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

Christophe JAILLET
2022-05-11T20:56:37Z

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.
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%)

(but patch looks great to me :)

CJ