Messages in this thread | | | From | Vincent Mailhol <> | Subject | [PATCH 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions | Date | Tue, 10 May 2022 23:25:48 +0900 |
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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
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