[Qemu-devel] [PATCH 1/7] qcow2: Make get_bits_from_size() common

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

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(-)

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);
-- 
1.7.1