[llvm] [SimpleLoopUnswitch] Fix bug in partially invariant unswitching (PR #222118)

[Ehsan Amiri via llvm-commits]

https://github.com/amehsan created https://github.com/llvm/llvm-project/pull/222118

First commit checks in the testcase and runs the CI to show the current implementation has a bug. I will commit the fix after CI finished for the first commit.

>From e25c5be1ec1bd99070da6d42e0da05474f155cb0 Mon Sep 17 00:00:00 2001
From: Ehsan Amiri <ehsan.amiri at huawei.com>
Date: Thu, 3 Sep 2026 15:25:41 -0400
Subject: [PATCH] add the failing testcase

---
 .../Transforms/SimpleLoopUnswitch/pr219510.ll | 34 +++++++++++++++++++
 1 file changed, 34 insertions(+)
 create mode 100644 llvm/test/Transforms/SimpleLoopUnswitch/pr219510.ll

diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/pr219510.ll b/llvm/test/Transforms/SimpleLoopUnswitch/pr219510.ll
new file mode 100644
index 0000000000000..555bfc1d992cc
--- /dev/null
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/pr219510.ll
@@ -0,0 +1,34 @@
+; RUN: opt -passes='loop-mssa(simple-loop-unswitch<nontrivial>)' -S < %s | FileCheck %s 
+
+declare void @barrier() memory(read)
+declare void @clobber()
+
+; CHECK: call void @barrier()
+; CHECK-NEXT: %lv = load i32, ptr %ptr
+
+
+define i32 @bad_unswitch(ptr %ptr, i32 %N) {
+entry:
+  br label %loop.header
+loop.header:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop.latch ]
+  call void @barrier()
+  %lv = load i32, ptr %ptr
+  %sc = icmp eq i32 %lv, 100
+  br i1 %sc, label %noclobber, label %clobber
+
+noclobber:
+  br label %loop.latch
+
+clobber:
+  call void @clobber()
+  br label %loop.latch
+
+loop.latch:
+  %c = icmp ult i32 %iv, %N
+  %iv.next = add i32 %iv, 1
+  br i1 %c, label %loop.header, label %exit
+
+exit:
+  ret i32 10
+}