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[GlobalISel] Rework more/fewer elements for vectors
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Authored by Petar.Avramovic on Nov 18 2021, 3:26 PM.

Details

Summary

Artifact combiner is not able to access individual elements after using
LCMTy style merge/unmerge, extract and insert to change vector number of
elements (pad with undef or split to sub-vector instructions).
Use unmerge to individual elements instead and then merge elements into
requested types.
Change argument lowering for vectors and moreElementsVector to use
buildPadVectorWithUndefElements and buildDeleteTrailingVectorElements.
FewerElementsVector had a few helpers that had different behaviour,
introduce new helper for most of the opcodes.
FewerElementsVector helper is more flexible since it can create leftover
instruction smaller then requested type (useful in case target wants to
avoid pad with undef and use fewer registers). If target does not want
leftover of different type it should call more elements first.
Some helpers were performing more elements first to have split without
leftover. Opcodes that used this helper use clampMaxNumElementsStrict
(does more elements first) in LegalizerInfo to avoid test changes.
Fixes failures caused by failing to combine artifacts created during
more/fewer elements vector.

Diff Detail

Event Timeline

Petar.Avramovic requested review of this revision.Nov 18 2021, 3:26 PM
Herald added a project: Restricted Project. · View Herald TranscriptNov 18 2021, 3:26 PM

Rebase and ping.

Rebase and ping.

Thanks, this mostly looks less ugly than before

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
995

Why this change? I thought this previously handled arbitrary cases, and this doesn't?

3634

I think the prevailing style is to use static functions instead of anonymous namespaces

3647

Not sure what the point of this check is

3825

Might as well hardcode G_PHI

6600

Use the default small size?

6870

I think it's more appropriate to lower the vector operation in terms of G_EXTRACT_VECTOR_ELT and not direct to the register extracts, G_SHUFFLE_VECTOR is not a legalization artifact

llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
231

4 seems too small, use the default or 8?

253

Ditto

llvm/lib/CodeGen/GlobalISel/Utils.cpp
938

I'm not sure what to do about scalable vectors, but no legalization has been added for those as far as I know

llvm/lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
2248–2251

Can you split this into a separate patch? I know I have one that does the same thing somewhere

2282

Can you also split this, as it's not dependent on the larger legalizer changes or new builder functions

2284

2 seems too small, most vectors are probably 4 or 8

4696–4712

This is a lot longer, is there a simpler way?

foad added inline comments.Dec 21 2021, 6:41 AM
llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
3665

is_contained(NonVecOpIndices, OpIdx)

6690

This is the same condition as two lines above.

llvm/lib/CodeGen/GlobalISel/Utils.cpp
938

Can you use LLT::scalarOrVector here, or OrigTy.changeElementCount?

Rebase and address comments.

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
995

reduceOperationWidth did number of elements splits and narrow scalar for freeze, here I extracted narrow scalar only.

3647

This is for load/store with vector pointer address. the whole function is annoying check that we don't try to split operands of non-intended Opcode.

6870

Sure, we do unmerge via custom legalize of G_EXTRACT_VECTOR_ELT

llvm/lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
4696–4712

I don't see it. Whole function is complicated, handles many cases, and packs all output operands into ResultRegs. And we no longer pad with undef in all cases.

arsenm added inline comments.Dec 21 2021, 9:14 AM
llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
3694

Convert if chain to else if and use llvm_unreachable instead of having to keep this list consistent

3752

Typo witout

llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
231

Same applies elsewhere in patch

Address comments, fix more spelling mistakes.

arsenm accepted this revision.Dec 22 2021, 1:51 PM
This revision is now accepted and ready to land.Dec 22 2021, 1:51 PM
This revision was landed with ongoing or failed builds.Dec 23 2021, 5:30 AM
This revision was automatically updated to reflect the committed changes.
mehdi_amini added inline comments.
llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
3658

This line does not seem right? (flagged by coverity by the way)

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
3658

Thanks, that line was pasted there by mistake.