use Inline C;

use strict;
use warnings;

# Assembly code via Inline::C to return the
# 1. number of leading zeros of the input integer
# 2. a number with only bit set where the MSSB is located
#
# by bliako
# for https://perlmonks.org/?node_id=11158279
# 21/03/2024

my $z = 17;
my $res = mssb($z);
print "Leading zeros for $z : ".$res->[0]."\n";
print "MSSB for $z : ".sprintf("%032b\n", $res->[1])."\n";
# result:
# Leading zeros for 17 : 27
# MSSB for 17 : 00000000000000000000000000010000

__END__
__C__
#include <stdio.h>

AV * mssb(int input){
	int num_leading_zeros;
	int mssb;
	asm volatile(
/* note: lzcnt inp, out
	 mov src, dst
	 add what, dst
         # set bit of value in dst at zero-based bitposition:
	 btsl bitposition, dst (it modifies dst)
*/
  "lzcnt %[input], %[num_leading_zeros]		\n\t\
   mov $32, %%eax				\n\t\
   sub %[num_leading_zeros], %%eax		\n\t\
   sub $1, %%eax				\n\t\
   xor %[mssb], %[mssb]				\n\t\
   bts %%eax, %[mssb]				\n\t\
  "
/* outputs */
  : [num_leading_zeros] "=r" (num_leading_zeros)
  , [mssb] "=r" (mssb)
/* inputs */
  : [input] "mr"  (input)
/* clobbers: we are messing with these registers */
  : "eax"
	);

 	// return an arrayref of the two outputs
	AV* ret = newAV();
	sv_2mortal((SV*)ret);
	av_push(ret, newSViv(num_leading_zeros));
	av_push(ret, newSViv(mssb));

	return ret;
}