How to do an integer log2() in C++?

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If you are on a recent-ish x86 or x86-64 platform (and you probably are), use the bsr instruction which will return the position of the highest set bit in an unsigned integer. It turns out that this is exactly the same as log2(). Here is a short C or C++ function that invokes bsr using inline ASM:

#include <stdint.h>
static inline uint32_t log2(const uint32_t x) {
  uint32_t y;
  asm ( "\tbsr %1, %0\n"
      : "=r"(y)
      : "r" (x)
  );
  return y;
}

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