Re: multiply by 2

Just for confirmation: Linux Mandrake 8.1, PHP 4.0.6, PIII 600MHz, using Jason's code: - Original code via HTTP: 1st attempt 3.4497429132462 3.1789749860764 [7.85% faster with bit op's] 2nd attempt (reload) 3.3336659669876 3.1573359966278 [5.29% faster with bit op's] - Original code from the command line: [bogdan@localhost bogdan]$ php -q ../httpd/mul.php 3.2482839822769 3.1069300174713 [4.35% faster with bit op's] [bogdan@localhost bogdan]$ php -q ../httpd/mul.php 3.2525320053101 3.1399850845337 [3.46% faster with bit op's] - Same code, but with $j*4 and $j<<2: [bogdan@localhost bogdan]$ php -q ../httpd/mul.php 3.23199903965 [3.0816890001297] 4.65% faster with bit op's - Same code with $j*256 and $j<<8: [bogdan@localhost bogdan]$ php -q ../httpd/mul.php 3.2386699914932 3.0807039737701 [4.88% faster with bit op's] - Original code (*2 and <<1), but also added $q=$p/2; and $q=$p>>1; respectively in the two loops: [bogdan@localhost bogdan]$ php -q ../httpd/mul.php 4.6924660205841 4.4853490591049 [4.41% faster with bit op's] Switching the loops (making the bit op's loop first and the multiplication loop second) doesn't change the results. Bogdan Jason Wong wrote:

On Monday 20 May 2002 12:03, Martin Towell wrote:

I was doing some timing of a function on the weekend and in one of the loops, I had to multiply a variable by 2 - easy enough - $i*2 but then I remembered that in C, it's quicker to do a left shift - so $i<<1 but this actually took longer to execute in php than the $i*2 - can anyone confirm my test? I iterated ~28,000 times. Each time, there was 2 places where I was doing the left shift. The time (including all the other stuff I was doing in the function) for $i*2 was ~12secs, and $i<<1 was ~19secs. I am running PHP4.0.6 (download cgi version from php.net) on a win98 system (500MHz Celeron if that changes anything)

My tests don't show much difference, shifting is slightly faster. <?php ini_set('max_execution_time', 600); function gettime() { list($usec, $sec) = explode(" ", microtime()); return ((float)$usec + (float)$sec); } $start=gettime(); $j = 24; for($i = 1; $i <= 1000000; $i++) { $p = $j * 2; #echo $p; } $end=gettime(); echo $end - $start, "\n"; $start=gettime(); $j = 24; for($i = 1; $i <= 1000000; $i++) { $p = $j << 1; #echo $p; } $end=gettime(); echo $end - $start, "\n"; ?> Results ======= [jason@x27 jason]$ php -q doo.php 7.4377170801163 7.2966409921646 [jason@x27 jason]$ php -q doo.php 7.4303779602051 7.2926670312881