Re: [PATCH 06/13] lib: extend the scope of small_const_nbits() macro

[Yury Norov <yury.norov@gmail.com>]

On Tue, Mar 16, 2021 at 09:56:49AM +0100, Rasmus Villemoes wrote:
> On 16/03/2021 02.54, Yury Norov wrote:
> > find_bit would also benefit from small_const_nbits() optimizations.
> > 
> > Signed-off-by: Yury Norov <yury.norov@gmail.com>
> > ---
> >  include/asm-generic/bitsperlong.h | 9 +++++++++
> >  include/linux/bitmap.h            | 3 ---
> >  2 files changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
> > index 3905c1c93dc2..96032f4f908f 100644
> > --- a/include/asm-generic/bitsperlong.h
> > +++ b/include/asm-generic/bitsperlong.h
> > @@ -23,4 +23,13 @@
> >  #define BITS_PER_LONG_LONG 64
> >  #endif
> >  
> > +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) < BITS_PER_LONG)
> > +
> > +/*
> > + * The valid number of bits for a bitmap to be small enough, or in other words,
> > + * fit into a single machine word is 1 to BITS_PER_LONG inclusively. 0 is not a
> > + * valid number for size, and most probably a sing of error.
> > + */
> > +#define small_const_nbits(n) SMALL_CONST((n) - 1)
> > +
> 
> I still don't see the point of introducing SMALL_CONST and still don't
> like the much-too-generic-name - especially since AFAICT you don't
> actually use it anywhere outside the definition of small_const_nbits()?
> 
> >  #endif /* __ASM_GENERIC_BITS_PER_LONG */
> > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > index adf7bd9f0467..bc13a890ecc1 100644
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
> >   * so make such users (should any ever turn up) call the out-of-line
> >   * versions.
> >   */
> 
> You added another comment near its new definition, but the left-behind
> comment in bitmap.h is now somewhat confusing, no? I suggest expanding
> your new comment a bit so it's clear why we're interested in whether a
> bitmap is known at compile-time to consist of exactly one word:
> 
> /*
> small_const_nbits(n) is true precisely when it is known at compile-time
> that BITMAP_SIZE(n) is 1, i.e. 1 <= n <= BITS_PER_LONG. This allows
> various bit/bitmap APIs to provide a fast inline implementation. Bitmaps
> of size 0 are very rare, and a compile-time-known-size 0 is most likely
> a sign of error. They will be handled correctly by the bit/bitmap APIs,
> but using the out-of-line functions, so that the inline implementations
> can unconditionally dereference the pointer(s).
> */

Ok, make sense

> > -#define small_const_nbits(nbits) \
> > -	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> > -
> >  static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> >  {
> >  	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> > 
> 
> Rasmus