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.
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