comment on

He was suggesting that you do something like:

my $firstbit = ($longsize * 8) - 1;
my $firstbit_mask = 1 << $firstbit;
[download]

then use $firstbit_mask instead of 1 << $firstbit_mask. This cuts down on the number of shifts to do.

As for the second paragraph:

use constant NUM_BITS = $Config{ivsize} * 8;
use constant SIGN_BIT = 1 << ( NUM_BITS - 1 );
use constant ALL_BITS = -1;

sub sr_signed {
   my ($p, $q) = @_;
   if ($p & SIGN_BIT) {
      return ($p >> $q) | ($ALL_BITS << (NUM_BITS - $q));
   } else {
      return $p >> $q;
   }
}
[download]

Example:

Shift 10101111...10101100 by 3

     10101111...10101100 >> 3
     ||||||||   |||||
      \\\\\\\\   \\\\\
       \\\\\\\\   \\\\\
        ||||||||   |||||
     00010101111...10101

     11111111...11111111 << (NUM_BITS-3)
                     |||
       _____________///
      ///
     |||
     11100000...00000000

     00010101111...10101
   | 11100000...00000000
     -------------------
     11110101111...10101
[download]

(Note that ivsize should be used, not longsize.)

In reply to Re^3: Implementing a signed/sign-propagating right bitwise shift by ikegami
in thread Implementing a signed/sign-propagating right bitwise shift by LonelyPilgrim

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