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

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Message-ID: <CAHk-=wjJEgjCYzHZFPxTs01p7FMEHKKqXyqwRVBk6KnvHB1qVA@mail.gmail.com>
Date:   Fri, 30 Apr 2021 15:06:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        lkml <linux-kernel@...r.kernel.org>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Heads up: gcc miscompiling initramfs zlib decompression code at -O3

On Fri, Apr 30, 2021 at 1:46 PM Vineet Gupta <Vineet.Gupta1@...opsys.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