Patchwork [v2,1/7] qcow2: Make get_bits_from_size() common

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Submitter Stefan Hajnoczi
Date Oct. 8, 2010, 3:48 p.m.
Message ID <1286552914-27014-2-git-send-email-stefanha@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/67270/
State New
Headers show

Comments

Stefan Hajnoczi - Oct. 8, 2010, 3:48 p.m.
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      |   18 ++++++++++++++++++
 qemu-common.h |    1 +
 3 files changed, 19 insertions(+), 22 deletions(-)

Patch

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);