Branch prediction at php

Question

Just read a great post about branch prediction. I was trying to reproduce it using php language.

<?php

function microtime_float()
{
    list($usec, $sec) = explode(" ", microtime());
    return ((float)$usec + (float)$sec);
}

$time_start = microtime_float();

$count = 300000;
$sum = 0;
for ($i = 0; $i <= $count; $i++) {
    $array[] = rand(0, $count);
}

sort($array);

for ($i = 0; $i <= $count; $i++) {
    if ($array[$i] <= 150000) {
        $sum += $array[$i];
    }
}

$time_end = microtime_float();
$time = $time_end - $time_start;

echo $sum . '<br />';
echo 'End:' . $time;
?>

But I always get the same results with sorting and without it. Maybe I'm doing something wrong? Or maybe php has built in optimization for branch predictor?

UPD:

I made code modifications according to comments and measure the time on my local machine.

Not sorted array: 1.108197927475

Sorted array: 1.6477839946747

Difference: 0.539586067.

I think this difference spent on sorting. It looks like true that branch predictor has no impact on speed.

Answer 1

You won't replicate this in PHP. End of story. The reason is that the Java RTS uses JiT compilation techniques to compile the Java intermediate code down to the underlying X86 order code. This underlying order code will expose these branch prediction artefacts.

The PHP runtime system compiles PHP down to a bytecode which is a pseudo machine code that is interpreted. This interpreter will execute of the order of 0.5M opcodes /sec on a typical single core -- that is each PHP opcode takes perhaps 2-6K native instructions. Any subtleties of branching will be lost in this.