[PATCH v3 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions

Vincent Mailhol
2022-05-12T00:04:15Z

The compilers provide 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 **

Patch 1/2 optimizes 26.7% of ffs() calls and patch 2/2 optimizes 27.9%
of __ffs() and ffz() calls (details of the calculation in each patch).

** Changelog **

v2 -> v3:

* Redacted out the instructions after ret and before next function
in the assembly output.

* Added a note and a link to Nick's message on the constant
propagation missed-optimization in clang:
https://lore.kernel.org/all/CAKwvOdnH_gYv4qRN9pKY7jNTQK95xNeH1w1KZJJmvCkh8xJLBg@mail.gmail.com/

* Fix copy/paste type in statistics of patch 1. Number of occurences
before patches are 1081 and not 3607 (percentage reduction of
26.7% remains correct)

* Rename the functions as follow:
- __varible_ffs() -> variable___ffs()
- __variable_ffz() -> variable_ffz()

v1 -> v2:

* Use the ORC unwinder for the produced assembly code in patch 1.

* Rename the functions as follow:
- __ffs_asm() -> variable_ffs()
- __ffs_asm_not_zero() -> __variable_ffs()
- ffz_asm() -> variable_ffs()

* fit #define ffs(x) in a single line.

* Correct the statistics for ffs() in patch 1 and add the statistics
for __ffs() and ffz() in patch 2.

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 | 64 +++++++++++++++++++++--------------
1 file changed, 38 insertions(+), 26 deletions(-)

--
2.35.1