[release-branch.go1.20] cmd/compile: fix findIndVar so it does not match disjointed loop headers

Fix #63983

parseIndVar, prove and maybe more are on the assumption that the loop header
is a single block. This can be wrong, ensure we don't match theses cases we
don't know how to handle.

In the future we could update them so that they know how to handle such cases
but theses cases seems rare so I don't think the value would be really high.
We could also run a loop canonicalization pass first which could handle this.

The repro case looks weird because I massaged it so it would crash with the
previous compiler.

Change-Id: I4aa8afae9e90a17fa1085832250fc1139c97faa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/539977
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 8b4e1259d0e82c8fe38a1456f997a4e9d63573a2)
Reviewed-on: https://go-review.googlesource.com/c/go/+/539936
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/compile/internal/ssa/loopbce.go

@@ -130,6 +130,13 @@ func findIndVar(f *Func) []indVar {
 			less = false
 		}
 
+		if ind.Block != b {
+			// TODO: Could be extended to include disjointed loop headers.
+			// I don't think this is causing missed optimizations in real world code often.
+			// See https://go.dev/issue/63955
+			continue
+		}
+
 		// Expect the increment to be a nonzero constant.
 		if inc.Op != OpConst64 {
 			continue

test/fixedbugs/issue63955.go

@@ -0,0 +1,22 @@
+// compile
+
+// Copyright 2023 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 j
+
+func f(try func() int, shouldInc func() bool, N func(int) int) {
+	var n int
+loop: // we want to have 3 preds here, the function entry and both gotos
+	if v := try(); v == 42 || v == 1337 { // the two || are to trick findIndVar
+		if n < 30 { // this aims to be the matched block
+			if shouldInc() {
+				n++
+				goto loop
+			}
+			n = N(n) // try to prevent some block joining
+			goto loop
+		}
+	}
+}