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 <stefa...@linux.vnet.ibm.com>
---
block/qcow2.c | 22 ----------------------
cutils.c | 18 ++++++++++++++++++
qemu-common.h | 1 +
3 files changed, 19 insertions(+), 22 deletions(-)
diff --git a/block/qcow2.c b/block/qcow2.c
index ee3481b..6e25812 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -794,28 +794,6 @@ 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 5883737..6c32198 100644
--- a/cutils.c
+++ b/cutils.c
@@ -283,3 +283,21 @@ 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)
+{
+ if (size == 0 || (size & (size - 1))) {
+ return -1;
+ }
+
+#if defined(_WIN32) && defined(__x86_64__)
+ return __builtin_ctzll(size);
+#else
+ return __builtin_ctzl(size);
+#endif
+}
diff --git a/qemu-common.h b/qemu-common.h
index 81aafa0..e0ca398 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -153,6 +153,7 @@ 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);
--
1.7.1
