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, 172060045@xxxxxxxxxx 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. Segher
