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 07:19:13 -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 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.

> Assuming some other
> architectures are not optimized yet, adding such a test might break
> some builds. I am fine with adding the test, however, I will not write
> patches for the other architecture because I do not have the
> environment to compile and test it.
> 
> Does it still make sense to add the test before fixing all the architectures?

All-arches fix should begin with changing the ffs design. Namely, there
should be a generic ffs() in include/linux/bitops.h, and arch-specific
arch__ffs() in arch/xxx/include/asm/bitops.h; like we do for the set_bit()
family. I have a feeling that it's far beyond the scope of your series.

The test is a different story. Good tests are always welcome, even if
they don't cover all the arches.

> > 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.
> 
> Agree. The fls() and fls64() can use __builtin_ctz() and
> __builtin_ctzll(). However, those two functions are a bit less
> trivial. I wanted to have this first series approved first before
> working on *fls*().

OK, the test and fls() can be a matter of a follow-up series, taking
into account how long are these 2 patches moving.

Thanks,
Yury