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

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Date:   Tue, 1 Oct 2019 14:14:46 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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 2:06 PM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> 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.

I assert that's just another case of 2, and should be investigated. (I
think I remember that from when I had to teach LLVM how to inline asm
goto; since the compiler can reject inlining if it doesn't know how to
do such a transform).

>
> 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).

Cool, so one bug in arm32, one bug in arm64, one bug in x86_64.
Doesn't sound like too much work to fix.
-- 
Thanks,
~Nick Desaulniers