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

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Message-ID: <CANiq72kbZVB4vdyQonMQzuRHdh=BnD6F=sv5NQsFey5_xAB-Zg@mail.gmail.com>
Date:   Tue, 1 Oct 2019 00:08:07 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     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>,
        Russell King <rmk+kernel@....linux.org.uk>,
        Stefan Wahren <wahrenst@....net>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

On Mon, Sep 30, 2019 at 11:50 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> So __attribute__((always_inline)) doesn't guarantee that code will be
> inlined.  [...] inline and __attribute__((always_inline))
> are a heuristic laden mess and should not be relied upon.

Small note: in GCC, __attribute__((always_inline)) is documented as
actually guaranteeing to either inline or error otherwise (although
see the remark for indirect calls):

    "Failure to inline such a function is diagnosed as an error. Note
that if such a function is called indirectly the compiler may or may
not inline it depending on optimization level and a failure to inline
an indirect call may or may not be diagnosed."

As for LLVM/Clang, no idea, since it does not say anything about it in
the docs -- but from what you say, it is a weaker guarantee.

Cheers,
Miguel