Why vectorization didn't turn on by -O2

[Segher Boessenkool]

On Mon, May 10, 2021 at 10:21:04AM +0200, Richard Biener wrote:
> 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@hdu.edu.cn 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.

Yeah, sorry, I glossed over those details, but they do matter.

My point is that -O2 opts "almost never" should degrade quality (code
speed, code size, compilation speed, and tradeoffs in that all --
nothing is a hard and fast rule here).

> Vectorization with the -O2 default
> cost model of "cheap" tends to mostly increase the first and the second
> whilst only moderately affecting the last.

One promising plan is to use very-cheap instead.  It should increase code
size even less (but see what Honza said).

> It's all of course hugely dependent on the source base you are
> working with.

Yeah.


Segher