Bit fiddling of low order bits. More...
Collaboration diagram for Bitfiddling:
Macros | |
| #define | P99_LOW0SET(X) ((X) | ((X) + 1)) |
| set least significant zero bit More... | |
| #define | P99_LOW2(X) ((X) & -(X)) |
| extract the least significant bit that is non zero More... | |
| #define | P99_LOW2CLEAR(X) ((X) & ((X) - 1)) |
| clear the least significant bit that is non zero More... | |
| #define | P99_LOW2FILL(X) ((X) | ((X) - 1)) |
| fill all bits below least significant bit that is non zero More... | |
| #define | P99_LOW2MASK0(X) (~(X) & ((X) - 1)) |
| mask consisting of bits below and including the least significant bit that is non zero More... | |
| #define | P99_LOW2MASK1(X) ((X) ^ ((X) - 1)) |
| mask consisting of bits strictly below the least significant bit that is non zero More... | |
| #define | P99_LOW2SHIFT(X) (P99_LIKELY(!!(X)) ? ((X) / ((X) & -(X))) : 0u) |
| shift number right such that the least significant bit that is non zero ends up at bit position 0 More... | |
| #define | P99_MASK2LOW0(X) ((X) ^ -(X)) |
| mask consisting of bits strictly above the least significant bit that is non zero More... | |
| #define | P99_MASK2LOW1(X) ((X) | -(X)) |
| mask consisting of bits above and including the least significant bit that is non zero More... | |
Functions | |
| uintmax_t | p99_low0set (uintmax_t p00_x) |
| function equivalent to P99_LOW0SET More... | |
| uintmax_t | p99_low2 (uintmax_t p00_x) |
| function equivalent to P99_LOW2 More... | |
| uintmax_t | p99_low2clear (uintmax_t p00_x) |
| function equivalent to P99_LOW2CLEAR More... | |
| uintmax_t | p99_low2fill (uintmax_t p00_x) |
| function equivalent to P99_LOW2FILL More... | |
| uintmax_t | p99_low2mask0 (uintmax_t p00_x) |
| function equivalent to P99_LOW2MASK0 More... | |
| uintmax_t | p99_low2mask1 (uintmax_t p00_x) |
| function equivalent to P99_LOW2MASK1 More... | |
| uintmax_t | p99_low2shift (uintmax_t p00_x) |
| function equivalent to P99_LOW2SHIFT More... | |
| uintmax_t | p99_mask2low0 (uintmax_t p00_x) |
| function equivalent to P99_MASK2LOW0 More... | |
| uintmax_t | p99_mask2low1 (uintmax_t p00_x) |
| function equivalent to P99_MASK2LOW1 More... | |
| uintmax_t | p99_next_popcount (uintmax_t p00_x) |
| function that returns the next higher value that has exactly the same popcount as x. More... | |
Detailed Description
Bit fiddling of low order bits.
The smart expression used in this group are taken as summarized in Christer Ericson's blog.
They work because of the special features of unsigned integer arithmetic that it is arithmetic modulo a power of 2.
The notation of the macro and function names has some menomics:
low2stands for the least significant bit resulting in the corresponding power of 2, something like1 << bitpositionmaskstands for a mask that is constructed from this, that is a sequence of bits that are all one. Whenmaskcomes afterlowas inlow2maskit means that the bit mask follows that least significant bit. Whenmaskcomes beforelowas inmask2lowit means that the bit mask precedes that least significant bit.- The last digit of
0or1indicates whether that least significant bit is cleared or left untouched.
Be careful when using the macros in this group:
- they may evaluate their arguments several times
- they must be only used with expressions that can be guaranteed to be unsigned
On the other hand prefer the macro when you known that you have compile time expressions. Then you may even use them as part of an #if preprocessing conditional.
