[llvm] [InstCombine]: Eliminate redundant modulus for urem (PR #157644)

[via llvm-commits]

https://github.com/kper created https://github.com/llvm/llvm-project/pull/157644

Folds `urem` when first modulus is redundant.

Alive: https://alive2.llvm.org/ce/z/b_GQJX

>From 7cabd0bd1196bb79304d3bdee75762fbcc9c7e8e 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      |  8 ++
 llvm/test/Transforms/InstCombine/urem.ll      | 82 +++++++++++++++++++
 2 files changed, 90 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..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
+}