Messages in this thread | | | Date | Thu, 9 Jun 2016 08:31:39 +0200 | From | Martin Schwidefsky <> | Subject | Re: [PATCH] bitmap: bitmap_equal memcmp optimization |
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On Wed, 8 Jun 2016 13:42:12 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
> On Tue, 7 Jun 2016 10:37:41 +0200 Martin Schwidefsky <schwidefsky@de.ibm.com> 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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