Messages in this thread | | | From | Vincent Mailhol <> | Subject | [PATCH v2 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions | Date | Thu, 12 May 2022 01:03:17 +0900 |
| |
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 **
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
| |