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.
> --- 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,
_
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