[PATCH] PR target/38201: -mfma/-mavx and -msse5/-msse4a don't work together

H.J. Lu hjl.tools@gmail.com
Mon Dec 15 18:10:00 GMT 2008


On Mon, Dec 15, 2008 at 7:23 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Mon, Dec 15, 2008 at 4:05 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>
>> Also, we have to deal with Intel FMA and SSE5. Since there is no silicon,
>> we can't use -mtune= to guide us. That leaves to -msse5 and -mfma.
>> If we allow this combination, we have to pick one arbitrarily.
>
> I think that the problematic insns are only various FMA instructions
> that fight for generic (a * b) + c expressions. We currently have
> -mno-fused-madd that disables generic AMD FMA while still leaves SSE5
> intrinsics version of FMA enabled. There is also -mfma that will
> enable Intel intrinsics and generic FMA.
>
> We can solve this by saying that -mfma implies -mno-fused-madd. This
> still leaves various intrinsic FMA implementations (Intel and AMD)
> enabled, while defaulting to Intel version of generic FMA when -mfma
> is in effect. This can further be refined with various -mtune options
> in future if some target processor prefers AMD generic FMA
> implementation over Intel one.
>
> This will also cover future processors that will eventually implement
> SSE5 and FMA.
>
> Does this looks OK?
>

There are

mfused-madd
Target Report InverseMask(NO_FUSED_MADD, FUSED_MADD) Save
Enable automatic generation of fused floating point multiply-add instructions
if the ISA supports such instructions.  The -mfused-madd option is on by
default.

Now, its comment is no long true for Intel FMA and -mfma will turn it off
by default.

What should happen with permutation of -mfused-madd/-mfma/-msse5?

We are making gcc more complicated for no good reasons.


-- 
H.J.



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