The following commit has been merged into the x86/asm branch of tip: Commit-ID: fdb6649ab7c142e497539a471e573c2593b9c923 Gitweb: https://git.kernel.org/tip/fdb6649ab7c142e497539a471e573c2593b9c923 Author: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx> AuthorDate: Wed, 07 Sep 2022 18:09:35 +09:00 Committer: Borislav Petkov <bp@xxxxxxx> CommitterDate: Tue, 20 Sep 2022 15:35:37 +02:00 x86/asm/bitops: Use __builtin_ctzl() to evaluate constant expressions If x is not 0, __ffs(x) is equivalent to: (unsigned long)__builtin_ctzl(x) And if x is not ~0UL, 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. Concerning the edge cases, __builtin_ctzl(0) is always undefined, whereas __ffs(0) and ffz(~0UL) may or may not be defined, depending on the processor. Regardless, for both functions, developers are asked to check against 0 or ~0UL so replacing __ffs() or ffz() by __builting_ctzl() is safe. 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 BSF instruction produces TZCNT when used with the REP prefix (which explain the use of `grep tzcnt' instead of `grep bsf' in above benchmark). c.f. [1] [1] e26a44a2d618 ("x86: Use REP BSF unconditionally") [ bp: Massage commit message. ] Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Reviewed-by: Yury Norov <yury.norov@xxxxxxxxx> Link: https://lore.kernel.org/r/20220511160319.1045812-1-mailhol.vincent@xxxxxxxxxx --- arch/x86/include/asm/bitops.h | 28 +++++++++++++++++++--------- 1 file changed, 19 insertions(+), 9 deletions(-) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 879238e..2edf684 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -247,17 +247,30 @@ arch_test_bit_acquire(unsigned long nr, const volatile unsigned long *addr) variable_test_bit(nr, addr); } +static __always_inline unsigned long variable__ffs(unsigned long word) +{ + asm("rep; bsf %1,%0" + : "=r" (word) + : "rm" (word)); + return 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. */ -static __always_inline unsigned long __ffs(unsigned long word) +#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) - : "rm" (word)); + : "r" (~word)); return word; } @@ -267,13 +280,10 @@ static __always_inline unsigned long __ffs(unsigned long word) * * Undefined if no zero exists, so code should check against ~0UL first. */ -static __always_inline unsigned long ffz(unsigned long word) -{ - asm("rep; bsf %1,%0" - : "=r" (word) - : "r" (~word)); - return word; -} +#define ffz(word) \ + (__builtin_constant_p(word) ? \ + (unsigned long)__builtin_ctzl(~word) : \ + variable_ffz(word)) /* * __fls: find last set bit in word
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