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

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Date:   Mon, 30 Sep 2019 15:34:41 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.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 3:08 PM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> 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

Not an error, but a warning at least: https://godbolt.org/z/_V5im1.

That's interesting, so it has multiple semantics, because it's also
documented to inline even when no optimizations are specified.  So
when someone uses __attribute__((always_inline)) without a comment,
it's not clear whether they mean for there to be a warning when this
is not inlined, or for it to be inlined at -O0 (guess not for the
kernel), or both.  If the kernel wants to enforce the former, why not
set `-Werror=attributes`?  Maybe that warning is too broad?  Seems
like a recipe for subtly broken code found at runtime, when we'd
rather have stronger compile time guarantees.

> 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.

Filed https://bugs.llvm.org/show_bug.cgi?id=43517
-- 
Thanks,
~Nick Desaulniers