Commit fdb6649a authored Sep 07, 2022 by Vincent Mailhol Committed by Borislav Petkov Sep 20, 2022

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] e26a44a2 ("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

parent 146034fe

arch/x86/include/asm/bitops.h

+19 −9

Original line number	Diff line number	Diff line
		@@ -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