Sanjay Patel 4a54e3eed3 [x86] try to replace 0.0 in fcmp with negated operand ...

This inverts a fold recently added to IR with:
3491f2f4b0

We can put -bidirectional on the Alive2 examples to show that
the reverse transforms work:
https://alive2.llvm.org/ce/z/8iVQwB

The motivation for the IR change was to improve matching to
'fabs' in IR (see https://github.com/llvm/llvm-project/issues/38828 ),
but it regressed x86 codegen for 'not-quite-fabs' patterns like
(X > -X) ? X : -X.
Ie, when there is no fast-math (nsz), the cmp+select is not a proper
fabs operation, but it does map nicely to the unusual NAN semantics
of MINSS/MAXSS.

I drafted this as a target-independent fold, but it doesn't appear to
help any other targets and seems to cause regressions for SystemZ at
least.

Differential Revision: https://reviews.llvm.org/D122726

2022-03-31 09:17:49 -04:00

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