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Re: MMX instruction and gcc
"Andrew Pinski" <pinskia@gmail.com> дÈëÏûÏ¢ÐÂÎÅ:de8d50360703150859s246a6ff6i20a1fb6c191d97cc@mail.gmail.com...
> On 15 Mar 2007 08:12:21 -0700, Ian Lance Taylor <iant@google.com> wrote:
>> Galloth <lordgalloth@gmail.com> writes:
>>
>> > I want ask, if there is posibilities that gcc will be automaticaly
>> > generates MMX instruction? I tried -mtune, -march -mmmx. But gcc stil
>> > generates code without MMX instruction. On net I found only
>> > information about inline assembler function or citation from man page.
>> > I tried to compile program for adding two arrays of 24 chars.
>>
>> This question is appropriate for the mailing list
>> gcc-help@gcc.gnu.org, not for gcc@gcc.gnu.org. Please take any
>> follow-ups to gcc-help. Thanks.
>
> Actually this one I think belongs still here because ...
>
>> In some cases gcc may be able to automatically generate MMX
>> instructions if you use the -ftree-vectorize option.
>
> This is not true right now. GCC is not able to automatically use the
> MMX instructions at all, this is already filed as PR 22076 and others.
> There was a patch posted as an RFC to fix this but it caused
> regression on x86_64 and the person who submitted has not updated it
> yet for the mainline.
I remembered that Uros said that the biggest problem is to determine which
mode (x87 or MMX) is active; MMX/3DNow! autovectorization isn't so useful
when SSE2/SSE is available so maybe we can turn it off with x86_64?
--
Zuxy