[llvm-branch-commits] [llvm] release/21.x: [CodeGenPrepare] Make sure that `AddOffset` is also a loop invariant (#150625) (PR #150646)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/150646

Backport 2d0ca09305fcece75e2c501f1ec74aa6eada69a0

Requested by: @dtcxzyw

>From 15fbe94cc3389b0fa8c820d06d83c42333b53631 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sat, 26 Jul 2025 00:23:56 +0800
Subject: [PATCH] [CodeGenPrepare] Make sure that `AddOffset` is also a loop
 invariant (#150625)

Closes https://github.com/llvm/llvm-project/issues/150611.

(cherry picked from commit 2d0ca09305fcece75e2c501f1ec74aa6eada69a0)
---
 llvm/lib/CodeGen/CodeGenPrepare.cpp           |  4 +++
 .../CodeGenPrepare/X86/fold-loop-of-urem.ll   | 34 +++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 9bbb89e37865d..3d1408256df8e 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -2096,6 +2096,10 @@ static bool isRemOfLoopIncrementWithLoopInvariant(
   if (!L->isLoopInvariant(RemAmt))
     return false;
 
+  // Only works if the AddOffset is a loop invaraint
+  if (AddOffset && !L->isLoopInvariant(AddOffset))
+    return false;
+
   // Is the PHI a loop increment?
   auto LoopIncrInfo = getIVIncrement(PN, LI);
   if (!LoopIncrInfo)
diff --git a/llvm/test/Transforms/CodeGenPrepare/X86/fold-loop-of-urem.ll b/llvm/test/Transforms/CodeGenPrepare/X86/fold-loop-of-urem.ll
index 7abc32e4f1cd8..f53127f015391 100644
--- a/llvm/test/Transforms/CodeGenPrepare/X86/fold-loop-of-urem.ll
+++ b/llvm/test/Transforms/CodeGenPrepare/X86/fold-loop-of-urem.ll
@@ -1065,3 +1065,37 @@ for.body:
   %exitcond.not = icmp eq i32 %inc, %N
   br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
 }
+
+define i64 @pr150611_add_offset_is_not_loop_invariant(i1 %cond) {
+; CHECK-LABEL: define i64 @pr150611_add_offset_is_not_loop_invariant(
+; CHECK-SAME: i1 [[COND:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    [[REMAMT:%.*]] = select i1 [[COND]], i64 2, i64 0
+; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
+; CHECK:       [[FOR_BODY]]:
+; CHECK-NEXT:    [[INDVARS:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[INDVARS_NEXT:%.*]], %[[FOR_BODY]] ]
+; CHECK-NEXT:    [[ADD_OFFSET:%.*]] = zext i1 [[COND]] to i64
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw i64 [[INDVARS]], [[ADD_OFFSET]]
+; CHECK-NEXT:    [[REM:%.*]] = urem i64 [[ADD]], [[REMAMT]]
+; CHECK-NEXT:    [[INDVARS_NEXT]] = add nuw i64 [[INDVARS]], 1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[INDVARS_NEXT]], 3
+; CHECK-NEXT:    br i1 [[EXITCOND]], label %[[FOR_EXIT:.*]], label %[[FOR_BODY]]
+; CHECK:       [[FOR_EXIT]]:
+; CHECK-NEXT:    ret i64 [[REM]]
+;
+entry:
+  %remamt = select i1 %cond, i64 2, i64 0
+  br label %for.body
+
+for.body:
+  %indvars = phi i64 [ 0, %entry ], [ %indvars.next, %for.body ]
+  %add.offset = zext i1 %cond to i64
+  %add = add nuw i64 %indvars, %add.offset
+  %rem = urem i64 %add, %remamt
+  %indvars.next = add nuw i64 %indvars, 1
+  %exitcond = icmp eq i64 %indvars.next, 3
+  br i1 %exitcond, label %for.exit, label %for.body
+
+for.exit:
+  ret i64 %rem
+}