[RESEND PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions

Vincent Mailhol
2022-07-23T15:15:58Z

__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