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Date: Wed, 11 May 2022 15:20:55 -0700 From: Nick Desaulniers <ndesaulniers@...gle.com> To: Vincent Mailhol <mailhol.vincent@...adoo.fr> Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>, Nathan Chancellor <nathan@...nel.org>, Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, David Howells <dhowells@...hat.com>, Jan Beulich <JBeulich@...e.com> Subject: Re: [PATCH v2 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions On Wed, May 11, 2022 at 9:04 AM Vincent Mailhol <mailhol.vincent@...adoo.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@...gle.com> > Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.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
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