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Subject[PATCH 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions
Date
__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.


Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
Usually, helper functions are prefixed by two underscores. But here,
__ffs() is already prefixed, instead of creating an ____ffs(), I
renamed it instead to __ffs_asm_not_zero(). The _not_zero() suffix is
to differentiate it from the __ffs_asm() helper function of the
previous patch.

If someone has better inspiration of the naming for this patch and the
previous one, please let me know().
---
arch/x86/include/asm/bitops.h | 36 ++++++++++++++++++++++-------------
1 file changed, 23 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 535a7a358c14..694d0d100399 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 __ffs_asm_not_zero(unsigned long word)
{
asm("rep; bsf %1,%0"
: "=r" (word)
@@ -239,12 +233,17 @@ static __always_inline unsigned long __ffs(unsigned long 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) : \
+ __ffs_asm_not_zero(word))
+
+static __always_inline unsigned long __ffz_asm(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) : \
+ __ffz_asm(word))
+
/*
* __fls: find last set bit in word
* @word: The word to search
--
2.35.1
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