[llvm] [InstCombine]: Eliminate redundant modulus for urem (PR #157644)
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Tue Sep 9 08:20:36 PDT 2025
https://github.com/kper updated https://github.com/llvm/llvm-project/pull/157644
>From d575ebb93776b695eedd17030a986883db860c04 Mon Sep 17 00:00:00 2001
From: Kevin Per <kevin.per at protonmail.com>
Date: Tue, 9 Sep 2025 10:53:11 +0000
Subject: [PATCH] [InstCombine]: Eliminate redundant modulus for urem
---
.../InstCombine/InstCombineMulDivRem.cpp | 10 +++
llvm/test/Transforms/InstCombine/urem.ll | 79 +++++++++++++++++++
2 files changed, 89 insertions(+)
create mode 100644 llvm/test/Transforms/InstCombine/urem.ll
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index a9aacc707cc20..84aab9b0142e9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -2473,6 +2473,16 @@ Instruction *InstCombinerImpl::visitURem(BinaryOperator &I) {
}
}
+ Value *A;
+ const APInt *Op1Cst, *BCst;
+ // urem(urem(A, BCst), Op1Cst) -> urem(A, Op1Cst)
+ // iff urem(BCst, Op1Cst) == 0
+ if (match(Op0, m_OneUse(m_URem(m_Value(A), m_APInt(BCst)))) &&
+ match(Op1, m_APInt(Op1Cst)) && BCst->urem(*Op1Cst).isZero()) {
+ return BinaryOperator::CreateURem(
+ A, ConstantInt::get(Op1->getType(), *Op1Cst));
+ }
+
return nullptr;
}
diff --git a/llvm/test/Transforms/InstCombine/urem.ll b/llvm/test/Transforms/InstCombine/urem.ll
new file mode 100644
index 0000000000000..36098576d9908
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/urem.ll
@@ -0,0 +1,79 @@
+; 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(i8 %v0) {
+; CHECK-LABEL: @fold_urem_constants(
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V0:%.*]], 5
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v1 = urem i8 %v0, 25
+ %v2 = urem i8 %v1, 5
+ ret i8 %v2
+}
+
+define <2 x i8> @fold_urem_vec_splat(<2 x i8> %v0) {
+; CHECK-LABEL: @fold_urem_vec_splat(
+; CHECK-NEXT: [[V2:%.*]] = urem <2 x i8> [[V0:%.*]], splat (i8 5)
+; CHECK-NEXT: ret <2 x i8> [[V2]]
+;
+ %v1 = urem <2 x i8> %v0, <i8 25, i8 25>
+ %v2 = urem <2 x i8> %v1, <i8 5, i8 5>
+ ret <2 x i8> %v2
+}
+
+define <2 x i8> @fold_urem_vec_splat_poison(<2 x i8> %v0) {
+; CHECK-LABEL: @fold_urem_vec_splat_poison(
+; CHECK-NEXT: ret <2 x i8> poison
+;
+ %v1 = urem <2 x i8> %v0, <i8 25, i8 poison>
+ %v2 = urem <2 x i8> %v1, <i8 5, i8 poison>
+ ret <2 x i8> %v2
+}
+
+define i8 @fold_urem_constants_multi_use(i8 %v0) {
+; CHECK-LABEL: @fold_urem_constants_multi_use(
+; CHECK-NEXT: [[V1:%.*]] = urem i8 [[V0:%.*]], 25
+; CHECK-NEXT: call void @use(i8 [[V1]])
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V1]], 5
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v1 = urem i8 %v0, 25
+ call void @use(i8 %v1)
+ %v2 = urem i8 %v1, 5
+ ret i8 %v2
+}
+
+define <2 x i8> @fold_urem_vec_splat_negative_test(<2 x i8> %v0) {
+; CHECK-LABEL: @fold_urem_vec_splat_negative_test(
+; CHECK-NEXT: [[V1:%.*]] = urem <2 x i8> [[V0:%.*]], splat (i8 26)
+; CHECK-NEXT: [[V2:%.*]] = urem <2 x i8> [[V1]], splat (i8 5)
+; CHECK-NEXT: ret <2 x i8> [[V2]]
+;
+ %v1 = urem <2 x i8> %v0, <i8 26, i8 26>
+ %v2 = urem <2 x i8> %v1, <i8 5, i8 5>
+ ret <2 x i8> %v2
+}
+
+define i8 @fold_urem_constants_negative_test(i8 %v0) {
+; CHECK-LABEL: @fold_urem_constants_negative_test(
+; CHECK-NEXT: [[V1:%.*]] = urem i8 [[V0:%.*]], 25
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V1]], 3
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v1 = urem i8 %v0, 25
+ %v2 = urem i8 %v1, 3 ; should not be folded because not known urem(25, 3) != 0
+ ret i8 %v2
+}
+
+define i8 @fold_urem_variables_negative_test(i8 %v0, i8 %A, i8 %B) {
+; CHECK-LABEL: @fold_urem_variables_negative_test(
+; CHECK-NEXT: [[V1:%.*]] = urem i8 [[V0:%.*]], [[A:%.*]]
+; CHECK-NEXT: [[V2:%.*]] = urem i8 [[V1]], [[B:%.*]]
+; CHECK-NEXT: ret i8 [[V2]]
+;
+ %v1 = urem i8 %v0, %A
+ %v2 = urem i8 %v1, %B ; should not be folded because not known urem(%B, %A) == 0
+ ret i8 %v2
+}
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