From 4ef68a16e21a2e63f2f6bdc498fe34c9ca994011 Mon Sep 17 00:00:00 2001
From: Jorropo <[email protected]>
Date: Sun, 5 Nov 2023 22:40:01 +0100
Subject: [PATCH 05/20] [release-branch.go1.21] cmd/compile: fix findIndVar so
 it does not match disjointed loop headers

Fix #63984

Conflict:NA
Reference:https://go-review.googlesource.com/c/go/+/539977

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 <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
(cherry picked from commit 8b4e1259d0e82c8fe38a1456f997a4e9d63573a2)
Reviewed-on: https://go-review.googlesource.com/c/go/+/540535
Reviewed-by: Jorropo <[email protected]>
Reviewed-by: Mauri de Souza Meneguzzo <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
---
 src/cmd/compile/internal/ssa/loopbce.go |  7 +++++++
 test/fixedbugs/issue63955.go            | 22 ++++++++++++++++++++++
 2 files changed, 29 insertions(+)
 create mode 100644 test/fixedbugs/issue63955.go

diff --git a/src/cmd/compile/internal/ssa/loopbce.go b/src/cmd/compile/internal/ssa/loopbce.go
index b7dfaa33e3bf..7f432e61ba50 100644
--- a/src/cmd/compile/internal/ssa/loopbce.go
+++ b/src/cmd/compile/internal/ssa/loopbce.go
@@ -127,6 +127,13 @@ 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.isGenericIntConst() {
 			continue
diff --git a/test/fixedbugs/issue63955.go b/test/fixedbugs/issue63955.go
new file mode 100644
index 000000000000..258e874220f0
--- /dev/null
+++ b/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
+		}
+	}
+}
-- 
2.33.0