cogl-bitmask: Use ffsl to speedup bitmask iteration

Instead of testing each bit when iterating a bitmask, we can use ffsl
to skip over unset bits in single instruction. That way it will scale
by the number of bits set, not the total number of bits.

ffsl is a non-standard function which glibc only provides by defining
GNUC_SOURCE. However if we are compiling with GCC we can avoid that
mess and just use the equivalent builtin. When not compiling for GCC
it will fall back to _cogl_util_ffs if the size of ints and longs are
the same (which is the case on i686). Otherwise it fallbacks to a slow
function implementation.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-util.c

@@ -77,3 +77,26 @@ _cogl_util_ffs (int num)
   return i;
 }
 #endif /* HAVE_FFS */
+
+/* The 'ffsl' is non-standard but when building with GCC we'll use its
+   builtin instead */
+#ifndef COGL_UTIL_HAVE_BUILTIN_FFSL
+
+int
+_cogl_util_ffsl_wrapper (long int num)
+{
+  int i = 1;
+
+  if (num == 0)
+    return 0;
+
+  while ((num & 1) == 0)
+    {
+      num >>= 1;
+      i++;
+    }
+
+  return i;
+}
+
+#endif /* COGL_UTIL_HAVE_BUILTIN_FFSL */