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Re: [PATCH,i386] fma,fma4 and xop flags


On 08/13/2012 12:33 PM, Uros Bizjak wrote:
> AFAIU fma3 is better than fma4 for bdver2 (the only CPU that
> implements both FMA sets). Current description of bdver2 doesn't even
> enable fma4 in processor_alias_table due to this fact.
> 
> The change you are referring to adds preference for fma3 insn set for
> generic code (not FMA4 builtins!), even when fma4 is enabled. So, no
> matter which combination and sequence of -mfmfa -mfma4 or -mxop user
> passes to the compiler, only fma3 instructions will be generated.

This rationale needs to appear as a comment above

> +      (eq_attr "isa" "fma4")
> +        (symbol_ref "TARGET_FMA4 && !TARGET_FMA")

Longer term we may well require some sort of

  (TARGET_FMA4 && !(TARGET_FMA && TARGET_PREFER_FMA3))

with an appropriate entry in ix86_tune_features to match.


r~


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