[tip: x86/asm] x86/asm/bitops: Use __builtin_ctzl() to evaluate constant expressions

tip-bot2 for Vincent Mailhol
2022-09-20T20:00:31Z

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@wanadoo.fr>
AuthorDate: Wed, 07 Sep 2022 18:09:35 +09:00
Committer: Borislav Petkov <bp@suse.de>
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@wanadoo.fr>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
Link: https://lore.kernel.org/r/20220511160319.1045812-1-mailhol.vincent@wanadoo.fr
---
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