三元式(ternary)性能优化

PHP 5.4 由Arnaud 引入了一个对三元式的优化方案.

我们都知道PHP用写时复制来对变量复制做性能优化, 而在以前的三元式中, 却每次都会复制, 这在操作数是大数组的情况下, 会造成性能问题:

<?php
$a = range(1, 1000);
$i = 0;
$start = microtime(true);
while (++$i < 1000) {
    $b = isset($a)? $a : NULL;
}
var_dump(microtime(true) - $start);

相比, 我们采用if-else来做同样的功能:

<?php
$a = range(1, 1000);
$i = 0;
$start = microtime(true);
while (++$i < 1000) {
    if (isset($a)) {
        $b = $a;
    } else {
        $b = NULL;
    }
}
var_dump(microtime(true) - $start);

前者在我的机器上, 运行时间为: float(0.0448620319366), 而采用if-else则是: float(0.000280006027222)

为此, Arnaud提供了一个patch, 来对三元式做了一个优化, 使得三元式不会每次都复制操作数, 在优化以后, 开头给的例子的运行时间降低为: float(0.00029182434082031)

> The ternary operator always copies its second or third operand, which is very
>
> slow compared to an if/else when the operand is an array for example:
>
> $a = range(0,9);
>
> // this takes 0.3 seconds here:
>
> for ($i = 0; $i < 5000000; ++$i) {
> if (true) {
> $b = $a;
> } else {
> $b = $a;
> }
> }
> // this takes 3.8 seconds:
> for ($i = 0; $i < 5000000; ++$i) {
> $b = true ? $a : $a;
> }
> I've tried to reduce the performance hit by avoiding the copy when possible
> (patch attached).
> Benchmark:
> Without patch: (the numbers are the time taken to run the code a certain
> amount of times)
> $int = 0;
> $ary = array(1,2,3,4,5,6,7,8,9);
> true ? 1 : 0 0.124
> true ? 1+0 : 0 0.109
> true ? $ary : 0 2.020 !
> true ? $int : 0 0.103
> true ? ${'ary'} : 0 2.290 !
> true ?: 0 0.091
> 1+0 ?: 0 0.086
> $ary ?: 0 2.151 !
> ${'var'} ?: 0 2.317 !
> With patch:
> true ? 1 : 0 0.124
> true ? 1+0 : 0 0.195
> true ? $ary : 0 0.103
> true ? $int : 0 0.089
> true ? ${'ary'} : 0 0.103
> true ?: 0 0.086
> 1+0 ?: 0 0.159
> $cv ?: 0 0.090
> ${'var'} ?: 0 0.089
> The array copying overhead is eliminated. There is however a slowdown in some
> of the cases, but overall there is no completely unexpected performance hit as
> it is the case currently.

不过, 还是要提醒下: PHP 5.4还处于开发阶段, 在最终release之前, 任何新特性都可能被调整或者更改. 如果大家有任何建议, 也欢迎反馈, 帮助我们使得PHP变得更好.

谢谢

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