add log base 2 function: log2_exact()

Add log2_exact() to get the base 2 logarithm of a value known
to be a power of 2.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/bits.h b/bits.h
index 63a663c2..9908190d 100644
--- a/bits.h
+++ b/bits.h
@@ -63,4 +63,11 @@ static inline int is_power_of_2(long long val)
 	return val && !(val & (val - 1));
 }
 
+///
+// log base 2 of an exact power-of-2
+static inline int log2_exact(unsigned long long val)
+{
+	return 8 * sizeof(val) - __builtin_clzl(val) - 1;
+}
+
 #endif