I want to write a function that returns the nearest next power of 2 number. For example if my input is 789, the output should be 1024. Is there any way of achieving this without using any loops but just using some bitwise operators?
Related: Algorithm for finding the smallest power of two that's greater or equal to a given value is a C++ question. C++20 introduced std:bit_ceil which lets the compiler do whatever's optimal for the target system, but nothing equivalent is yet available in portable ISO C for bit-scan, popcount or other common bit operations that most CPUs have. Portable C code has to be less efficient and/or more complicated.
Given an integer, how do I find the next largest power of two using bit-twiddling? is a language-agnostic version of the question with some C++11 and 17 constexpr using GNU extensions.
Answers to this question don't need to be portable; fast versions for various platforms are useful.
Check the Bit Twiddling Hacks. You need to get the base 2 logarithm, then add 1 to that. Example for a 32-bit value:
Round up to the next highest power of 2
unsigned int v; // compute the next highest power of 2 of 32-bit v v--; v |= v >> 1; v |= v >> 2; v |= v >> 4; v |= v >> 8; v |= v >> 16; v++;
The extension to other widths should be obvious.
An answer on Given an integer, how do I find the next largest power of two using bit-twiddling? presents some explanation of how it works, and examples of the bit-patterns for a couple inputs.