[PATCH] D25291: [AArch64] Optionally use the reciprocal estimation machinery

[James Molloy via llvm-commits]

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Hi,

Yes, I've said multiple times that I'm opposed to enabling this by feature and I stand by that. If someone can show a good reason for it, fair enough but I haven't seen a good reasoning (for AArch64/ARM) so far.

If you want to just enable reciprocal selection and test it, then a cl::opt flag seems most appropriate because that's how we enable experimental stuff broad-brush for testing. A CPU feature really isn't right as it ignores the important context that should go into deciding whether to use these instructions (on ARM/AArch64).

Alternatively there may exist a target with such a slow SQRT unit that RSQRTE/RSQRTS is always better regardless of context, but I haven't seen any evidence for that either.

Cheers,

James


Repository:
  rL LLVM

https://reviews.llvm.org/D25291