> On Sun, 9 May 2021, Segher Boessenkool wrote: > > > 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". > > In particular -O2 is a balance of compile-time, generated code size > and resulting performance. Vectorization with the -O2 default > cost model of "cheap" tends to mostly increase the first and the second > whilst only moderately affecting the last. Last time I ran -O2 with cheap model enabled, the code size was actually decreasing at average for SPEC which was a bit of a surprise. I plan to re-do the benchmarks soon now gcc11 branched. Honza > > It's all of course hugely dependent on the source base you are > working with. > > Richard. > > > > 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
