2021-03-24T17:54:03.000Z
We need to be careful that when doing value graph surgery, we not re-substitute a value that has already been substituted. That can lead to confusing a previous iteration's value with the current iteration's value. The simple fix in this CL just aborts the optimization if it detects intertwined phis (a phi which is the argument to another phi). It might be possible to keep the optimization with a more complicated CL, but: 1) This CL is clearly safe to backport. 2) There were no instances of this abort triggering in all.bash, prior to the test introduced in this CL. Fixes #45175 Change-Id: I2411dca03948653c053291f6829a76bec0c32330 Reviewed-on: https://go-review.googlesource.com/c/go/+/304251 Trust: Keith Randall <khr@golang.org> Trust: Josh Bleecher Snyder <josharian@gmail.com> Run-TryBot: Keith Randall <khr@golang.org> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/ssa/shortcircuit.go b/src/cmd/compile/internal/ssa/shortcircuit.go
@@ -138,6 +138,24 @@ func shortcircuitBlock(b *Block) bool {
 	if len(b.Values) != nval+nOtherPhi {
 		return false
 	}
+	if nOtherPhi > 0 {
+		// Check for any phi which is the argument of another phi.
+		// These cases are tricky, as substitutions done by replaceUses
+		// are no longer trivial to do in any ordering. See issue 45175.
+		m := make(map[*Value]bool, 1+nOtherPhi)
+		for _, v := range b.Values {
+			if v.Op == OpPhi {
+				m[v] = true
+			}
+		}
+		for v := range m {
+			for _, a := range v.Args {
+				if a != v && m[a] {
+					return false
+				}
+			}
+		}
+	}
 
 	// Locate index of first const phi arg.
 	cidx := -1
diff --git a/test/fixedbugs/issue45175.go b/test/fixedbugs/issue45175.go
@@ -0,0 +1,29 @@
+// run
+
+// Copyright 2021 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 main
+
+//go:noinline
+func f(c bool) int {
+	b := true
+	x := 0
+	y := 1
+	for b {
+		b = false
+		y = x
+		x = 2
+		if c {
+			return 3
+		}
+	}
+	return y
+}
+
+func main() {
+	if got := f(false); got != 0 {
+		panic(got)
+	}
+}
