Messages in this thread | | | From | Vincent MAILHOL <> | Date | Thu, 12 May 2022 08:30:45 +0900 | Subject | Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions |
| |
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
| |