[release-branch.go1.17] cmd/compile: fix fuse pass to do CFG surgery correctly

removePred and removeArg do different things. removePred moves the last
predecessor to index k, whereas removeArg slides all the args k or
greater down by 1 index.

Kind of unfortunate different behavior in things named similarly.

Fixes #49129

Change-Id: I9ae409bdac744e713f4c121f948e43db6fdc8542
Reviewed-on: https://go-review.googlesource.com/c/go/+/358117
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
(cherry picked from commit 8dbf3e9393400d72d313e5616c88873e07692c70)
Reviewed-on: https://go-review.googlesource.com/c/go/+/358118
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>

2 files changed, 21 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/ssa/fuse_branchredirect.go b/src/cmd/compile/internal/ssa/fuse_branchredirect.go
index 1b8b307bca..ba5220bd87 100644
--- a/src/cmd/compile/internal/ssa/fuse_branchredirect.go
+++ b/src/cmd/compile/internal/ssa/fuse_branchredirect.go
@@ -78,7 +78,11 @@ func fuseBranchRedirect(f *Func) bool {
 					if v.Op != OpPhi {
 						continue
 					}
-					v.RemoveArg(k)
+					n := len(v.Args)
+					v.Args[k].Uses--
+					v.Args[k] = v.Args[n-1]
+					v.Args[n-1] = nil
+					v.Args = v.Args[:n-1]
 					phielimValue(v)
 				}
 				// Fix up child to have one more predecessor.
diff --git a/test/fixedbugs/issue49122.go b/test/fixedbugs/issue49122.go
new file mode 100644
index 0000000000..c62a627dae
--- /dev/null
+++ b/test/fixedbugs/issue49122.go
@@ -0,0 +1,16 @@
+// compile
+
+// 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
+
+var B []bool
+var N int
+
+func f(p bool, m map[bool]bool) bool {
+	var q bool
+	_ = p || N&N < N || B[0] || B[0]
+	return p && q && m[q]
+}