Don't fold (select C, (gep Ptr, Idx), Ptr) if C is vector but Idx is scalar

[kongyi]

Summary

The folding rule (select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C, Idx, 0)) creates a malformed select if C is a vector while Idx is scalar.

SELECT VecC, ScalarIdx, 0

We could splat Idx to a vector but it defeats the purpose of optimisation. Don't apply the folding rule in this case.

This fixes a regression from commit d561b6fbdbe6d1da05fd92003a4ac1e37bf4b8bc.

[RKSimon]

LGTM with one minor

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

260

newline

[kongyi]

Landed d0746f2e9bbf08f52196ae12f25d0ef7edcbbe4c (forgot to add the Phabricator link)

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   // Fold (select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C, Idx, 0))
   // Fold (select C, Ptr, (gep Ptr, Idx)) -> (gep Ptr, (select C, 0, Idx))
   auto SelectGepWithBase = [&](GetElementPtrInst *Gep, Value *Base,
                                bool Swap) -> GetElementPtrInst * {
     Value *Ptr = Gep->getPointerOperand();
     if (Gep->getNumOperands() != 2 || Gep->getPointerOperand() != Base ||
         !Gep->hasOneUse())
       return nullptr;
-    Type *ElementType = Gep->getResultElementType();
     Value *Idx = Gep->getOperand(1);
+    if (isa<VectorType>(CondVal->getType()) && !isa<VectorType>(Idx->getType()))
+      return nullptr;
+    Type *ElementType = Gep->getResultElementType();
     Value *NewT = Idx;
     Value *NewF = Constant::getNullValue(Idx->getType());
     if (Swap)
       std::swap(NewT, NewF);
     Value *NewSI =
         Builder.CreateSelect(CondVal, NewT, NewF, SI.getName() + ".idx", &SI);
     return GetElementPtrInst::Create(ElementType, Ptr, {NewSI});
   };

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

;
   %gep1 = getelementptr inbounds i32, i32* %p, i64 %x
   %gep2 = getelementptr inbounds i32, i32* %gep1, i64 %y
   %icmp = icmp ugt i64 %x, %y
   %sel = select i1 %icmp, i32* %gep2, i32* %gep1
   call void @use_i32p(i32* %gep1)
   ret i32* %sel
 }
 declare void @use_i32p(i32*)
+
+; We cannot create a select-with-idx with a vector condition but scalar idx.
+
+define <2 x i64*> @test7(<2 x i64*> %p1, i64 %idx, <2 x i1> %cc) {
+; CHECK-LABEL: @test7(
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i64, <2 x i64*> [[P1:%.*]], i64 [[IDX]]
+; CHECK-NEXT:    [[SELECT:%.*]] = select <2 x i1> [[CC:%.*]], <2 x i64*> [[P1:%.*]], <2 x i64*> [[GEP]]
+; CHECK-NEXT:    ret <2 x i64*> [[SELECT]]
+;
+  %gep = getelementptr i64, <2 x i64*> %p1, i64 %idx
+  %select = select <2 x i1> %cc, <2 x i64*> %p1, <2 x i64*> %gep
+  ret <2 x i64*> %select
+}
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