linux-kernel - Re: [PATCH] linux/compiler-clang.h: define HAVE_BUILTIN

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Message-ID: <CANiq72=5t1maOv1FerMLRVCRos_yv-iQGLzB21Z+aYxTXLy8gA@mail.gmail.com>
Date:   Fri, 26 Feb 2021 04:58:38 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Miguel Ojeda <ojeda@...nel.org>,
        Marco Elver <elver@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Randy Dunlap <rdunlap@...radead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] linux/compiler-clang.h: define HAVE_BUILTIN_BSWAP*

On Thu, Feb 25, 2021 at 9:18 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Indeed. Any wins that we can get with compile time, we should take.
> Clang is being widely used in production systems now so I feel like with
> a trivial change plus user visible impact, it should be backported.
>
> Not to mention that the generated code in theory should be better
> because it is the compiler's builtin, rather than a hand rolled one, AND
> this is technically a regression, given that it worked before compiler.h
> was split.

Compilation speed shouldn't be an argument for a stable change unless
it is something egregious like a 50% that may affect users or tightly
timed CIs.

Fixing an important runtime regression is a stronger argument, but the
patch doesn't show what the effects are, so it isn't justified (yet).

Please note that this kind of change touches a lot of code all over
the place, which could always trigger other runtime regressions or
even bad codegen (yes, very unlikely, but it happens).

Cheers,
Miguel