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 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/[email protected]
Reviewed-by: Nick Desaulniers <[email protected]>
Reviewed-by: Yury Norov <[email protected]>
Signed-off-by: Vincent Mailhol <[email protected]>
---
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..95591310c080 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