[llvm] [InstCombine]: Eliminate redundant modulus for urem (PR #157644)
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Tue Sep 9 03:58:53 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: None (kper)
<details>
<summary>Changes</summary>
Folds `urem` when first modulus is redundant.
Alive: https://alive2.llvm.org/ce/z/b_GQJX
Fixes https://github.com/llvm/llvm-project/issues/157370
---
Full diff: https://github.com/llvm/llvm-project/pull/157644.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (+8)
- (added) llvm/test/Transforms/InstCombine/urem.ll (+82)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index a9aacc707cc20..5288be9c84383 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -2473,6 +2473,14 @@ Instruction *InstCombinerImpl::visitURem(BinaryOperator &I) {
}
}
+ Value *A;
+ Value *B;
+ // urem(urem(A, B), Op1) -> urem(A, Op1)
+ if (match(Op0, m_URem(m_Value(A), m_Value(B)))) {
+ Value *Fold = Builder.CreateURem(A, Op1);
+ return replaceInstUsesWith(I, Fold);
+ }
+
return nullptr;
}
diff --git a/llvm/test/Transforms/InstCombine/urem.ll b/llvm/test/Transforms/InstCombine/urem.ll
new file mode 100644
index 0000000000000..751dc4991907a
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/urem.ll
@@ -0,0 +1,82 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+declare void @use(i8)
+
+define i8 @fold_urem_constants(i64 %arg0) {
+; CHECK-LABEL: @fold_urem_constants(
+; CHECK-NEXT: [[V0:%.*]] = trunc i64 [[ARG0:%.*]] to i8
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V0]], 5
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v0 = trunc i64 %arg0 to i8
+ %v1 = urem i8 %v0, 25
+ %v2 = urem i8 %v1, 5
+ ret i8 %v2
+}
+
+define i8 @fold_urem_variables(i64 %arg0, i8 %A, i8 %B) {
+; CHECK-LABEL: @fold_urem_variables(
+; CHECK-NEXT: [[V0:%.*]] = trunc i64 [[ARG0:%.*]] to i8
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V0]], [[B:%.*]]
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v0 = trunc i64 %arg0 to i8
+ %v1 = urem i8 %v0, %A
+ %v2 = urem i8 %v1, %B
+ ret i8 %v2
+}
+
+define i8 @fold_urem_constants_multi_use(i64 %arg0) {
+; CHECK-LABEL: @fold_urem_constants_multi_use(
+; CHECK-NEXT: [[V0:%.*]] = trunc i64 [[ARG0:%.*]] to i8
+; CHECK-NEXT: [[V1:%.*]] = urem i8 [[V0]], 25
+; CHECK-NEXT: call void @use(i8 [[V1]])
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V0]], 5
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v0 = trunc i64 %arg0 to i8
+ %v1 = urem i8 %v0, 25
+ call void @use(i8 %v1)
+ %v2 = urem i8 %v1, 5
+ ret i8 %v2
+}
+
+define i8 @fold_urem_variables_multi_use(i64 %arg0, i8 %A, i8 %B) {
+; CHECK-LABEL: @fold_urem_variables_multi_use(
+; CHECK-NEXT: [[V0:%.*]] = trunc i64 [[ARG0:%.*]] to i8
+; CHECK-NEXT: [[V1:%.*]] = urem i8 [[V0]], [[A:%.*]]
+; CHECK-NEXT: call void @use(i8 [[V1]])
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V0]], [[B:%.*]]
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v0 = trunc i64 %arg0 to i8
+ %v1 = urem i8 %v0, %A
+ call void @use(i8 %v1)
+ %v2 = urem i8 %v1, %B
+ ret i8 %v2
+}
+
+define i8 @fold_urem_constants_negative_test(i64 %arg0) {
+; CHECK-LABEL: @fold_urem_constants_negative_test(
+; CHECK-NEXT: [[V0:%.*]] = trunc i64 [[ARG0:%.*]] to i8
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V0]], 25
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v0 = trunc i64 %arg0 to i8
+ %v1 = urem i8 %v0, 25
+ %v2 = urem i8 %v0, 5 ; removes this because not used
+ ret i8 %v1
+}
+
+define i8 @fold_urem_variables_negative_test(i64 %arg0, i8 %A, i8 %B) {
+; CHECK-LABEL: @fold_urem_variables_negative_test(
+; CHECK-NEXT: [[V0:%.*]] = trunc i64 [[ARG0:%.*]] to i8
+; CHECK-NEXT: [[V1:%.*]] = urem i8 [[V0]], [[A:%.*]]
+; CHECK-NEXT: ret i8 [[V1]]
+;
+ %v0 = trunc i64 %arg0 to i8
+ %v1 = urem i8 %v0, %A
+ %v2 = urem i8 %v0, %B ; removes this because not used
+ ret i8 %v1
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/157644
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