Cleanup and optimize attribute value reading (php#13897)

When the attribute has a single text child, we can avoid an allocating
call to libxml2 and read the contents directly.

On my i7-4790, I tested the optimization with both the $value and
$nodeValue property.

```
Summary
  ./sapi/cli/php bench_value.php ran
    1.82 ± 0.09 times faster than ./sapi/cli/php_old bench_value.php

Summary
  ./sapi/cli/php bench_nodeValue.php ran
    1.78 ± 0.10 times faster than ./sapi/cli/php_old bench_nodeValue.php
```

Test code:
```
$dom = new DOMDocument;
$dom->loadXML('<root attrib="this is a relatively short text"/>');
$attrib = $dom->documentElement->attributes[0];

for ($i=0; $i<1000*1000; $i++) {
	$attrib->value; // or nodeValue
}
```

Author: nielsdos

ext/dom/attr.c

@@ -113,19 +113,9 @@ URL: http://www.w3.org/TR/2003/WD-DOM-Level-3-Core-20030226/DOM3-Core.html#ID-22
 */
 zend_result dom_attr_value_read(dom_object *obj, zval *retval)
 {
-	DOM_PROP_NODE(xmlAttrPtr, attrp, obj);
-	xmlChar *content;
-
-	/* Can't avoid a content copy because it's an attribute node */
-	if ((content = xmlNodeGetContent((xmlNodePtr) attrp)) != NULL) {
-		ZVAL_STRING(retval, (char *) content);
-		xmlFree(content);
-	} else {
-		ZVAL_EMPTY_STRING(retval);
-	}
-
+	DOM_PROP_NODE(xmlNodePtr, attrp, obj);
+	php_dom_get_content_into_zval(attrp, retval, false);
 	return SUCCESS;
-
 }
 
 zend_result dom_attr_value_write(dom_object *obj, zval *newval)

ext/dom/php_dom.c

@@ -2255,36 +2255,60 @@ void dom_remove_all_children(xmlNodePtr nodep)
 	}
 }
 
-void php_dom_get_content_into_zval(const xmlNode *nodep, zval *retval, bool default_is_null)
+void php_dom_get_content_into_zval(const xmlNode *nodep, zval *return_value, bool null_on_failure)
 {
 	ZEND_ASSERT(nodep != NULL);
 
-	if (nodep->type == XML_TEXT_NODE
-		|| nodep->type == XML_CDATA_SECTION_NODE
-		|| nodep->type == XML_PI_NODE
-		|| nodep->type == XML_COMMENT_NODE) {
-		char *str = (char * ) nodep->content;
-		if (str != NULL) {
-			ZVAL_STRING(retval, str);
-		} else {
-			goto failure;
+	switch (nodep->type) {
+		case XML_TEXT_NODE:
+		case XML_CDATA_SECTION_NODE:
+		case XML_PI_NODE:
+		case XML_COMMENT_NODE: {
+			char *str = (char * ) nodep->content;
+			if (str != NULL) {
+				RETURN_STRING(str);
+			}
+
+			break;
 		}
-		return;
-	}
 
-	char *str = (char *) xmlNodeGetContent(nodep);
+		case XML_ATTRIBUTE_NODE: {
+			/* For attributes we can also have an optimized fast-path.
+			 * This fast-path is only possible in the (common) case where the attribute
+			 * has a single text child. Note that if the child or the content is NULL, this
+			 * is equivalent to not having content (i.e. the attribute has the empty string as value). */
 
-	if (str != NULL) {
-		ZVAL_STRING(retval, str);
-		xmlFree(str);
-		return;
+			if (nodep->children == NULL) {
+				RETURN_EMPTY_STRING();
+			}
+
+			if (nodep->children->type == XML_TEXT_NODE && nodep->children->next == NULL) {
+				if (nodep->children->content == NULL) {
+					RETURN_EMPTY_STRING();
+				} else {
+					RETURN_STRING((const char *) nodep->children->content);
+				}
+			}
+
+			ZEND_FALLTHROUGH;
+		}
+
+		default: {
+			char *str = (char *) xmlNodeGetContent(nodep);
+			if (str != NULL) {
+				RETVAL_STRING(str);
+				xmlFree(str);
+				return;
+			}
+
+			break;
+		}
 	}
 
-failure:
-	if (default_is_null) {
-		ZVAL_NULL(retval);
+	if (null_on_failure) {
+		RETURN_NULL();
 	} else {
-		ZVAL_EMPTY_STRING(retval);
+		RETURN_EMPTY_STRING();
 	}
 }