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

[Uros Bizjak <ubizjak at gmail dot com>]

On Mon, Aug 13, 2012 at 9:50 PM, Richard Henderson <rth@redhat.com> wrote:
> 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")

I plan to commit following patch:

--cut here--
Index: i386.md
===================================================================
--- i386.md     (revision 190362)
+++ i386.md     (working copy)
@@ -659,6 +659,9 @@
         (eq_attr "isa" "noavx2") (symbol_ref "!TARGET_AVX2")
         (eq_attr "isa" "bmi2") (symbol_ref "TARGET_BMI2")
         (eq_attr "isa" "fma") (symbol_ref "TARGET_FMA")
+        ;; Disable generation of FMA4 instructions for generic code
+        ;; since FMA3 is preferred for targets that implement both
+        ;; instruction sets.
         (eq_attr "isa" "fma4")
           (symbol_ref "TARGET_FMA4 && !TARGET_FMA")
        ]
--cut here--

> 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.

This won't work, since we have to prefer FMA3 also in case when only
"-mfma -mfma4" without -mtune=XX is used. We can add TARGET_FMA_BOTH
though, but I doubt there will ever be target that implements both
insn sets without preferences.

Uros.