[PATCH 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions

[Vincent Mailhol]

The compilers provides some builtin expression equivalent to the
ffs(), __ffs() and ffz() function of the kernel. The kernel uses
optimized assembly which produces better code than the builtin
functions. However, such assembly code can not be optimized when used
on constant expression.

This series relies on __builtin_constant_p to select the optimal solution:

  * use kernel assembly for non constant expressions

  * use compiler's __builtin function for constant expressions.

I also think that the fls() and fls64() can be optimized in a similar
way, using __builtin_ctz() and __builtin_ctzll() but it is a bit less
trivial so I want to focus on this series first. If it get accepted, I
will then work on those two additionnal function.


** Statistics **

On a allyesconfig, before applying this series, I get:

| $ objdump -d vmlinux.o | grep bsf | wc -l
| 1081

After applying this series:

| $ objdump -d vmlinux.o | grep bsf | wc -l
| 792

So, roughly 26.7% of the call to either ffs() or __ffs() were using
constant expression and can be optimized (I did not produce the
figures for ffz()).

(tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1)


Vincent Mailhol (2):
  x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant
    expressions
  x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant
    expressions

 arch/x86/include/asm/bitops.h | 65 +++++++++++++++++++++--------------
 1 file changed, 40 insertions(+), 25 deletions(-)

-- 
2.35.1