1 files changed, 41 insertions, 7 deletions
diff --git a/include/my_bit.h b/include/my_bit.h
index ccdf5a069e1..ca51c9825b6 100644
--- a/include/my_bit.h
+++ b/include/my_bit.h
@@ -25,16 +25,50 @@ C_MODE_START
 
 extern const uchar _my_bits_reverse_table[256];
 
+
 /*
-  Find smallest X in 2^X >= value
-  This can be used to divide a number with value by doing a shift instead
-*/
+  my_bit_log2_xxx()
+
+  In the given value, find the highest bit set,
+  which is the smallest X that satisfies the condition: (2^X >= value).
+  Can be used as a reverse operation for (1<<X), to find X.
+
+  Examples:
+  - returns 0 for (1<<0)
+  - returns 1 for (1<<1)
+  - returns 2 for (1<<2)
+  - returns 2 for 3, which has (1<<2) as the highest bit set.
 
-static inline uint my_bit_log2(ulong value)
+  Note, the behaviour of log2(0) is not defined.
+  Let's return 0 for the input 0, for the code simplicity.
+  See the 000x branch. It covers both (1<<0) and 0.
+*/
+static inline CONSTEXPR uint my_bit_log2_hex_digit(uint8 value)
 {
-  uint bit;
-  for (bit=0 ; value > 1 ; value>>=1, bit++) ;
-  return bit;
+  return value & 0x0C ? /*1100*/ (value & 0x08 ? /*1000*/ 3 : /*0100*/ 2) :
+                        /*0010*/ (value & 0x02 ? /*0010*/ 1 : /*000x*/ 0);
+}
+static inline CONSTEXPR uint my_bit_log2_uint8(uint8 value)
+{
+  return value & 0xF0 ? my_bit_log2_hex_digit(value >> 4) + 4:
+                        my_bit_log2_hex_digit(value);
+}
+static inline CONSTEXPR uint my_bit_log2_uint16(uint16 value)
+{
+  return value & 0xFF00 ? my_bit_log2_uint8((uint8) (value >> 8)) + 8 :
+                          my_bit_log2_uint8((uint8) value);
+}
+static inline CONSTEXPR uint my_bit_log2_uint32(uint32 value)
+{
+  return value & 0xFFFF0000UL ?
+         my_bit_log2_uint16((uint16) (value >> 16)) + 16 :
+         my_bit_log2_uint16((uint16) value);
+}
+static inline CONSTEXPR uint my_bit_log2_uint64(ulonglong value)
+{
+  return value & 0xFFFFFFFF00000000ULL ?
+         my_bit_log2_uint32((uint32) (value >> 32)) + 32 :
+         my_bit_log2_uint32((uint32) value);
 }