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Re: [PATCH] x86-64: Compile branred.c with -mprefer-vector-width=128 [BZ #24603]
* H. J. Lu:
> On Tue, Jun 11, 2019 at 9:07 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Tue, Jun 11, 2019 at 9:02 AM Florian Weimer <fweimer@redhat.com> wrote:
>> >
>> > * H. J. Lu:
>> >
>> > > Fixed. I also added check for -mprefer-vector-width=128 which is new in GCC 8.
>> > > GCC 7 doesn't have this problem since it doesn't vectorize the second loop.
>> > >
>> > > Here is the updated patch.
>> > >
>> > > OK for master?
>> >
>> > I still don't understand why we need a workaround for a GCC performance
>> > bug in glibc. Is there any precedent for this?
>> >
>>
>> I don't know if we ever investigated performance impacts of different GCC
>> optimizations on glibc.
>>
>
> I found:
>
> sysdeps/powerpc/powerpc64/power4/Makefile:CFLAGS-memmove.c += --param
> max-variable-expansions-in-unroller=2 --param max-unroll-times=2
> -funroll-loops -fpeel-loops
That's been there since before 2012. This reflects my main concern
about such tweaks: That they will never be reviewed and removed when
they are no longer necessary (and probably have negative impact).
Thanks,
Florian