[llvm] [InstCombine] Use intersectForOffsetAdd() in CommonPointerBase (PR #157851)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/157851

Transforms using this helper will add up all the offsets, so we should use intersectForOffsetAdd() instead of plain intersection.

Annoyingly, this requires specially handling the first GEP to avoid losing flags in that case.

Fixes https://github.com/llvm/llvm-project/issues/157714.

>From 2daa3626c1c88aca27b9ad0f9774a5a8f1a89666 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Wed, 10 Sep 2025 14:40:15 +0200
Subject: [PATCH] [InstCombine] Use intersectForOffsetAdd() in
 CommonPointerBase

Transform using this will add up all the offsets, so we should use
intersectForOffsetAdd() instead of plain intersection.

Annoyingly, this requires specially handling the first GEP to
avoid losing flags in that case.

Fixes https://github.com/llvm/llvm-project/issues/157714.
---
 .../Transforms/InstCombine/InstCombineAddSub.cpp | 16 ++++++++++++++--
 llvm/test/Transforms/InstCombine/icmp-gep.ll     |  6 +++---
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index d934638c15e75..f9155cc660317 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2115,6 +2115,7 @@ CommonPointerBase CommonPointerBase::compute(Value *LHS, Value *RHS) {
   }
 
   // Find common base and collect RHS GEPs.
+  bool First = true;
   while (true) {
     if (Ptrs.contains(RHS)) {
       Base.Ptr = RHS;
@@ -2123,7 +2124,12 @@ CommonPointerBase CommonPointerBase::compute(Value *LHS, Value *RHS) {
 
     if (auto *GEP = dyn_cast<GEPOperator>(RHS)) {
       Base.RHSGEPs.push_back(GEP);
-      Base.RHSNW &= GEP->getNoWrapFlags();
+      if (First) {
+        First = false;
+        Base.RHSNW = GEP->getNoWrapFlags();
+      } else {
+        Base.RHSNW = Base.RHSNW.intersectForOffsetAdd(GEP->getNoWrapFlags());
+      }
       RHS = GEP->getPointerOperand();
     } else {
       // No common base.
@@ -2132,13 +2138,19 @@ CommonPointerBase CommonPointerBase::compute(Value *LHS, Value *RHS) {
   }
 
   // Collect LHS GEPs.
+  First = true;
   while (true) {
     if (LHS == Base.Ptr)
       break;
 
     auto *GEP = cast<GEPOperator>(LHS);
     Base.LHSGEPs.push_back(GEP);
-    Base.LHSNW &= GEP->getNoWrapFlags();
+    if (First) {
+      First = false;
+      Base.LHSNW = GEP->getNoWrapFlags();
+    } else {
+      Base.LHSNW = Base.LHSNW.intersectForOffsetAdd(GEP->getNoWrapFlags());
+    }
     LHS = GEP->getPointerOperand();
   }
 
diff --git a/llvm/test/Transforms/InstCombine/icmp-gep.ll b/llvm/test/Transforms/InstCombine/icmp-gep.ll
index 1385dc3f625f1..10d7b6df8eaf3 100644
--- a/llvm/test/Transforms/InstCombine/icmp-gep.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-gep.ll
@@ -838,9 +838,9 @@ define i1 @gep_mugtiple_ugt_not_all_nuw(ptr %base, i64 %idx, i64 %idx2) {
 
 define i1 @gep_mugtiple_ugt_inbounds_nusw(ptr %base, i64 %idx, i64 %idx2) {
 ; CHECK-LABEL: @gep_mugtiple_ugt_inbounds_nusw(
-; CHECK-NEXT:    [[GEP1_IDX1:%.*]] = add i64 [[IDX:%.*]], [[IDX2:%.*]]
-; CHECK-NEXT:    [[TMP1:%.*]] = shl i64 [[GEP1_IDX1]], 2
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i64 [[TMP1]], 0
+; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr inbounds i32, ptr [[BASE:%.*]], i64 [[IDX:%.*]]
+; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr nusw i32, ptr [[GEP1]], i64 [[IDX2:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt ptr [[GEP2]], [[BASE]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %gep1 = getelementptr inbounds i32, ptr %base, i64 %idx