use strict;
use warnings;
use feature 'say';
use Benchmark 'cmpthese';

use constant L => 1e5;
my $str = '1' x L;
substr $str, rand L, 1 , '0' for 1 .. L;

use Inline C => << 'END_OF_C';

int test_c_3( SV* str ) {
    int i, tmp, acc, max;
    
    STRLEN len;
    char* buf = SvPVbyte( str, len );
    len --;
    
    acc = 0;
    max = -2;
    for( i = 0; i < len; i ++ ) {
        acc += ( buf[ i ] == '1' );         // ***
        tmp = i - 2 * acc;
        if ( tmp > max ) max = tmp;
    }
    
    return max + acc + ( buf[ len ] == '1' ) + 1;
}

int test_c_4( SV* str ) {
    int i, tmp, acc, max;
    
    STRLEN len;
    char* buf = SvPVbyte( str, len );
    len --;
    
    acc = 0;
    max = -2;
    for( i = 0; i < len; i ++ ) {
        if ( buf[ i ] == '1' ) acc ++;      // ***
        tmp = i - 2 * acc;
        if ( tmp > max ) max = tmp;
    }
    
    return max + acc + ( buf[ len ] == '1' ) + 1;
}

END_OF_C

say $^V;
say 'String length: ', L;

cmpthese -2, {
    c3 => sub { test_c_3( $str )},
    c4 => sub { test_c_4( $str )},
};

__END__

v5.32.1
String length: 100000
     Rate  c4  c3
c4 8800/s  -- -0%
c3 8817/s  0%  --

v5.42.0
String length: 100000
     Rate   c4   c3
c4 2511/s   -- -71%
c3 8755/s 249%   --