On Wed 23-05-12 21:12:18, Akinobu Mita wrote:
> 2012/5/23 Jan Kara <jack@xxxxxxx>:
> > On Sun 20-05-12 22:23:14, Akinobu Mita wrote:
> >> memweight() is the function that counts the total number of bits set
> >> in memory area. The memory area doesn't need to be aligned to
> >> long-word boundary unlike bitmap_weight().
> > Thanks for the patch. I have some comments below.
>
> Thanks for the review.
>
> >> @@ -824,3 +825,39 @@ void *memchr_inv(const void *start, int c, size_t bytes)
> >> return check_bytes8(start, value, bytes % 8);
> >> }
> >> EXPORT_SYMBOL(memchr_inv);
> >> +
> >> +/**
> >> + * memweight - count the total number of bits set in memory area
> >> + * @ptr: pointer to the start of the area
> >> + * @bytes: the size of the area
> >> + */
> >> +size_t memweight(const void *ptr, size_t bytes)
> >> +{
> >> + size_t w = 0;
> >> + size_t longs;
> >> + union {
> >> + const void *ptr;
> >> + const unsigned char *b;
> >> + unsigned long address;
> >> + } bitmap;
> > Ugh, this is ugly and mostly unnecessary. Just use "const unsigned char
> > *bitmap".
> >
> >> +
> >> + for (bitmap.ptr = ptr; bytes > 0 && bitmap.address % sizeof(long);
> >> + bytes--, bitmap.address++)
> >> + w += hweight8(*bitmap.b);
> > This can be:
> > count = ((unsigned long)bitmap) % sizeof(long);
>
> The count should be the size of unaligned area and it can be greater than
> bytes. So
>
> count = min(bytes,
> sizeof(long) - ((unsigned long)bitmap) % sizeof(long));
You are right, I didn't quite think this through.
> > while (count--) {
> > w += hweight(*bitmap);
> > bitmap++;
> > bytes--;
> > }
> >> +
> >> + for (longs = bytes / sizeof(long); longs > 0; ) {
> >> + size_t bits = min_t(size_t, INT_MAX & ~(BITS_PER_LONG - 1),
> >> + longs * BITS_PER_LONG);
> > I find it highly unlikely that someone would have such a large bitmap
> > (256 MB or more on 32-bit). Also the condition as you wrote it can just
> > overflow so it won't have the desired effect. Just do
> > BUG_ON(longs >= ULONG_MAX / BITS_PER_LONG);
>
> The bits argument of bitmap_weight() is int type. So this should be
>
> BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
OK, I didn't check and thought it's size_t.
> > and remove the loop completely. If someone comes with such a huge bitmap,
> > the code can be modified easily (after really closely inspecting whether
> > such a huge bitmap is really well justified).
>
> size_t memweight(const void *ptr, size_t bytes)
> {
> size_t w = 0;
> size_t longs;
> const unsigned char *bitmap = ptr;
>
> for (; bytes > 0 && ((unsigned long)bitmap) % sizeof(long);
> bytes--, bitmap++)
> w += hweight8(*bitmap);
>
> longs = bytes / sizeof(long);
> BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
> w += bitmap_weight((unsigned long *)bitmap, longs * BITS_PER_LONG);
> bytes -= longs * sizeof(long);
> bitmap += longs * sizeof(long);
>
> for (; bytes > 0; bytes--, bitmap++)
> w += hweight8(*bitmap);
>
> return w;
> }
Yup, this looks much more readable. Thanks!
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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