[PATCH] D57066: [TargetLowering] try harder to determine undef elements of vector binops

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: RKSimon, craig.topper, andreadb, efriedma.
Herald added a subscriber: mcrosier.

This might be the start of tracking all vector element constants generally if we take it to its logical conclusion, but I thought I'd better stop here and see if there's support for the general direction.

The affected tests require a convoluted path before they get simplified currently because we don't call SimplifyDemandedVectorElts() from binops directly and don't modify the binop operands directly in SimplifyDemandedVectorElts().

That's why the tests all have a trailing shuffle to induce a chain reaction of transforms. So something like this is happening:

1. Improve the knowledge of undefs in the binop via a SimplifyDemandedVectorElts() call that originates from a shuffle.
2. Transfer that undef knowledge back to the shuffle mask user as more undef lanes.
3. Combine the modified shuffle by calling SimplifyDemandedVectorElts() again.
4. Translate the improved shuffle mask as undemanded lanes of build vector constants causing those to become full undef constants.
5. Simplify the binop now that it has a full undef operand.

As we can see from the unchanged 'and' and 'or' tests, tracking undefs alone isn't a full solution. We would need to track zero and all-ones constants to improve those opcodes. We'd probably need to track NaN for FP ops too (assuming we don't have fast-math-flags set).


https://reviews.llvm.org/D57066

Files:
  lib/CodeGen/SelectionDAG/TargetLowering.cpp
  test/CodeGen/X86/vector-partial-undef.ll

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