Re: [PATCH v2 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions

Nick Desaulniers
2022-05-11T22:21:21Z

On Wed, May 11, 2022 at 9:04 AM Vincent Mailhol
<mailhol.vincent@wanadoo.fr> wrote:
>
> __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 call to either __ffs() or ffz() were using
> constant expression and were 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
>
> CC: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>

Patch LGTM, though I find the location of the double unscores in the
names slightly against my taste.

> ---
> 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..7cf5374ce403 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)

How about `variable___ffs`? Patch 1/2 used `variable_ffs` for `ffs`?

> {
> 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)

`ffz` had no underscore. Regardless of `__ffs`, this should definitely
be `variable_ffz` IMO.

> {
> 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
>

--
Thanks,
~Nick Desaulniers