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

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Date:   Thu, 3 Oct 2019 22:21:11 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Will Deacon <will@...nel.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>, Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

On Thu, Oct 3, 2019 at 7:29 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Oct 3, 2019 at 10:24 AM Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> >
> > I just want to annotate __always_inline for the case
> > "2. code that if not inlined is somehow not correct."
>
> Oh, I support that entirely - if only for documentation.
>
> But I do *not* support the dismissal of the architecture maintainers
> concerns about "does it work?" and apparently known compiler bugs.
>
> > Again, not saying "use a macro".
>
> Other people did, though.
>
> And there seemed to be little balancing of the pain vs the gain. The
> gain really isn't that obvious. If the code shrinks by a couple of kB,
> is that good or bad? Maybe it is smaller, but is it _better_?

I think both positions that people have shown are important to take
into account.

We should minimize our usage of macros wherever possible and certainly
not write new ones when another solution is available. But we should
*also* minimize our dependence on code that "must-be-inlined" to work
as much as possible.

In particular, I think we should allow to use __always_inline only if
it doesn't work otherwise, as an alternative before trying the next
worst solution (macros). And avoid using only "inline" when we
actually require inlining, of course.

And the reasoning for each usage of __always_inline should have a
comment (be it "bad codegen", "performance tanks without it",
"compiler X <= 4.2 refuses to compile"...). Which is also useful for
compiler folks to grep for cases to improve/fix in their compiler!

Cheers,
Miguel