zend_vm: Add OPcode specialization for === []

[TimWolla]

Checking whether an array is empty with a strict comparison against the empty array is a common pattern in PHP. A GitHub search for "=== []" language:PHP reveals 44k hits. From the set of !$a, count($a) === 0, empty($a) and $a === [] it however is also the slowest option.

A test script:

<?php

$variable = array_fill(0, 10, random_int(1, 2));

$f = true;
for ($i = 0; $i < 50_000_000; $i++) {
	$isEmpty = $variable === [];
	$f = $f && $isEmpty;
}

var_dump($f);

with the $isEmpty = …; statement appropriately replaced results in:

Benchmark 1: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 count.php
  Time (mean ± σ):     467.6 ms ±   2.3 ms    [User: 463.3 ms, System: 3.4 ms]
  Range (min … max):   464.6 ms … 473.4 ms    10 runs

Benchmark 2: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 empty.php
  Time (mean ± σ):     305.3 ms ±   0.3 ms    [User: 302.0 ms, System: 3.1 ms]
  Range (min … max):   304.9 ms … 305.7 ms    10 runs

Benchmark 3: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 identical.php
  Time (mean ± σ):     630.3 ms ±   3.9 ms    [User: 624.8 ms, System: 3.8 ms]
  Range (min … max):   627.4 ms … 637.6 ms    10 runs

Benchmark 4: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 not.php
  Time (mean ± σ):     311.8 ms ±   3.4 ms    [User: 307.9 ms, System: 3.6 ms]
  Range (min … max):   308.7 ms … 320.7 ms    10 runs

Summary
  sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 empty.php ran
    1.02 ± 0.01 times faster than sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 not.php
    1.53 ± 0.01 times faster than sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 count.php
    2.06 ± 0.01 times faster than sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 identical.php

This patch adds another OPcode specialization for ZEND_IS_IDENTICAL that specifically matches a comparison against the empty array. With this specialization the === [] check becomes the fastest of them all, which is not surprising given how specific it is:

Benchmark 1: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 count.php
  Time (mean ± σ):     384.1 ms ±   2.3 ms    [User: 379.3 ms, System: 3.8 ms]
  Range (min … max):   382.2 ms … 389.8 ms    10 runs

Benchmark 2: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 empty.php
  Time (mean ± σ):     305.8 ms ±   3.2 ms    [User: 301.7 ms, System: 3.8 ms]
  Range (min … max):   304.4 ms … 314.9 ms    10 runs

  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.

Benchmark 3: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 identical.php
  Time (mean ± σ):     293.9 ms ±   2.9 ms    [User: 289.7 ms, System: 3.3 ms]
  Range (min … max):   291.5 ms … 299.4 ms    10 runs

Benchmark 4: sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 not.php
  Time (mean ± σ):     306.8 ms ±   0.4 ms    [User: 303.8 ms, System: 2.7 ms]
  Range (min … max):   306.3 ms … 307.3 ms    10 runs

Summary
  sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 identical.php ran
    1.04 ± 0.01 times faster than sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 empty.php
    1.04 ± 0.01 times faster than sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 not.php
    1.31 ± 0.02 times faster than sapi/cli/php -d zend_extension=modules/opcache.so -d opcache.enable_cli=1 count.php

As a follow-up optimization it might be possible to transform the other emptiness checks, such as count($arr) === 0 into $arr === [] if $arr is known to be MAY_BE_ARRAY only.

[TimWolla]

Will follow up with !== [] if this one is good.

[TimWolla]

Why do I manually need to check op->op2_type == IS_CONST here? Shouldn't that be the job of the CONST in ZEND_IS_IDENTICAL_EMPTY_ARRAY, TMPVARCV, CONST?

[TimWolla]

More generally speaking, I'm not at all sure if the “flags” I specified here are correct, since I clearly don't understand the implications. So please carefully check this 😄

[iluuu1994]

You can look at the generated zend_vm_set_opcode_handler_ex(). It assumes op types are correct. If it works only for some op types, checking the op type should be the right approach. The signature looks correct to me.

[iluuu1994]

I benchmarked this against Symfony Demo and saw varying but consistent improvements of ~0.1% on average. Given the 3 additional handlers are quite small, I would personally support this.

[TimWolla]

Given the 3 additional handlers are quite small,

Without the SMART_BRANCH it's only one handler. I don't know what exactly that does and whether or not it provides a value-add in this case.

I've also just added the corresponding !== [] handler, since you considered the existing one to be good. So we're at additional 6 handlers total.

[iluuu1994]

The smart branch is used when the comparison is immediately followed by a conditional branch. Instead of advancing to the opcode to branch, it branches directly.

[iluuu1994]

I can't measure more improvements for this change, but Symfony+vendors only has ~20 uses of [] !== or !== [], so not a big improvement is expected.