| Date | Wed, 11 May 2022 22:56:15 +0200 | Subject | Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions | From | Christophe JAILLET <> |
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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
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