bit_scan_f.pl

#!/usr/bin/perl
use 5.16.1;
use strict;
use warnings;
use Inline C => <<'...';
void bit_scan_f(int n, int start) {
  int res[] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};
  __asm__ __volatile__ (
       "xorl %%eax, %%eax\n"
       "leaq (%[res]), %%rdi\n"
       "movl %1, %%ecx\n"
       "movl %0, %%edx\n"
       "inloop:\n"
       "shrl %%cl, %%edx\n"
       "orl %%edx, %%edx\n"
       "jz outloop\n"
       "bsf %%edx, %%ecx\n"
       "addl %%ecx, %%eax\n"
       "movl %%eax, (%%rdi)\n"
       "leaq 4(%%rdi), %%rdi\n"
       "incl %%eax\n"
       "incl %%ecx\n"
       "jmp inloop\n"
       "outloop:\n"
       "cmpl $32, %%eax\n"
       "jge skip128\n"
       "movl $128, (%%rdi)\n"
       "skip128:\n"
      :
      : "r"(n),"r"(start),[res]"r"(&res)
      : "%eax", "%ecx", "%edx", "%rdi"
  );
  Inline_Stack_Vars;
  Inline_Stack_Reset;
  for (int i=0; i<32; i++) {
    if (res[i] == 128) { break; }
    Inline_Stack_Push(sv_2mortal(newSViv(res[i])));
//    printf("%d: %d\n", i, res[i]);
  }
  Inline_Stack_Done;
}
...

say '[' .
    join(', ' => bit_scan_f($ARGV[0] // 0 + 0, $ARGV[1] // 0))
    . ']';