linux-kernel - Re: [PATCH 0/2] Kconfig: -O3 enablement

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Date:   Fri, 24 Jun 2022 11:29:49 -0700
From:   Linus Torvalds <linus@...ux-foundation.org>
To:     arnd@...db.de
Cc:     bigeasy@...utronix.de, chris@...isdown.name,
        dmitry.torokhov@...il.com, gustavoars@...nel.org,
        john.ogness@...utronix.de, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, masahiroy@...nel.org,
        mikoxyzzz@...il.com, nathan@...nel.org, ndesaulniers@...gle.com,
        oleksandr@...hat.com, quic_eberman@...cinc.com, seanjc@...gle.com,
        tglx@...utronix.de, vbabka@...e.cz, x86@...nel.org
Subject: Re: [PATCH 0/2] Kconfig: -O3 enablement

On Thu, Jun 23 at 7:27:29 PM  Arnd Bergmann <arnd@...db.de> wrote:
>
> This probably also needs a dependency on !COMPILE_TEST so we don't
> report compile-time problems that are specific to -O3.

Honestly, let's just remove -O3 entirely.

Enabling it, and then not even build-testing the result, is just about
the *worst* possible case. That's just horrible.

The argument that "but ARC uses it" is not an argument. It was always
a bad argument, and ARC needs to just fix whatever it is that made it
an issue (likely already fixed with a compiler upgrade).

And there is no way I would ever accept this as a "let people try it" when

 - as mentioned, just use KCFLAGS=-O3 if you want to

 - -O3 has a *loong* history of generating worse code than -O2

so I will *not* be taking these kinds of patches without some very
serious explanations of why -O3 has suddenly become acceptable again.

Those explanations had better be more than "let people try". They
should have in-depth actual performance numbers for a real load, not
some made-up "bigger is better" logic.

                 Linus