x86: simplify ceil(log2(x)) function

A function called mask_width in one place and log2 in the other
calculates its value in a more complex way than necessary. A simpler
implementation offered here saves a few bytes in the functions that
call it.

Reviewed by:	alc, avg
Differential Revision:	https://reviews.freebsd.org/D45483

sys/amd64/vmm/x86.c

@@ -61,14 +61,13 @@ SYSCTL_INT(_hw_vmm_topology, OID_AUTO, cpuid_leaf_b, CTLFLAG_RDTUN,
     &cpuid_leaf_b, 0, NULL);
 
 /*
- * Round up to the next power of two, if necessary, and then take log2.
- * Returns -1 if argument is zero.
+ * Compute ceil(log2(x)).  Returns -1 if x is zero.
  */
 static __inline int
 log2(u_int x)
 {
 
-	return (fls(x << (1 - powerof2(x))) - 1);
+	return (x == 0 ? -1 : fls(x - 1));
 }
 
 int

sys/x86/x86/mp_x86.c

@@ -183,15 +183,13 @@ mem_range_AP_init(void)
 }
 
 /*
- * Round up to the next power of two, if necessary, and then
- * take log2.
- * Returns -1 if argument is zero.
+ * Compute ceil(log2(x)).  Returns -1 if x is zero.
  */
 static __inline int
 mask_width(u_int x)
 {
 
-	return (fls(x << (1 - powerof2(x))) - 1);
+	return (x == 0 ? -1 : fls(x - 1));
 }
 
 /*