minor #15434 [property-access] Improvement for Accessing Reference Chain (zerustech)

This PR was merged into the 2.7 branch.

Discussion
----------

[property-access] Improvement for Accessing Reference Chain

Improve performance for the following scenarios:

- Example 1:
```php
$a = array(
    'a' => array(
        'b' => array(
            'c' => 'old-value'
        )
    )
);
$pa->setValue($a, '[a][b][c]', 'new-value');
// The PropertyAccessor will try to set values for
// $a['a']['b']['c'], $a['a']['b'] and $a['a'],
// but in fact it may terminate the loop
// right after the value of $a[a][b][c] is set,
// because $a, $[a], $[a][b] and $[a][b][c]
// are all passed as reference - the reference chain is not broken.
```

- Example 2
```php
$b = array(
    'a' => array(
        'b' => array(
            'c' => 'old-value'
        )
    )
)

$a = new Foo($b);
// In this example, the reference chain of $b is broken,
// because it's passed to $a.value as value
// But its elements are all passed as reference,
// so after setting the value for $b[a][b][c], there is no need
// to set value for $b[a][b] and $b[a]

$pa->setValue($a, 'value[a][b][c]', 'new-value');
```

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

e24a798 [property-access] Improvement for Accessing Reference Chain

src/Symfony/Component/PropertyAccess/PropertyAccessor.php

@@ -25,6 +25,7 @@ class PropertyAccessor implements PropertyAccessorInterface
 {
     const VALUE = 0;
     const IS_REF = 1;
+    const IS_REF_CHAINED = 2;
 
     /**
      * @var bool
@@ -75,6 +76,7 @@ class PropertyAccessor implements PropertyAccessorInterface
         array_unshift($propertyValues, array(
             self::VALUE => &$objectOrArray,
             self::IS_REF => true,
+            self::IS_REF_CHAINED => true,
         ));
 
         for ($i = count($propertyValues) - 1; $i >= 0; --$i) {
@@ -82,14 +84,33 @@ class PropertyAccessor implements PropertyAccessorInterface
 
             $property = $propertyPath->getElement($i);
 
-            if ($propertyPath->isIndex($i)) {
-                $this->writeIndex($objectOrArray, $property, $value);
-            } else {
-                $this->writeProperty($objectOrArray, $property, $value);
-            }
+            // You only need set value for current element if:
+            // 1. it's the parent of the last index element
+            // OR
+            // 2. its child is not passed by reference
+            //
+            // This may avoid uncessary value setting process for array elements.
+            // For example:
+            // '[a][b][c]' => 'old-value'
+            // If you want to change its value to 'new-value',
+            // you only need set value for '[a][b][c]' and it's safe to ignore '[a][b]' and '[a]'
+            //
+            if ($i === count($propertyValues) - 1 || !$propertyValues[$i + 1][self::IS_REF]) {
+                if ($propertyPath->isIndex($i)) {
+                    $this->writeIndex($objectOrArray, $property, $value);
+                } else {
+                    $this->writeProperty($objectOrArray, $property, $value);
+                }
+
+                // if current element is an object
+                // OR
+                // if current element's reference chain is not broken - current element
+                // as well as all its ancients in the property path are all passed by reference,
+                // then there is no need to continue the value setting process
+                if (is_object($propertyValues[$i][self::VALUE]) || $propertyValues[$i][self::IS_REF_CHAINED]) {
+                    return;
+                }
 
-            if ($propertyValues[$i][self::IS_REF] && is_object($objectOrArray)) {
-                return;
             }
 
             $value = &$objectOrArray;
@@ -132,6 +153,7 @@ class PropertyAccessor implements PropertyAccessorInterface
             array_unshift($propertyValues, array(
                 self::VALUE => $objectOrArray,
                 self::IS_REF => true,
+                self::IS_REF_CHAINED => true,
             ));
 
             for ($i = count($propertyValues) - 1; $i >= 0; --$i) {
@@ -149,7 +171,7 @@ class PropertyAccessor implements PropertyAccessorInterface
                     }
                 }
 
-                if ($propertyValues[$i][self::IS_REF] && is_object($objectOrArray)) {
+                if (is_object($propertyValues[$i][self::VALUE]) || $propertyValues[$i][self::IS_REF_CHAINED]) {
                     return true;
                 }
             }
@@ -232,6 +254,13 @@ class PropertyAccessor implements PropertyAccessorInterface
                 throw new UnexpectedTypeException($objectOrArray, $propertyPath, $i + 1);
             }
 
+            // Set the IS_REF_CHAINED flag to true if:
+            // current property is passed by reference and
+            // it is the first element in the property path or
+            // the IS_REF_CHAINED flag of its parent element is true
+            // Basically, this flag is true only when the reference chain from the top element to current element is not broken
+            $propertyValue[self::IS_REF_CHAINED] = $propertyValue[self::IS_REF] && ($i == 0 || $propertyValues[$i - 1][self::IS_REF_CHAINED]);
+
             $propertyValues[] = &$propertyValue;
         }