Answer by eraoul for How to check if a number is a power of 2

In C, I tested the i && !(i & (i - 1) trick and compared it with __builtin_popcount(i), using gcc on Linux, with the -mpopcnt flag to be sure to use the CPU's POPCNT instruction. My test program counted the # of integers between 0 and 2^31 that were a power of two.

At first I thought that i && !(i & (i - 1) was 10% faster, even though I verified that POPCNT was used in the disassembly where I used__builtin_popcount.

However, I realized that I had included an if statement, and branch prediction was probably doing better on the bit twiddling version. I removed the if and POPCNT ended up faster, as expected.

Results:

Intel(R) Core(TM) i7-4771 CPU max 3.90GHz

Timing (i & !(i & (i - 1))) trick30real    0m13.804suser    0m13.799ssys     0m0.000sTiming POPCNT30real    0m11.916suser    0m11.916ssys     0m0.000s

AMD Ryzen Threadripper 2950X 16-Core Processor max 3.50GHz

Timing (i && !(i & (i - 1))) trick30real    0m13.675suser    0m13.673ssys 0m0.000sTiming POPCNT30real    0m13.156suser    0m13.153ssys 0m0.000s

Note that here the Intel CPU seems slightly slower than AMD with the bit twiddling, but has a much faster POPCNT; the AMD POPCNT doesn't provide as much of a boost.

popcnt_test.c:

#include "stdio.h"// Count # of integers that are powers of 2 up to 2^31;int main() {  int n;  for (int z = 0; z < 20; z++){      n = 0;      for (unsigned long i = 0; i < 1<<30; i++) {       #ifdef USE_POPCNT        n += (__builtin_popcount(i)==1); // Was: if (__builtin_popcount(i) == 1) n++;       #else        n += (i && !(i & (i - 1)));  // Was: if (i && !(i & (i - 1))) n++;       #endif      }  }  printf("%d\n", n);  return 0;}

Run tests:

gcc popcnt_test.c -O3 -o test.exegcc popcnt_test.c -O3 -DUSE_POPCNT -mpopcnt -o test-popcnt.exeecho "Timing (i && !(i & (i - 1))) trick"time ./test.exeechoecho "Timing POPCNT"time ./test-opt.exe