[llvm] [InstCombine] Preserve nuw in canonicalizeGEPOfConstGEPI8() (PR #151533)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/151533

Proof: https://alive2.llvm.org/ce/z/vC239S

>From d1497fee26038be968094b19c143c3fd10ee9065 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Thu, 31 Jul 2025 16:59:41 +0200
Subject: [PATCH] [InstCombine] Preserve nuw in canonicalizeGEPOfConstGEPI8()

Proof: https://alive2.llvm.org/ce/z/vC239S
---
 .../InstCombine/InstructionCombining.cpp      | 10 ++-
 .../Transforms/InstCombine/gepofconstgepi8.ll | 75 +++++++++++++++++++
 2 files changed, 83 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 9e333202fb2e7..d5896a3041ff6 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2652,9 +2652,15 @@ static Instruction *canonicalizeGEPOfConstGEPI8(GetElementPtrInst &GEP,
   APInt NewOffset = TypeSize * *C2 + *C1;
   if (NewOffset.isZero() ||
       (Src->hasOneUse() && GEP.getOperand(1)->hasOneUse())) {
+    GEPNoWrapFlags Flags = GEPNoWrapFlags::none();
+    if (GEP.hasNoUnsignedWrap() &&
+        cast<GEPOperator>(Src)->hasNoUnsignedWrap() &&
+        match(GEP.getOperand(1), m_NUWAddLike(m_Value(), m_Value())))
+      Flags |= GEPNoWrapFlags::noUnsignedWrap();
+
     Value *GEPConst =
-        IC.Builder.CreatePtrAdd(Base, IC.Builder.getInt(NewOffset));
-    return GetElementPtrInst::Create(BaseType, GEPConst, VarIndex);
+        IC.Builder.CreatePtrAdd(Base, IC.Builder.getInt(NewOffset), "", Flags);
+    return GetElementPtrInst::Create(BaseType, GEPConst, VarIndex, Flags);
   }
 
   return nullptr;
diff --git a/llvm/test/Transforms/InstCombine/gepofconstgepi8.ll b/llvm/test/Transforms/InstCombine/gepofconstgepi8.ll
index a92e0c263d357..63553189cca11 100644
--- a/llvm/test/Transforms/InstCombine/gepofconstgepi8.ll
+++ b/llvm/test/Transforms/InstCombine/gepofconstgepi8.ll
@@ -293,3 +293,78 @@ entry:
   %p2 = getelementptr <vscale x 4 x i32>, ptr %p1, i64 %index
   ret ptr %p2
 }
+
+define ptr @test_all_nuw(ptr %base, i64 %a) {
+; CHECK-LABEL: define ptr @test_all_nuw(
+; CHECK-SAME: ptr [[BASE:%.*]], i64 [[A:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr nuw i8, ptr [[BASE]], i64 9
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr nuw i32, ptr [[TMP0]], i64 [[A]]
+; CHECK-NEXT:    ret ptr [[P2]]
+;
+entry:
+  %p1 = getelementptr nuw i8, ptr %base, i64 1
+  %index = add nuw i64 %a, 2
+  %p2 = getelementptr nuw i32, ptr %p1, i64 %index
+  ret ptr %p2
+}
+
+define ptr @test_all_partial_nuw1(ptr %base, i64 %a) {
+; CHECK-LABEL: define ptr @test_all_partial_nuw1(
+; CHECK-SAME: ptr [[BASE:%.*]], i64 [[A:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i8, ptr [[BASE]], i64 9
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr i32, ptr [[TMP0]], i64 [[A]]
+; CHECK-NEXT:    ret ptr [[P2]]
+;
+entry:
+  %p1 = getelementptr i8, ptr %base, i64 1
+  %index = add nuw i64 %a, 2
+  %p2 = getelementptr nuw i32, ptr %p1, i64 %index
+  ret ptr %p2
+}
+
+define ptr @test_all_partial_nuw2(ptr %base, i64 %a) {
+; CHECK-LABEL: define ptr @test_all_partial_nuw2(
+; CHECK-SAME: ptr [[BASE:%.*]], i64 [[A:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i8, ptr [[BASE]], i64 9
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr i32, ptr [[TMP0]], i64 [[A]]
+; CHECK-NEXT:    ret ptr [[P2]]
+;
+entry:
+  %p1 = getelementptr nuw i8, ptr %base, i64 1
+  %index = add i64 %a, 2
+  %p2 = getelementptr nuw i32, ptr %p1, i64 %index
+  ret ptr %p2
+}
+
+define ptr @test_all_partial_nuw3(ptr %base, i64 %a) {
+; CHECK-LABEL: define ptr @test_all_partial_nuw3(
+; CHECK-SAME: ptr [[BASE:%.*]], i64 [[A:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i8, ptr [[BASE]], i64 9
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr i32, ptr [[TMP0]], i64 [[A]]
+; CHECK-NEXT:    ret ptr [[P2]]
+;
+entry:
+  %p1 = getelementptr nuw i8, ptr %base, i64 1
+  %index = add nuw i64 %a, 2
+  %p2 = getelementptr i32, ptr %p1, i64 %index
+  ret ptr %p2
+}
+
+define ptr @test_all_nuw_disjoint(ptr %base, i64 %a) {
+; CHECK-LABEL: define ptr @test_all_nuw_disjoint(
+; CHECK-SAME: ptr [[BASE:%.*]], i64 [[A:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr nuw i8, ptr [[BASE]], i64 9
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr nuw i32, ptr [[TMP0]], i64 [[A]]
+; CHECK-NEXT:    ret ptr [[P2]]
+;
+entry:
+  %p1 = getelementptr nuw i8, ptr %base, i64 1
+  %index = or disjoint i64 %a, 2
+  %p2 = getelementptr nuw i32, ptr %p1, i64 %index
+  ret ptr %p2
+}