Generated IPC decoders check the decode success twice

https://bugs.webkit.org/show_bug.cgi?id=258604
rdar://problem/111434133

Reviewed by Alex Christensen.

The check would be like:
  decode property
  check success
  decode property
  check success

On first decode error, the subsequent decodes will fail as per design.
Thus we can omit the redundant caller checks and check only once after
series of decodes, before using the decoded properties.

Instead of checking each decode operation result, check as follows:

  decode properties
  if (!decoder.isValid())
    return std::nullopt;
  use properties
  decode more properties
  if (!decoder.isValid())
    return std::nullopt;
  use properties

This should, very slightly, optimize for the standard case of all
decodes succeeding.

* Source/WebKit/Scripts/generate-serializers.py:
(decode_type):
(generate_impl):
* Source/WebKit/Scripts/webkit/tests/GeneratedSerializers.cpp:
(IPC::ArgumentCoder<Namespace::Subnamespace::StructName>::decode):
(IPC::ArgumentCoder<Namespace::OtherClass>::decode):
(IPC::ArgumentCoder<Namespace::ReturnRefClass>::decode):
(IPC::ArgumentCoder<Namespace::EmptyConstructorStruct>::decode):
(IPC::ArgumentCoder<Namespace::EmptyConstructorNullable>::decode):
(IPC::ArgumentCoder<WithoutNamespace>::decode):
(IPC::ArgumentCoder<WithoutNamespaceWithAttributes>::decode):
(IPC::ArgumentCoder<WebCore::InheritsFrom>::decode):
(IPC::ArgumentCoder<WebCore::InheritanceGrandchild>::decode):
(IPC::ArgumentCoder<WTF::Seconds>::decode):
(IPC::ArgumentCoder<WTF::CreateUsingClass>::decode):
(IPC::ArgumentCoder<WebCore::FloatBoxExtent>::decode):
(IPC::ArgumentCoder<NullableSoftLinkedMember>::decode):
(IPC::ArgumentCoder<WebCore::TimingFunction>::decode):
(IPC::ArgumentCoder<Namespace::ConditionalCommonClass>::decode):
(IPC::ArgumentCoder<Namespace::CommonClass>::decode):
(IPC::ArgumentCoder<Namespace::AnotherCommonClass>::decode):
(IPC::ArgumentCoder<WebCore::MoveOnlyBaseClass>::decode):
(IPC::ArgumentCoder<WebCore::MoveOnlyDerivedClass>::decode):

Canonical link: https://commits.webkit.org/265717@main

Source/WebKit/Scripts/generate-serializers.py

@@ -419,7 +419,7 @@ def decode_type(type):
 
     if type.members_are_subclasses:
         result.append('    auto type = decoder.decode<' + type.subclass_enum_name() + '>();')
-        result.append('    if (!type)')
+        result.append('    if (UNLIKELY(!decoder.isValid()))')
         result.append('        return std::nullopt;')
         result.append('')
 
@@ -433,13 +433,11 @@ def decode_type(type):
             match = re.search("RetainPtr<(.*)>", member.type)
             assert match
             result.append('    auto ' + sanitized_variable_name + ' = IPC::decode<' + match.groups()[0] + '>(decoder, @[ ' + decodable_classes[0] + ' ]);')
-            result.append('    if (!' + sanitized_variable_name + ')')
-            result.append('        return std::nullopt;')
         elif member.is_subclass:
             result.append('    if (type == ' + type.subclass_enum_name() + "::" + member.name + ') {')
             typename = member.namespace + "::" + member.name
             result.append('        auto result = decoder.decode<Ref<' + typename + '>>();')
-            result.append('        if (!result)')
+            result.append('        if (UNLIKELY(!decoder.isValid()))')
             result.append('            return std::nullopt;')
             result.append('        return WTFMove(*result);')
             result.append('    }')
@@ -454,38 +452,34 @@ def decode_type(type):
                 if 'Nullable' in member.attributes:
                     indentation = 2
                     result.append('    auto has' + sanitized_variable_name + ' = decoder.decode<bool>();')
-                    result.append('    if (!has' + sanitized_variable_name + ')')
+                    result.append('    if (UNLIKELY(!decoder.isValid()))')
                     result.append('        return std::nullopt;')
                     result.append('    std::optional<' + member.type + '> ' + sanitized_variable_name + ';')
                     result.append('    if (*has' + sanitized_variable_name + ') {')
                 auto_specifier = '' if 'Nullable' in member.attributes else 'auto '
                 result.append(indent(indentation) + auto_specifier + sanitized_variable_name + ' = IPC::decode<' + match.groups()[0] + '>(decoder, ' + soft_linked_classes[0] + ');')
-                result.append(indent(indentation) + 'if (!' + sanitized_variable_name + ')')
-                result.append(indent(indentation) + '    return std::nullopt;')
                 if 'Nullable' in member.attributes:
                     result.append('    } else')
                     result.append('        ' + sanitized_variable_name + ' = std::optional<' + member.type + '> { ' + member.type + ' { } };')
             elif 'Nullable' in member.attributes:
                 result.append('    auto has' + sanitized_variable_name + ' = decoder.decode<bool>();')
-                result.append('    if (!has' + sanitized_variable_name + ')')
+                result.append('    if (UNLIKELY(!decoder.isValid()))')
                 result.append('        return std::nullopt;')
                 result.append('    std::optional<' + member.type + '> ' + sanitized_variable_name + ';')
                 result.append('    if (*has' + sanitized_variable_name + ') {')
                 # FIXME: This should be below
                 result.append('        ' + sanitized_variable_name + ' = decoder.decode<' + member.type + '>();')
-                result.append('        if (!' + sanitized_variable_name + ')')
-                result.append('            return std::nullopt;')
                 result.append('    } else')
                 result.append('        ' + sanitized_variable_name + ' = std::optional<' + member.type + '> { ' + member.type + ' { } };')
             else:
                 assert len(soft_linked_classes) == 0
                 result.append('    auto ' + sanitized_variable_name + ' = decoder.decode<' + member.type + '>();')
-                result.append('    if (!' + sanitized_variable_name + ')')
-                result.append('        return std::nullopt;')
         for attribute in member.attributes:
             match = re.search(r'Validator=\'(.*)\'', attribute)
             if match:
                 validator, = match.groups()
+                result.append('    if (UNLIKELY(!decoder.isValid()))')
+                result.append('        return std::nullopt;')
                 result.append('')
                 result.append('    if (!(' + validator + '))')
                 result.append('        return std::nullopt;')
@@ -495,7 +489,6 @@ def decode_type(type):
                 assert not match
         if member.condition is not None:
             result.append('#endif')
-        result.append('')
     return result
 
 
@@ -624,6 +617,8 @@ def generate_impl(serialized_types, serialized_enums, headers):
         result.append('{')
         result = result + decode_type(type)
         if not type.members_are_subclasses:
+            result.append('    if (UNLIKELY(!decoder.isValid()))')
+            result.append('        return std::nullopt;')
             if type.populate_from_empty_constructor:
                 result.append('    ' + type.namespace_and_name() + ' result;')
                 for member in type.serialized_members():