Possible implementations of strlen.

strlen.c

#include <string.h>

// Compile with: clang -O3 -fno-builtin

__attribute__((noinline)) int my_strlen(const char *ptr) {
  int len;
  asm("xorl %%ecx, %%ecx\n\t"
      "notl %%ecx\n\t"
      "xorl %%eax, %%eax\n\t"
      "cld\n\t"
      "repne scasb\n\t"
      "notl %%ecx\n\t"
      : "=c"(len)
      : "D"(ptr)
      : "%eax");
  return len;
}

__attribute__((noinline)) int fast_strlen(const char *ptr) {
  // Slightly less efficient version of code generated for:
  //    int len = 0;
  //    while(*ptr++) ++len;
  long len;
  asm("xorq %0, %0\n\t"
      "1:\n\t"
      "cmpb $0, (%1,%0)\n\t"
      "leaq 1(%0), %0\n\t"
      "jne 1b\n\t"
      : "=r"(len)
      : "r"(ptr));
  return len;
}


const char *str = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

int main() {
  for (long i = 0; i < 100000000; ++i) {
    my_strlen(str);
    // Or: fast_strlen, or strlen.
  }
  return 0;
}