[InstCombine] Restore splat gep support in OptimizePointerDifference() (#143906)

When looking for the common base pointer, support the case where the
type changes because the GEP goes from pointer to vector of pointers.
This was supported prior to #142958.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -2088,8 +2088,6 @@ CommonPointerBase CommonPointerBase::compute(Value *LHS, Value *RHS) {
   // Find common base and collect RHS GEPs.
   while (true) {
     if (Ptrs.contains(RHS)) {
-      if (LHS->getType() != RHS->getType())
-        return Base;
       Base.Ptr = RHS;
       break;
     }
@@ -2132,12 +2130,15 @@ Value *InstCombinerImpl::OptimizePointerDifference(Value *LHS, Value *RHS,
   // TODO: We should probably do this even if there is only one GEP.
   bool RewriteGEPs = !Base.LHSGEPs.empty() && !Base.RHSGEPs.empty();
 
-  Type *IdxTy = DL.getIndexType(Base.Ptr->getType());
+  Type *IdxTy = DL.getIndexType(LHS->getType());
   auto EmitOffsetFromBase = [&](ArrayRef<GEPOperator *> GEPs,
                                 GEPNoWrapFlags NW) -> Value * {
     Value *Sum = nullptr;
     for (GEPOperator *GEP : reverse(GEPs)) {
       Value *Offset = EmitGEPOffset(GEP, RewriteGEPs);
+      if (Offset->getType() != IdxTy)
+        Offset = Builder.CreateVectorSplat(
+            cast<VectorType>(IdxTy)->getElementCount(), Offset);
       if (Sum)
         Sum = Builder.CreateAdd(Sum, Offset, "", NW.hasNoUnsignedWrap(),
                                 NW.isInBounds());

llvm/test/Transforms/InstCombine/sub-gep.ll

@@ -995,3 +995,33 @@ define i64 @multiple_geps_inbounds_nuw(ptr %base, i64 %idx, i64 %idx2) {
   %d = sub i64 %i2, %i1
   ret i64 %d
 }
+
+define <2 x i64> @splat_geps(ptr %base, <2 x i64> %idx1, <2 x i64> %idx2) {
+; CHECK-LABEL: @splat_geps(
+; CHECK-NEXT:    [[D:%.*]] = sub nsw <2 x i64> [[IDX2:%.*]], [[IDX1:%.*]]
+; CHECK-NEXT:    ret <2 x i64> [[D]]
+;
+  %gep1 = getelementptr inbounds i8, ptr %base, <2 x i64> %idx1
+  %gep2 = getelementptr inbounds i8, ptr %base, <2 x i64> %idx2
+  %gep1.int = ptrtoint <2 x ptr> %gep1 to <2 x i64>
+  %gep2.int = ptrtoint <2 x ptr> %gep2 to <2 x i64>
+  %d = sub <2 x i64> %gep2.int, %gep1.int
+  ret <2 x i64> %d
+}
+
+define <2 x i64> @splat_geps_multiple(ptr %base, i64 %idx0, <2 x i64> %idx1, <2 x i64> %idx2) {
+; CHECK-LABEL: @splat_geps_multiple(
+; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x i64> poison, i64 [[IDX0:%.*]], i64 0
+; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <2 x i64> [[DOTSPLATINSERT]], <2 x i64> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP1:%.*]] = add nsw <2 x i64> [[DOTSPLAT]], [[IDX1:%.*]]
+; CHECK-NEXT:    [[D:%.*]] = sub nsw <2 x i64> [[IDX2:%.*]], [[TMP1]]
+; CHECK-NEXT:    ret <2 x i64> [[D]]
+;
+  %gep0 = getelementptr inbounds i8, ptr %base, i64 %idx0
+  %gep1 = getelementptr inbounds i8, ptr %gep0, <2 x i64> %idx1
+  %gep2 = getelementptr inbounds i8, ptr %base, <2 x i64> %idx2
+  %gep1.int = ptrtoint <2 x ptr> %gep1 to <2 x i64>
+  %gep2.int = ptrtoint <2 x ptr> %gep2 to <2 x i64>
+  %d = sub <2 x i64> %gep2.int, %gep1.int
+  ret <2 x i64> %d
+}