Messages in this thread Patch in this message | | | From | Vincent Mailhol <> | Subject | [PATCH v7 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions | Date | Mon, 5 Sep 2022 09:37:32 +0900 |
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__ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x) is equivalent to (unsigned long)__builtin_ctzl(~x). Because __builting_ctzl() returns an int, a cast to (unsigned long) is necessary to avoid potential warnings on implicit casts.
For x86_64, the current __ffs() and ffz() implementations do not produce optimized code when called with a constant expression. On the contrary, the __builtin_ctzl() folds into a single instruction.
However, for non constant expressions, the __ffs() and ffz() asm versions of the kernel remains slightly better than the code produced by GCC (it produces a useless instruction to clear eax).
Use __builtin_constant_p() to select between the kernel's __ffs()/ffz() and the __builtin_ctzl() depending on whether the argument is constant or not.
** Statistics **
On a allyesconfig, before...:
$ objdump -d vmlinux.o | grep tzcnt | wc -l 3607
...and after:
$ objdump -d vmlinux.o | grep tzcnt | wc -l 2600
So, roughly 27.9% of the calls to either __ffs() or ffz() were using constant expressions and could be optimized out.
(tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1)
Note: on x86_64, the asm bsf instruction produces tzcnt when used with the ret prefix (which explain the use of `grep tzcnt' instead of `grep bsf' in above benchmark). c.f. [1]
[1] commit e26a44a2d618 ("x86: Use REP BSF unconditionally") Link: http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Reviewed-by: Yury Norov <yury.norov@gmail.com> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr> --- arch/x86/include/asm/bitops.h | 38 ++++++++++++++++++++++------------- 1 file changed, 24 insertions(+), 14 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 879238e5a6a0..08eca2938f27 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -247,13 +247,7 @@ arch_test_bit_acquire(unsigned long nr, const volatile unsigned long *addr) variable_test_bit(nr, addr); } -/** - * __ffs - find first set bit in word - * @word: The word to search - * - * Undefined if no bit exists, so code should check against 0 first. - */ -static __always_inline unsigned long __ffs(unsigned long word) +static __always_inline unsigned long variable__ffs(unsigned long word) { asm("rep; bsf %1,%0" : "=r" (word) @@ -261,13 +255,18 @@ static __always_inline unsigned long __ffs(unsigned long word) return word; } -/** - * ffz - find first zero bit in word - * @word: The word to search - * - * Undefined if no zero exists, so code should check against ~0UL first. - */ -static __always_inline unsigned long ffz(unsigned long word) +/** + * __ffs - find first set bit in word + * @word: The word to search + * + * Undefined if no bit exists, so code should check against 0 first. + */ +#define __ffs(word) \ + (__builtin_constant_p(word) ? \ + (unsigned long)__builtin_ctzl(word) : \ + variable__ffs(word)) + +static __always_inline unsigned long variable_ffz(unsigned long word) { asm("rep; bsf %1,%0" : "=r" (word) @@ -275,6 +274,17 @@ static __always_inline unsigned long ffz(unsigned long word) return word; } +/** + * ffz - find first zero bit in word + * @word: The word to search + * + * Undefined if no zero exists, so code should check against ~0UL first. + */ +#define ffz(word) \ + (__builtin_constant_p(word) ? \ + (unsigned long)__builtin_ctzl(~word) : \ + variable_ffz(word)) + /* * __fls: find last set bit in word * @word: The word to search -- 2.35.1
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