cutils: extract buffer_is_zero() from qemu-img.c

The qemu-img.c:is_not_zero() function checks if a buffer contains all
zeroes.  This function will come in handy for zero-detection in the
block layer, so clean it up and move it to cutils.c.

Note that the function now returns true if the buffer is all zeroes.
This avoids the double-negatives (i.e. !is_not_zero()) that the old
function can cause in callers.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

cutils.c

@@ -303,6 +303,41 @@ void qemu_iovec_memset_skip(QEMUIOVector *qiov, int c, size_t count,
     }
 }
 
+/*
+ * Checks if a buffer is all zeroes
+ *
+ * Attention! The len must be a multiple of 4 * sizeof(long) due to
+ * restriction of optimizations in this function.
+ */
+bool buffer_is_zero(const void *buf, size_t len)
+{
+    /*
+     * Use long as the biggest available internal data type that fits into the
+     * CPU register and unroll the loop to smooth out the effect of memory
+     * latency.
+     */
+
+    size_t i;
+    long d0, d1, d2, d3;
+    const long * const data = buf;
+
+    assert(len % (4 * sizeof(long)) == 0);
+    len /= sizeof(long);
+
+    for (i = 0; i < len; i += 4) {
+        d0 = data[i + 0];
+        d1 = data[i + 1];
+        d2 = data[i + 2];
+        d3 = data[i + 3];
+
+        if (d0 || d1 || d2 || d3) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
 #ifndef _WIN32
 /* Sets a specific flag */
 int fcntl_setfl(int fd, int flag)

qemu-common.h

@@ -335,6 +335,8 @@ void qemu_iovec_memset(QEMUIOVector *qiov, int c, size_t count);
 void qemu_iovec_memset_skip(QEMUIOVector *qiov, int c, size_t count,
                             size_t skip);
 
+bool buffer_is_zero(const void *buf, size_t len);
+
 void qemu_progress_init(int enabled, float min_skip);
 void qemu_progress_end(void);
 void qemu_progress_print(float delta, int max);

qemu-img.c

@@ -514,40 +514,6 @@ static int img_commit(int argc, char **argv)
     return 0;
 }
 
-/*
- * Checks whether the sector is not a zero sector.
- *
- * Attention! The len must be a multiple of 4 * sizeof(long) due to
- * restriction of optimizations in this function.
- */
-static int is_not_zero(const uint8_t *sector, int len)
-{
-    /*
-     * Use long as the biggest available internal data type that fits into the
-     * CPU register and unroll the loop to smooth out the effect of memory
-     * latency.
-     */
-
-    int i;
-    long d0, d1, d2, d3;
-    const long * const data = (const long *) sector;
-
-    len /= sizeof(long);
-
-    for(i = 0; i < len; i += 4) {
-        d0 = data[i + 0];
-        d1 = data[i + 1];
-        d2 = data[i + 2];
-        d3 = data[i + 3];
-
-        if (d0 || d1 || d2 || d3) {
-            return 1;
-        }
-    }
-
-    return 0;
-}
-
 /*
  * Returns true iff the first sector pointed to by 'buf' contains at least
  * a non-NUL byte.
@@ -557,20 +523,22 @@ static int is_not_zero(const uint8_t *sector, int len)
  */
 static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum)
 {
-    int v, i;
+    bool is_zero;
+    int i;
 
     if (n <= 0) {
         *pnum = 0;
         return 0;
     }
-    v = is_not_zero(buf, 512);
+    is_zero = buffer_is_zero(buf, 512);
     for(i = 1; i < n; i++) {
         buf += 512;
-        if (v != is_not_zero(buf, 512))
+        if (is_zero != buffer_is_zero(buf, 512)) {
             break;
         }
+    }
     *pnum = i;
-    return v;
+    return !is_zero;
 }
 
 /*
@@ -955,7 +923,7 @@ static int img_convert(int argc, char **argv)
             if (n < cluster_sectors) {
                 memset(buf + n * 512, 0, cluster_size - n * 512);
             }
-            if (is_not_zero(buf, cluster_size)) {
+            if (!buffer_is_zero(buf, cluster_size)) {
                 ret = bdrv_write_compressed(out_bs, sector_num, buf,
                                             cluster_sectors);
                 if (ret != 0) {