[ScalarizeMaskedMemIntrin] Use the element type to calculate alignment for gather/scatter when alignment operand is 0.

[craig.topper]

Summary

Previously we used the vector type, but we're loading/storing
invididual elements so I think only element alignment should matter.

Noticed while looking at the code for something else so I don't
have a test case.

[lebedev.ri]

I think this is correct.
I guess for that you need to pass 0 into alignment argument of these intrinsics.

[craig.topper]

In D105220#2850946, @lebedev.ri wrote:

I think this is correct.
I guess for that you need to pass 0 into alignment argument of these intrinsics.

Yeah. And to have an observable change you need a target with a cost model that returns a different answer if you give it element alignment versus vector alignment. But since gather works on elements, I would expect that any cost model that cares about alignment is checking whether it is element aligned or not.

[efriedma]

Instead of messing with this, can we just change the verifier to require that the alignment argument to masked_gather is nonzero? (And teach autoupgrade to fix old code.) This is just a lesser version of the same mess we used to have with other load/store operations.

If you're going to fix it this way, though, please also fix SelectionDAGBuilder.

[craig.topper]

In D105220#2851174, @efriedma wrote:

Instead of messing with this, can we just change the verifier to require that the alignment argument to masked_gather is nonzero? (And teach autoupgrade to fix old code.) This is just a lesser version of the same mess we used to have with other load/store operations.

If you're going to fix it this way, though, please also fix SelectionDAGBuilder.

I stared at the autoupgrade code for intrinsics. I can't find any cases of autoupgrading an intrinsic by changing the value of an argument. It looks like the interface with the LLParser and BitcodeReader don't quite work for this. I think we need to return true with NewFn = nullptr from UpgradeIntrinsicFunction. That will enable UpgradeCallsToIntrinsic to call UpgradeIntrinsicCall which will enable LLParser. BitCodeReader calls UpgradeIntrinsicCall directly. Then in UpgradeIntrinsicCall we would need to modify the argument and generate a new call with the correct arguments. But I think UpgradeCallsToIntrinsic or the BitCodeReader will then try to delete the intrinsic declaration after all calls are upgraded because they don't expect AutoUpgrade to replace a call with a call to the same intrinsic. They think the declaration should be dead after upgrading all the calls.

[craig.topper]

Update SelectionDAGBuilder as well.

[efriedma]

LGTM

llvm/lib/Transforms/Scalar/ScalarizeMaskedMemIntrin.cpp

 //===- ScalarizeMaskedMemIntrin.cpp - Scalarize unsupported masked mem ----===//
 //                                    instrinsics
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
case Intrinsic::masked_store:
               cast<ConstantInt>(CI->getArgOperand(2))->getAlignValue()))
         return false;
       scalarizeMaskedStore(DL, CI, DTU, ModifiedDT);
       return true;
     case Intrinsic::masked_gather: {
       unsigned AlignmentInt =
           cast<ConstantInt>(CI->getArgOperand(1))->getZExtValue();
       Type *LoadTy = CI->getType();
-      Align Alignment =
-          DL.getValueOrABITypeAlignment(MaybeAlign(AlignmentInt), LoadTy);
+      Align Alignment = DL.getValueOrABITypeAlignment(MaybeAlign(AlignmentInt),
+                                                      LoadTy->getScalarType());
       if (TTI.isLegalMaskedGather(LoadTy, Alignment))
         return false;
       scalarizeMaskedGather(DL, CI, DTU, ModifiedDT);
       return true;
     }
     case Intrinsic::masked_scatter: {
       unsigned AlignmentInt =
           cast<ConstantInt>(CI->getArgOperand(2))->getZExtValue();
       Type *StoreTy = CI->getArgOperand(0)->getType();
-      Align Alignment =
-          DL.getValueOrABITypeAlignment(MaybeAlign(AlignmentInt), StoreTy);
+      Align Alignment = DL.getValueOrABITypeAlignment(MaybeAlign(AlignmentInt),
+                                                      StoreTy->getScalarType());
       if (TTI.isLegalMaskedScatter(StoreTy, Alignment))
         return false;
       scalarizeMaskedScatter(DL, CI, DTU, ModifiedDT);
       return true;
     }
     case Intrinsic::masked_expandload:
       if (TTI.isLegalMaskedExpandLoad(CI->getType()))
         return false;