Why vectorization didn't turn on by -O2
Sun May 9 13:44:37 GMT 2021
On Sun, May 09, 2021 at 12:54:08AM +0800, Xi Ruoyao via Gcc-help wrote:
> On Sat, 2021-05-08 at 20:07 +0800, email@example.com wrote:
> > Recently I noticed that gcc -O2 didn't turn on vectorization
> > optimization，
> > which it turns on in clang -O2.
> > Does GCC think it involves the trade-off of space speed, or other
> > considerations?
-O2 is for optimisations that (almost) never degrade code quality. -O3
is for those that are only beneficial "on average".
> It's just a decision I think. The "original" reason may be that
> vectorization can make code *slower* on some cases.
Yup. Vectorisation always causes hugely different code.
> There was some discussion about enabling -ftree-loop-vectorization at -
> O2 for x86, but that was too late (for GCC 9):
AFAIK the current plan is to enable vectorisation at -O2 with a more
conservative cost model. This will be a generic change, for all
architectures, and hopefully will arrive in GCC 12.
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