Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions

Vincent MAILHOL
2022-05-11T23:31:07Z

On Thu. 12 May 2022 at 05:56, Christophe JAILLET
<christophe.jaillet@wanadoo.fr> wrote:
> Le 11/05/2022 à 18:03, Vincent Mailhol a écrit :
> > 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.
> >
>
> [...]
>
> >
> > ** Statistics **
> >
> > On a allyesconfig, before applying this patch...:
> >
> > | $ objdump -d vmlinux.o | grep bsf | wc -l
> > | 3607
> >
> > ...and after:
> >
> > | $ objdump -d vmlinux.o | grep bsf | wc -l
> > | 792
> >
> > So, roughly 26.7% of the call to ffs() were using constant expression
> > and were optimized out.
> >
> >
> nitpicking: numbers look odd.

And they are. Thanks for spotting the issue.

> 3607 is the exact same number as in patch 2/2. (ok, could be)
> 26.7% is surprising with these numbers. (I guess it is (total_before
> - remaining) / total_before x 100 = (3607-792)/36.07 = 78.0%)

The 3607 is incorrect (copy/paste issue, sorry). The correct figure is
1081. And:

(1081-792)/1081 = 26.7%

Will amend the comment and send v3 right away.

> (but patch looks great to me :)

Thanks! :)

> CJ