Question

From MSDN
Search the mask data from most significant bit (MSB) to least significant bit (LSB) for a set bit (1).

unsigned char _BitScanReverse
(
unsigned long * Index,
unsigned long Mask
);

Parameters
[out] Index
Loaded with the bit position of the first set bit (1) found.

[in] Mask
The 32-bit or 64-bit value to search.

Return Value
0 if the mask is zero; nonzero otherwise.

Remarks
If a set bit is found, the bit position of the first set bit found is returned in the first parameter. If no set bit is found, 0 is returned; otherwise, 1 is returned.

Please tell me how to implement a safe and fast _BitScanReverse() function on OS X? Do I have to use assembly or is there more simple way?

Answer 1

GCC has some similar built-ins:

— Built-in Function: int __builtin_clz (unsigned int x)

Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.

— Built-in Function: int __builtin_ctz (unsigned int x)

Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined

If you have the number of zeros, you should be able to figure out where the first 1 is. 🙂