Question

I have the following code:

<?php
$start = 1;

$timestart = microtime(1);
for ($i = 0; $i < 1000000; $i++) {
    $result1 = $start * 4;
}
echo "\n";
echo microtime(1) - $timestart;
echo "\n";

$timestart = microtime(1);
for ($i = 0; $i < 1000000; $i++) {
    $result2 = $start << 2;
}
echo "\n";
echo microtime(1) - $timestart;
echo "\n";

This outputs:

0.14027094841003

0.12061500549316

I found on the Internet a Google interview question (which I wanted to apply for a developer, but I realize I can’t), and one of the questions asked what the fastest way was to multiply a number. My first thought was to use the * sign, so I tested it.

My question is, why is shifting bits faster than multiplication?

Answer 1

Because bit shifting is something the computer does all the time in hardware, it’s a no-brainer for the CPU. Multiplying arbitrary numbers is something more difficult, because it can’t necessarily be done using simple bit shifting but requires actual work. Multiplying a small integer by 4 happens to be an operation that’s identical to a left-shift by 2. But even if the compiler/runtime/CPU optimizes this operation down to a bit shift, some code first needs to recognize that it can be optimized this way, which is more work than a simple bit shift itself.

Either way, it’s simply more work because the two operations do entirely different things, even if the outcome of certain operations is the same.