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

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Date:   Wed, 31 Aug 2022 20:49:27 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc:     Borislav Petkov <bp@...en8.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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 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.

Also, what about fls? Is there any difference with ffs/ffz wrt compile
time optimizations? If not, would be great if the series will take
care of it too.

Thanks,
Yury