Re: [PATCH 01/10] string: introduce memweight
2012/5/23 Matthew Wilcox matt...@wil.cx:
On Wed, May 23, 2012 at 09:12:18PM +0900, Akinobu Mita wrote:
size_t memweight(const void *ptr, size_t bytes)
Why should this return size_t instead of unsigned long?
I just use the same type as the bytes argument without mature
consideration. If unsigned long is better than size_t, I'll
change the return type.
{
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;
}
bitmap_weight copes with a bitmask that isn't a multiple of BITS_PER_LONG
in size already. So I think this can be done as:
unsigned long memweight(const void *s, size_t n)
{
const unsigned char *ptr = s;
unsigned long r = 0;
while (n 0 (unsigned long)ptr % sizeof(long)) {
r += hweight8(*ptr);
n--;
ptr++;
}
BUG_ON(n = INT_MAX / 8)
return r + bitmap_weight((unsigned long *)ptr, n * 8);
}
This works perfectly on little-endian machines. But it doesn't work
on big-endian machines, if the bottom edge of memory area is not
aligned on long word boundary.
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Re: [PATCH 01/10] string: introduce memweight
2012/5/23 Jan Kara j...@suse.cz:
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));
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);
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;
}
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Re: [PATCH 01/10] string: introduce memweight
On Wed 23-05-12 21:12:18, Akinobu Mita wrote:
2012/5/23 Jan Kara j...@suse.cz:
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 j...@suse.cz
SUSE Labs, CR
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[PATCH 01/10] string: introduce memweight
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().
Signed-off-by: Akinobu Mita akinobu.m...@gmail.com
Cc: Anders Larsen a...@alarsen.net
Cc: Alasdair Kergon a...@redhat.com
Cc: dm-de...@redhat.com
Cc: linux-fsde...@vger.kernel.org
Cc: Laurent Pinchart laurent.pinch...@ideasonboard.com
Cc: linux-media@vger.kernel.org
Cc: Mark Fasheh mfas...@suse.com
Cc: Joel Becker jl...@evilplan.org
Cc: ocfs2-de...@oss.oracle.com
Cc: Jan Kara j...@suse.cz
Cc: linux-e...@vger.kernel.org
Cc: Andrew Morton a...@linux-foundation.org
Cc: Andreas Dilger adilger.ker...@dilger.ca
Cc: Theodore Ts'o ty...@mit.edu
---
include/linux/string.h |3 +++
lib/string.c | 37 +
2 files changed, 40 insertions(+), 0 deletions(-)
diff --git a/include/linux/string.h b/include/linux/string.h
index e033564..ffe0442 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -145,4 +145,7 @@ static inline bool strstarts(const char *str, const char
*prefix)
return strncmp(str, prefix, strlen(prefix)) == 0;
}
#endif
+
+extern size_t memweight(const void *ptr, size_t bytes);
+
#endif /* _LINUX_STRING_H_ */
diff --git a/lib/string.c b/lib/string.c
index e5878de..c8b92a0 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -26,6 +26,7 @@
#include linux/export.h
#include linux/bug.h
#include linux/errno.h
+#include linux/bitmap.h
#ifndef __HAVE_ARCH_STRNICMP
/**
@@ -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;
+
+ for (bitmap.ptr = ptr; bytes 0 bitmap.address % sizeof(long);
+ bytes--, bitmap.address++)
+ w += hweight8(*bitmap.b);
+
+ for (longs = bytes / sizeof(long); longs 0; ) {
+ size_t bits = min_t(size_t, INT_MAX ~(BITS_PER_LONG - 1),
+ longs * BITS_PER_LONG);
+
+ w += bitmap_weight(bitmap.ptr, bits);
+ bytes -= bits / BITS_PER_BYTE;
+ bitmap.address += bits / BITS_PER_BYTE;
+ longs -= bits / BITS_PER_LONG;
+ }
+
+ for (; bytes 0; bytes--, bitmap.address++)
+ w += hweight8(*bitmap.b);
+
+ return w;
+}
+EXPORT_SYMBOL(memweight);
--
1.7.7.6
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