linux-kernel - Re: [PATCH] compiler: enable CONFIG_OPTIMIZE

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Date:   Tue, 1 Oct 2019 23:06:18 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Stefan Wahren <wahrenst@....net>,
        Kees Cook <keescook@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

On Tue, Oct 1, 2019 at 10:53 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> 1. is clearly the most common case, but there is also
>
> 4. Some compiler version (possibly long gone, possibly still current)
> makes bad inlining decisions that result in horrible but functionally
> correct object code for a particular function, and forcing a function to
> be inlined results in what we had expected the compiler to do already.

There is also 5. code that does not even compile without it, e.g.
_static_cpu_has() in x86_64 which requires
__attribute__((always_inline)), at least on GCC 9.2.

For x64_64 it is the only one case I found, though. If you disable
__always_inline everything else compiles and links (in a defconfig).

Cheers,
Miguel