| Message ID | 1285256514-21138-2-git-send-email-stefanha@linux.vnet.ibm.com |
|---|---|
| State | New |
| Headers | show |
On Thu, 23 Sep 2010, Stefan Hajnoczi wrote: > The get_bits_from_size() calculates the log base-2 of a number. This is > useful in bit manipulation code working with power-of-2s. > > Currently used by qcow2 and needed by qed in a follow-on patch. int ilog2 (size_t size) { if (size & (size - 1)) return -1; return __builtin_ctzl (size); } Ifdef WIN64 omitted for brevity. [..snip..]
On Fri, Sep 24, 2010 at 01:50:39AM +0400, malc wrote: > On Thu, 23 Sep 2010, Stefan Hajnoczi wrote: > > > The get_bits_from_size() calculates the log base-2 of a number. This is > > useful in bit manipulation code working with power-of-2s. > > > > Currently used by qcow2 and needed by qed in a follow-on patch. > > int ilog2 (size_t size) > { > if (size & (size - 1)) > return -1; > > return __builtin_ctzl (size); > } > > Ifdef WIN64 omitted for brevity. What is the situation with WIN64? Stefan
On 09/24/2010 10:37 AM, Stefan Hajnoczi wrote: > On Fri, Sep 24, 2010 at 01:50:39AM +0400, malc wrote: >> On Thu, 23 Sep 2010, Stefan Hajnoczi wrote: >> >>> The get_bits_from_size() calculates the log base-2 of a number. This is >>> useful in bit manipulation code working with power-of-2s. >>> >>> Currently used by qcow2 and needed by qed in a follow-on patch. >> >> int ilog2 (size_t size) >> { >> if (size& (size - 1)) >> return -1; >> >> return __builtin_ctzl (size); >> } >> >> Ifdef WIN64 omitted for brevity. > > What is the situation with WIN64? long is only 32 bit, so you need __builtin_ctzll. Paolo
On Fri, Sep 24, 2010 at 1:56 PM, Paolo Bonzini <pbonzini@redhat.com> wrote: > On 09/24/2010 10:37 AM, Stefan Hajnoczi wrote: >> >> On Fri, Sep 24, 2010 at 01:50:39AM +0400, malc wrote: >>> >>> On Thu, 23 Sep 2010, Stefan Hajnoczi wrote: >>> >>>> The get_bits_from_size() calculates the log base-2 of a number. This is >>>> useful in bit manipulation code working with power-of-2s. >>>> >>>> Currently used by qcow2 and needed by qed in a follow-on patch. >>> >>> int ilog2 (size_t size) >>> { >>> if (size& (size - 1)) >>> return -1; >>> >>> return __builtin_ctzl (size); >>> } >>> >>> Ifdef WIN64 omitted for brevity. >> >> What is the situation with WIN64? > > long is only 32 bit, so you need __builtin_ctzll. Thanks Paolo and malc. It should be easy enough to use the builtin instead. Stefan
diff --git a/block/qcow2.c b/block/qcow2.c index a53014d..72c923a 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -767,28 +767,6 @@ static int qcow2_change_backing_file(BlockDriverState *bs, return qcow2_update_ext_header(bs, backing_file, backing_fmt); } -static int get_bits_from_size(size_t size) -{ - int res = 0; - - if (size == 0) { - return -1; - } - - while (size != 1) { - /* Not a power of two */ - if (size & 1) { - return -1; - } - - size >>= 1; - res++; - } - - return res; -} - - static int preallocate(BlockDriverState *bs) { uint64_t nb_sectors; diff --git a/cutils.c b/cutils.c index 036ae3c..f9812d5 100644 --- a/cutils.c +++ b/cutils.c @@ -251,3 +251,29 @@ int fcntl_setfl(int fd, int flag) } #endif +/** + * Get the number of bits for a power of 2 + * + * The following is true for powers of 2: + * n == 1 << get_bits_from_size(n) + */ +int get_bits_from_size(size_t size) +{ + int res = 0; + + if (size == 0) { + return -1; + } + + while (size != 1) { + /* Not a power of two */ + if (size & 1) { + return -1; + } + + size >>= 1; + res++; + } + + return res; +} diff --git a/qemu-common.h b/qemu-common.h index dfd3dc0..3170c64 100644 --- a/qemu-common.h +++ b/qemu-common.h @@ -137,6 +137,7 @@ time_t mktimegm(struct tm *tm); int qemu_fls(int i); int qemu_fdatasync(int fd); int fcntl_setfl(int fd, int flag); +int get_bits_from_size(size_t size); /* path.c */ void init_paths(const char *prefix);
The get_bits_from_size() calculates the log base-2 of a number. This is useful in bit manipulation code working with power-of-2s. Currently used by qcow2 and needed by qed in a follow-on patch. Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> --- block/qcow2.c | 22 ---------------------- cutils.c | 26 ++++++++++++++++++++++++++ qemu-common.h | 1 + 3 files changed, 27 insertions(+), 22 deletions(-)