On Fri, Apr 30, 2021 at 1:46 PM Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx> wrote:
>
> I've hit a mainline gcc 10.2 (also gcc 9.3) bug which triggers at -O3
> causing wrong codegen.
I'd be more than happy to just disable CC_OPTIMIZE_FOR_PERFORMANCE_O3 entirely.
The advantages are very questionable - with a lot of the optimizations
at O3 being about loops, something which the kernel to a close
approximation doesn't have.
Most kernel loops are "count on one hand" iterations, and loop
optimizations generally just make things worse.
And we've had problems with -O3 before, because not only are the
optimizations a bit esoteric, they are often relatively untested. If
you look around at various projects (outside the kernel), -O2 is
generally the "default".
And that's entirely ignoring the gcc history - where -O3 has often
been very buggy indeed. It's gotten much better, but I just don't see
the upside of using -O3.
In fact, it looks like we already have that
depends on ARC
for -O3, exactly because nobody really wants to use this.
So this bug seems to be entirely ARC-specific, in that only ARC can
use -O3 for the kernel already.
Linus
