[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 #157714.

Author: nikic

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();
   }
 

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