[PATCH] D104247: [DAGCombine] reassoc flag shouldn't enable contract

Fri Jun 18 07:53:20 PDT 2021

spatel added a comment.

In code that I've looked at (mostly C compiled with -ffast-math), we always have `contract` when we have `reassoc`, so I don't see much practical difference.
Can you explain more how we could benefit from this change - in the example in the description, we would have 4 instructions rather than 3 if we use FMA - is that better?
We should duplicate (instead of edit) at least a few of the tests, so we can verify that the existing tests do not form FMA now. That could also be used to show an advantage from not forming FMA.

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Comment at: llvm/test/CodeGen/PowerPC/fmf-propagation.ll:107

 ; The fadd is now fully 'fast'. This implies that contraction is allowed.

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This comment was not valid even before this patch.

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https://reviews.llvm.org/D104247