linux-kernel - Re: [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions

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Date:   Thu, 1 Sep 2022 10:06:48 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "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>,
        Christophe Jaillet <christophe.jaillet@...adoo.fr>,
        Joe Perches <joe@...ches.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for
 constant expressions

On Thu, Sep 1, 2022 at 7:19 AM Yury Norov <yury.norov@...il.com> wrote:
>
> On Thu, Sep 01, 2022 at 07:30:10PM +0900, Vincent MAILHOL wrote:
> > On Tue. 1 sept. 2022 at 12:49, Yury Norov <yury.norov@...il.com> wrote:
> > > On Wed, Aug 31, 2022 at 01:54:01AM -0700, Yury Norov wrote:
> > > > On Wed, Aug 31, 2022 at 04:57:40PM +0900, Vincent Mailhol wrote:
> > > > > The compilers provide some builtin expression equivalent to the ffs(),
> > > > > __ffs() and ffz() functions of the kernel. The kernel uses optimized
> > > > > assembly which produces better code than the builtin
> > > > > functions. However, such assembly code can not be folded when used
> > > > > with constant expressions.
> > > > >
> > > > > This series relies on __builtin_constant_p to select the optimal solution:
> > > > >
> > > > >   * use kernel assembly for non constant expressions
> > > > >
> > > > >   * use compiler's __builtin function for constant expressions.
> > > > >
> > > > >
> > > > > ** Statistics **
> > > > >
> > > > > Patch 1/2 optimizes 26.7% of ffs() calls and patch 2/2 optimizes 27.9%
> > > > > of __ffs() and ffz() calls (details of the calculation in each patch).
> > > >
> > > > Hi Vincent,
> > > >
> > > > Can you please add a test for this? We've recently added a very similar
> > > > test_bitmap_const_eval() in lib/test_bitmap.c.
> > > >
> > > > dc34d5036692c ("lib: test_bitmap: add compile-time optimization/evaluations
> > > > assertions")
> > > >
> > > > Would be nice to have something like this for ffs() and ffz() in
> > > > lib/test_bitops.c.
> > > >
> > > > Please keep me in loop in case of new versions.
> >
> > Hi Yury,
> >
> > My patch only takes care of the x86 architecture.
>
> OK, I just realized that you started submitting this at least back in May.
>
> For me, v6 is good enough and well-described. So, for the series:
> Reviewed-by: Yury Norov <yury.norov@...il.com>
>
> How are you going to merge it? If you haven't a specific tree in mind
> already, I can take it in my bitmap tree because  ffs and ffz are closely
> related to find_bit() functions.

Unless Boris feels strong enough to nack the series, I think Yury
accepting the series is the way to go.
-- 
Thanks,
~Nick Desaulniers