cmd/compile: reset branch prediction when deleting a branch

When we go from a branch block to a plain block, reset the
branch prediction bit. Downstream passes asssume that if the
branch prediction is set, then the block has 2 successors.

Fixes #23504

Change-Id: I2898ec002228b2e34fe80ce420c6939201c0a5aa
Reviewed-on: https://go-review.googlesource.com/88955
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

diff --git a/src/cmd/compile/internal/ssa/check.go b/src/cmd/compile/internal/ssa/check.go
index de77aff480310fce892f20fe2f1473135155850a..721e451f05164b315b0a3a58548ce915c4b7107d 100644 (file)
--- a/src/cmd/compile/internal/ssa/check.go
+++ b/src/cmd/compile/internal/ssa/check.go
@@ -102,7 +102,7 @@ func checkFunc(f *Func) {
                                f.Fatalf("plain/dead block %s has a control value", b)
                        }
                }
-               if len(b.Succs) > 2 && b.Likely != BranchUnknown {
+               if len(b.Succs) != 2 && b.Likely != BranchUnknown {
                        f.Fatalf("likeliness prediction %d for block %s with %d successors", b.Likely, b, len(b.Succs))
                }
 

diff --git a/src/cmd/compile/internal/ssa/fuse.go b/src/cmd/compile/internal/ssa/fuse.go
index f00356a7b23b37042bed4a59df31646ea9456aba..45b13a050d939c71364315dba0edadd5948b041c 100644 (file)
--- a/src/cmd/compile/internal/ssa/fuse.go
+++ b/src/cmd/compile/internal/ssa/fuse.go
@@ -92,6 +92,7 @@ func fuseBlockIf(b *Block) bool {
                b.removeEdge(1)
        }
        b.Kind = BlockPlain
+       b.Likely = BranchUnknown
        b.SetControl(nil)
 
        // Trash the empty blocks s0 & s1.

diff --git a/test/fixedbugs/issue23504.go b/test/fixedbugs/issue23504.go
new file mode 100644 (file)
index 0000000..77f3184
--- /dev/null
+++ b/test/fixedbugs/issue23504.go
@@ -0,0 +1,15 @@
+// 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 p
+
+func f() {
+       var B bool
+       B2 := (B || B && !B) && !B
+       B3 := B2 || B
+       for (B3 || B2) && !B2 && B {
+       }
+}