As the answers above point out, there are a number of ways to determine the most significant bit. However, as was also pointed out, the methods are likely to be unique to either 32bit or 64bit registers. The stanford.edu bithacks page provides solutions that work for both 32bit and 64bit computing. With a little work, they can be combined to provide a solid cross-architecture approach to obtaining the MSB. The solution I arrived at that compiled/worked across 64 & 32 bit computers was:
#if defined(__LP64__) || defined(_LP64)# define BUILD_64 1#endif#include <stdio.h>#include <stdint.h> /* for uint32_t *//* CHAR_BIT (or include limits.h) */#ifndef CHAR_BIT#define CHAR_BIT 8#endif /* CHAR_BIT *//* * Find the log base 2 of an integer with the MSB N set in O(N) * operations. (on 64bit & 32bit architectures) */intgetmsb (uint32_t word){ int r = 0; if (word < 1) return 0;#ifdef BUILD_64 union { uint32_t u[2]; double d; } t; // temp t.u[__FLOAT_WORD_ORDER==LITTLE_ENDIAN] = 0x43300000; t.u[__FLOAT_WORD_ORDER!=LITTLE_ENDIAN] = word; t.d -= 4503599627370496.0; r = (t.u[__FLOAT_WORD_ORDER==LITTLE_ENDIAN] >> 20) - 0x3FF;#else while (word >>= 1) { r++; }#endif /* BUILD_64 */ return r;}