linux-kernel - Re: [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin

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Message-ID: <CAMZ6RqJ_zU0DL1hiXE_xkOmHQFTxHPPt=c_bPUzGXWrjVJEKJg@mail.gmail.com>
Date:   Thu, 12 May 2022 10:18:40 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, David Howells <dhowells@...hat.com>,
        Jan Beulich <JBeulich@...e.com>
Subject: Re: [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate
 constant expressions

On Thu. 12 May 2022 at 09:28, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> On Wed, May 11, 2022 at 5:04 PM Vincent Mailhol
> <mailhol.vincent@...adoo.fr> wrote:
> >
> > And finally, Nick Desaulniers pointed out in [2] that this also fixes
> > a constant propagation missed-optimization in clang.
> >
> > [2] https://lore.kernel.org/all/CAKwvOdnH_gYv4qRN9pKY7jNTQK95xNeH1w1KZJJmvCkh8xJLBg@mail.gmail.com/
>
> Regarding
> https://github.com/llvm/llvm-project/issues/55394
> it seems that functions with static linkage cannot be considered
> library functions, so libcall optimization will not run on calls to
> them. So the compiler might be able to do a better job for constants
> if ffs() and friends indeed were not defined in a header as static
> inline.  But that relies on the compiler knowing these tricks; I think
> the kernel's approach is just fine (better in fact, because we should
> inline these tiny functions, regardless of LTO), but like this series
> shows, there may be room for improvement for other functions within
> the kernel that are defined as static inline in headers that are
> normally found in a libc.
>
> So I no longer think there's a missed optimization here, but at this
> point, it's not worth a respin of the series IMO to just let sleeping
> dogs lie.
>
> Unless the x86 maintainers wouldn't mind dropping that line and link
> when applying?

Let me send the v4, this will save the x86 maintainers some manual
editing effort (add will add your "Review-by" tag in patch 2 while
doing so).