compiled programs significantly slower after upgrading

Jack Stalnaker jack.stalnaker@gmail.com
Tue Apr 24 13:00:00 GMT 2018


I appreciate the information. I did try with O2 and the results were more
or less the same, unfortunately. Still about 5x slower than 4.8.2.

On Tue, Apr 24, 2018, 7:51 AM Mason <slash.tmp@free.fr> wrote:

> On 18/04/2018 23:55, Jack Stalnaker wrote:
>
> > Yeah, I realize it's a difficult question with no example. It would be
> > quite difficult to chop out a chunk of the code showing the 5x slowdown,
> > though, I think. I was probing for something big or obvious I may have
> > missed, or some known issue with the later versions, since this is so
> > dramatic, and since the only thing that has changed is the compiler. The
> > compilation options are minimal. I'm using -g -Og, and that's it. I have
> > tried a release build with some finer tuning, but that doesn't show much
> > improvement.
>
> A 5x slowdown sounds like SIMD-vs-integer code, for embarrassingly-parallel
> source (auto-vectorized vs scalar).
>
> Although, I think GCC is just now getting some auto-vectorization caps,
> and it is almost certain it did not in 4.8 o_O
>
> As Marc points out, you should test with -O2 instead of -Og
>
> Regards.
>



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