zend_vm: Add OPcode specialization for === []

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.

Author: TimWolla

Zend/zend_vm_def.h

@@ -10062,6 +10062,19 @@ ZEND_VM_HOT_TYPE_SPEC_HANDLER(ZEND_IS_NOT_EQUAL|ZEND_IS_NOT_IDENTICAL, (op1_info
 	ZEND_VM_SMART_BRANCH(result, 0);
 }
 
+ZEND_VM_TYPE_SPEC_HANDLER(ZEND_IS_IDENTICAL, op->op2_type == IS_CONST && (Z_TYPE_P(RT_CONSTANT(op, op->op2)) == IS_ARRAY && zend_hash_num_elements(Z_ARR_P(RT_CONSTANT(op, op->op2))) == 0), ZEND_IS_IDENTICAL_EMPTY_ARRAY, TMPVARCV, CONST, SPEC(SMART_BRANCH,COMMUTATIVE))
+{
+	USE_OPLINE
+	zval *op1;
+	bool result;
+
+	op1 = GET_OP1_ZVAL_PTR_DEREF(BP_VAR_R);
+	result = Z_TYPE_P(op1) == IS_ARRAY && zend_hash_num_elements(Z_ARR_P(op1)) == 0;
+	FREE_OP1();
+	FREE_OP2();
+	ZEND_VM_SMART_BRANCH(result, 0);
+}
+
 ZEND_VM_TYPE_SPEC_HANDLER(ZEND_IS_IDENTICAL, op->op1_type == IS_CV && (op->op2_type & (IS_CONST|IS_CV)) && !(op1_info & (MAY_BE_UNDEF|MAY_BE_REF)) && !(op2_info & (MAY_BE_UNDEF|MAY_BE_REF)), ZEND_IS_IDENTICAL_NOTHROW, CV, CONST|CV, SPEC(COMMUTATIVE))
 {
 	/* This is declared below the specializations for MAY_BE_LONG/MAY_BE_DOUBLE so those will be used instead if possible. */