dawn_wire: Use memcpy in Serializer / Deserializer when possible

This patch uses memcpy to copy the arrays in basic types instead
of iterating every elements in a for-loop.

In the next step we will copy specific structures with memcpy.

With this patch, the performance of dawn_perf_tests
BufferUploadPerf.Run/*_WriteBuffer_BufferSize_* with "-w" will
be greatly improved (~30%).

BUG=chromium:1266727

Change-Id: I7c6fb0fafa63bd6b602eeef8cf2c0ae0cfc7b0be
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/71180
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>

generator/dawn_json_generator.py

@@ -84,6 +84,7 @@ class Type:
         self.dict_name = name
         self.name = Name(name, native=native)
         self.category = json_data['category']
+        self.is_wire_transparent = False
 
 
 EnumValue = namedtuple('EnumValue', ['name', 'value', 'valid', 'json_data'])
@@ -113,6 +114,7 @@ class EnumType(Type):
                 raise Exception("Duplicate value {} in enum {}".format(
                     value.value, name))
             all_values.add(value.value)
+        self.is_wire_transparent = True
 
 
 BitmaskValue = namedtuple('BitmaskValue', ['name', 'value', 'json_data'])
@@ -128,6 +130,7 @@ class BitmaskType(Type):
         self.full_mask = 0
         for value in self.values:
             self.full_mask = self.full_mask | value.value
+        self.is_wire_transparent = True
 
 
 class CallbackType(Type):
@@ -145,6 +148,7 @@ class TypedefType(Type):
 class NativeType(Type):
     def __init__(self, is_enabled, name, json_data):
         Type.__init__(self, name, json_data, native=True)
+        self.is_wire_transparent = True
 
 
 # Methods and structures are both "records", so record members correspond to

generator/templates/dawn_wire/WireCmd.cpp

@@ -269,12 +269,18 @@
                 {{member_transfer_type(member)}}* memberBuffer;
                 WIRE_TRY(buffer->NextN(memberLength, &memberBuffer));
 
+                {% if member.type.is_wire_transparent %}
+                    memcpy(
+                        memberBuffer, record.{{memberName}},
+                        {{member_transfer_sizeof(member)}} * memberLength);
+                {% else %}
                     //* This loop cannot overflow because it iterates up to |memberLength|. Even if
-                //* memberLength were the maximum integer value, |i| would become equal to it just before
+                    //* memberLength were the maximum integer value, |i| would become equal to it
-                //* exiting the loop, but not increment past or wrap around.
+                    //* just before exiting the loop, but not increment past or wrap around.
                     for (decltype(memberLength) i = 0; i < memberLength; ++i) {
                         {{serialize_member(member, "record." + memberName + "[i]", "memberBuffer[i]" )}}
                     }
+                {% endif %}
             }
         {% endfor %}
         return WireResult::Success;
@@ -392,12 +398,23 @@
                     record->{{memberName}} = copiedMembers;
                 {% endif %}
 
+                {% if member.type.is_wire_transparent %}
+                    //* memcpy is not allowed to copy from volatile objects. However, these arrays
+                    //* are just used as plain data, and don't impact control flow. So if the
+                    //* underlying data were changed while the copy was still executing, we would
+                    //* get different data - but it wouldn't cause unexpected downstream effects.
+                    memcpy(
+                        copiedMembers,
+                        const_cast<const {{member_transfer_type(member)}}*>(memberBuffer),
+                        {{member_transfer_sizeof(member)}} * memberLength);
+                {% else %}
                     //* This loop cannot overflow because it iterates up to |memberLength|. Even if
-                //* memberLength were the maximum integer value, |i| would become equal to it just before
+                    //* memberLength were the maximum integer value, |i| would become equal to it
-                //* exiting the loop, but not increment past or wrap around.
+                    //* just before exiting the loop, but not increment past or wrap around.
                     for (decltype(memberLength) i = 0; i < memberLength; ++i) {
                         {{deserialize_member(member, "memberBuffer[i]", "copiedMembers[i]")}}
                     }
+                {% endif %}
             }
         {% endfor %}