[TableGen] Fix computeRegUnitLaneMasks for ad hoc aliasing #79835
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With ad hoc aliasing (using the Aliases field in register definitions) we can have subregisters with disjoint lane masks that nevertheless share a regunit. In this case we need to take the union of their lane masks, not the intersection. This was inadvertently broken by https://reviews.llvm.org/D157864
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Hi @jayfoad , thank you for this PR. I prepared a test for this PR like you asked me in #79642. What is the best way for you to merge the test I made? 1) merge this PR as usual, and I submit another PR for a test? 2) cherry-pick my test to this PR? Either is fine. Please let me know when you have time. Thanks! The test is available at https://github.com/kaz7/llvm-project/commit/b8d3ad4f9ece967ae0512b974893af95575d1af9. |
This test is based on VE implementation. VE has two subregisters. Both are disjointed. However, many instructions contaminate other register very often. Therefore, VE defines them as aliased to avoid register allocator to allocate both registers simultaneously. This trick conflicts wht recent computeRegUnitLaneMasks optimization. The optimization is reverted now. And I'm adding test case for that.
I cherry-picked your test, thanks. But I am no longer sure this is the best approach for fixing the problem. How about #79831 as an alternative? |
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Hi @jayfoad , Is it possible to ask review others about this PR in order to merge this? As I said at #79831 (comment), I think #79831 is not enough to cover the problem. Recently the modification on live range (#88892) broke VE backend. I don't inspect the patch deeply, but the patch itself seems reasonable. And, if I apply this patch, it solves the problem. I think that it just comes naturally to treat a lanemask == None as empty. So, I'd like to fix the problem from the source. |
Sure, you can ask opinion from others, but personally I am not convinced that this is the best way to fix it.
Do you have a test case for the new breakage? I'd like to take a look. |
With ad hoc aliasing (using the Aliases field in register definitions) we can have subregisters with disjoint lane masks that nevertheless share a regunit. In this case we need to take the union of their lane masks, not the intersection.
This was inadvertently broken by https://reviews.llvm.org/D157864