Messages in this thread | | | From | Vincent MAILHOL <> | Date | Wed, 11 May 2022 08:24:34 +0900 | Subject | Re: [PATCH 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions |
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On Wed. 11 May 2022 at 07:14, Nick Desaulniers <ndesaulniers@google.com> wrote: > On Tue, May 10, 2022 at 7:26 AM Vincent Mailhol > <mailhol.vincent@wanadoo.fr> wrote: > > > > 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()). > > These stats are interesting; consider putting them on patch 1/2 commit > message though (in addition to the cover letter). (Sending thoughts on > 1/2 next).
The fact is that patch 1/2 changes ffs() and patch 2/2 changes __ffs(). For v2, I will run the stats on each patch separately in order not to mix the results.
> > > > (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1) > > Here's the same measure of x86_64 allyesconfig (./scripts/config -d > CONFIG_HINIC) at 9be9ed2612b5aedb52a2c240edb1630b6b743cb6 with ToT > LLVM (~clang-15): > > Before: > $ objdump -d vmlinux.o | grep bsf | wc -l > 1454 > > After: > $ objdump -d vmlinux.o | grep bsf | wc -l > 1070 > > -26.4% :)
Roughly same ratio. I am just surprise that the absolute number are different:
* GCC before: 1081, after 792 * clang before 1454, after 1070
I wonder why clang produces more bsf instructions than GCC?
Also, on a side note, I am not the first one to realize that __builtin_ffs() is able to optimize the constant variable. Some people already used it to locally:
| $ git grep __builtin_ffs | wc -l | 80
> > > > > > 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 > > > > > -- > Thanks, > ~Nick Desaulniers
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