[AArch64] Don't use tbl lowering if ZExt can be folded into user.

[fhahn]

Summary

If the ZExt can be lowered to a single ZExt to the next power-of-2 and
the remaining ZExt folded into the user, don't use tbl lowering.

Fixes #62620.

[efriedma]

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

1832

Does this code do the right thing when the "Args" includes a cast with the wrong source type? (In this case, the source type is i16, but the type of the operand of the cast would actually be i8.)

[fhahn]

Generalize code to properly handling mull case with casts with different source types, thanks!

[fhahn]

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

1832

I don't think so, thanks! I updated the code to pass both source types in that case, not sure if there's a better way of handling that.

[fhahn]

ping :)

[efriedma]

LGTM

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

14461

Maybe just explicitly check "DstWidth != 16 && DstWidth != 32"?

[fhahn]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

14461

The current codegen also works for like i24 and there are existing tests for that; I left the check as is for now to avoid changing behavior unrelated to the patch

[v01dXYZ]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

14653

Sorry the question is dumb but why does it use TruncType instead of DoubleSrcType := SrcType with Element type width multiplied by 2 ?

14660

This assignment is not necessary.

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

1883

There is a typo (I think).

[v01dXYZ]

@fhahn ping

[v01dXYZ]

ping

[fhahn]

Thanks for taking a look. Comments should be addressed in baebe719a52acbe0a5aaa04dcf1abcfd4035bf1f and response inline.

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

14653

TBL lowering is not profitable if only one zext step is saved. If the zext to the next smaller type is free (i.e. it can be folded into the user), then only 1 step will be needed and no tbl should be generated

14660

Thanks, removed!

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

1883

Yes, this should check I's source type, should be fixed.