Heads up: gcc miscompiling initramfs zlib decompression code at -O3

[Linus Torvalds]

On Fri, Apr 30, 2021 at 1:46 PM Vineet Gupta <Vineet.Gupta1 at synopsys.com> 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