Re: [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions

Vincent MAILHOL
2022-08-12T11:55:30Z

Hi Borislav,

On Thu. 11 Aug 2022 at 23:59, Borislav Petkov <bp@alien8.de> wrote:
> On Sun, Jul 24, 2022 at 12:15:20AM +0900, Vincent Mailhol wrote:
> > For x86_64, the current ffs() implementation does not produce
> > optimized code when called with a constant expression. On the
> > contrary, the __builtin_ffs() function of both GCC and clang is able
> > to simplify the expression into a single instruction.
> >
> > * Example *
> >
> > Let's consider two dummy functions foo() and bar() as below:
> >
> > | #include <linux/bitops.h>
> > | #define CONST 0x01000000
>
> Those code examples you can simply indent with two spaces.
>
> > In both examples, we clearly see the benefit of using __builtin_ffs()
>
> Who's "we"?
>
> Please use passive voice in your commit message: no "we" or "I", etc,
> and describe your changes in imperative mood.
>
> > instead of the kernel's asm implementation for constant expressions.
> >
> > However, for non constant expressions, the ffs() asm version of the
> > kernel remains better for x86_64 because, contrary to GCC, it doesn't
> > emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is
> > able optimize out the CMOV call).
> >
> > This patch uses the __builtin_constant_p() to select between the
>
> Avoid having "This patch" or "This commit" in the commit message. It is
> tautologically useless.
>
> Also, do
>
> $ git grep 'This patch' Documentation/process
>
> for more details.
>
> > kernel's ffs() and the __builtin_ffs() depending on whether the
> > argument is constant or not.
>
> In general, you don't have to say what the patch does - that should be
> visible from the diff. The more important part is the *why*. And that
> you do.
>
> Rest looks ok.

Thank you for the review!
I addressed all your comments and sent a v5:
https://lore.kernel.org/all/20220812114438.1574-1-mailhol.vincent@wanadoo.fr/

Yours sincerely,
Vincent Mailhol