Re: [PATCH] bitmap: bitmap_equal memcmp optimization

[Martin Schwidefsky]

On Wed, 8 Jun 2016 13:42:12 -0700
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Tue,  7 Jun 2016 10:37:41 +0200 Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote:
> 
> > The bitmap_equal function has optimized code for small bitmaps with less
> > than BITS_PER_LONG bits. For larger bitmaps the out-of-line function
> > __bitmap_equal is called.
> > 
> > For a constant number of bits divisible by BITS_PER_LONG the memcmp
> > function can be used. For s390 gcc knows how to optimize this function,
> > memcmp calls with up to 256 bytes / 2048 bits are translated into a
> > single instruction.
> 
> Patch looks simple enough, although it would benefit from a comment
> explaining what's special about s390.

The explanation from the git commit could be transfered into a comment.

> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -267,6 +267,10 @@ static inline int bitmap_equal(const unsigned long *src1,
> >  {
> >  	if (small_const_nbits(nbits))
> >  		return ! ((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits));
> > +#ifdef CONFIG_S390
> > +	else if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0)
> > +		return !memcmp(src1, src2, nbits / 8);
> > +#endif
> >  	else
> >  		return __bitmap_equal(src1, src2, nbits);
> >  }
> 
> Those elses are a bit daffy.  This:
> 
> --- a/include/linux/bitmap.h~bitmap-bitmap_equal-memcmp-optimization-fix
> +++ a/include/linux/bitmap.h
> @@ -266,13 +266,12 @@ static inline int bitmap_equal(const uns
>  			const unsigned long *src2, unsigned int nbits)
>  {
>  	if (small_const_nbits(nbits))
> -		return ! ((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits));
> +		return !((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits));
>  #ifdef CONFIG_S390
> -	else if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0)
> +	if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0)
>  		return !memcmp(src1, src2, nbits / 8);
>  #endif
> -	else
> -		return __bitmap_equal(src1, src2, nbits);
> +	return __bitmap_equal(src1, src2, nbits);
>  }
> 
>  static inline int bitmap_intersects(const unsigned long *src1,
> _
> 

Yeah that looks better. Thanks for picking this up!

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.