Messages in this thread Patch in this message | | | From | Vincent Mailhol <> | Subject | [RESEND PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions | Date | Sat, 25 Jun 2022 16:26:45 +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() gets simplified 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).
This patch uses the __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 applying this patch...:
| $ 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 is why we grep tzcnt instead of bsf in above benchmark). c.f. [1]
[1] commit e26a44a2d618 ("x86: Use REP BSF unconditionally") http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com
Reviewed-by: Nick Desaulniers <ndesaulniers@google.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 6ed979547086..f88c55b8b37c 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -224,13 +224,7 @@ static __always_inline bool variable_test_bit(long nr, volatile const unsigned l ? constant_test_bit((nr), (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) @@ -238,13 +232,18 @@ 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) @@ -252,6 +251,17 @@ 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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