sub _i_max {{
    my $h = shift;
    return $h unless @_;
    $_[0] = $h if $h > $_[0];
    redo;
}}
sub i_max{ _i_max( @{[ @_ ]} ) }
__END__
C:\test>junk3 -N=5e3
           Rate      amp    r_max unrolled    i_max
amp      70.4/s       --     -48%     -52%     -73%
r_max     136/s      94%       --      -7%     -47%
unrolled  147/s     110%       8%       --     -43%
i_max     257/s     265%      88%      74%       --

C:\test>junk3 -N=5e4
           Rate      amp    r_max unrolled    i_max
amp      6.79/s       --     -47%     -53%     -72%
r_max    12.7/s      88%       --     -12%     -47%
unrolled 14.5/s     113%      14%       --     -39%
i_max    23.9/s     252%      87%      65%       --

C:\test>junk3 -N=5e5
            (warning: too few iterations for a reliable count)

            Rate      amp    r_max unrolled    i_max
amp      0.678/s       --     -47%     -53%     -72%
r_max     1.27/s      88%       --     -12%     -47%
unrolled  1.44/s     112%      13%       --     -39%
i_max     2.38/s     251%      87%      65%       --