Add heuristic to order non bit fields in ComputedStyleBase.

third_party/WebKit/Source/build/scripts/make_computed_style_base.py

Index: third_party/WebKit/Source/build/scripts/make_computed_style_base.py
diff --git a/third_party/WebKit/Source/build/scripts/make_computed_style_base.py b/third_party/WebKit/Source/build/scripts/make_computed_style_base.py
index b52ab45c015cddf0ee10ad7f4a8b266460fa3d80..bff95db7c6d4593b300c3d775aeab00952cf1f5b 100755
--- a/third_party/WebKit/Source/build/scripts/make_computed_style_base.py
+++ b/third_party/WebKit/Source/build/scripts/make_computed_style_base.py
@@ -17,6 +17,19 @@ from name_utilities import (
 from collections import defaultdict, OrderedDict
 from itertools import chain
 
+# We want to sort fields by their alignment sizes. Specifying the exact alignment requirements

[Bugs Nash, 2017/04/28 00:16:51]

Add to the comment where the types hard coded here have come from.

Also perhaps reword to "Heuristic ordering of types from largest to smallest,
used to sort fields by their alignment sizes. Specifying the exact blah blah".  

[shend, 2017/04/28 01:00:31]

done

+# for each type is impossible because it's platform specific. Instead, we will define an ordering
+# on types, from largest alignment size to smallest. This heuristic should work most of the time.
+ALIGNMENT_ORDER = [
+    'FillLayer', 'BorderData',  # Aligns like a void *
+    'double',
+    'LengthBox', 'Length', 'float',
+    'Color', 'unsigned', 'int',
+    'short',
+    'char',
+    'bool'
+]
+
 # TODO(shend): Improve documentation and add docstrings.
 
 
@@ -272,14 +285,7 @@ def _create_fields(properties):
     return fields
 
 
-def _reorder_fields(fields):
-    """
-    Returns a list of fields ordered to minimise padding.
-    """
-    # Separate out bit fields from non bit fields
-    bit_fields = [field for field in fields if field.is_bit_field]
-    non_bit_fields = [field for field in fields if not field.is_bit_field]
-
+def _reorder_bit_fields(bit_fields):
     # Since fields cannot cross word boundaries, in order to minimize
     # padding, group fields into buckets so that as many buckets as possible
     # are exactly 32 bits. Although this greedy approach may not always
@@ -305,8 +311,28 @@ def _reorder_fields(fields):
         if not added_to_bucket:
             field_buckets.append([field])
 
+    return _flatten_list(field_buckets)
+
+
+def _reorder_non_bit_fields(non_bit_fields):
+    # A general rule of thumb is to sort members by their alignment requirement
+    # (from biggest aligned to smallest). If we don't know its alignment, we
+    # assume the worst and put it at the front.
+    known_alignment = [field for field in non_bit_fields if field.type_name in ALIGNMENT_ORDER]

[Bugs Nash, 2017/04/28 00:16:51]

nit: swap these lines so that unknown_alignment comes first (as it does in the
return statement)

[shend, 2017/04/28 01:00:30]

done

+    unknown_alignment = [field for field in non_bit_fields if field.type_name not in ALIGNMENT_ORDER]
+    return unknown_alignment + list(sorted(known_alignment, key=lambda f: ALIGNMENT_ORDER.index(f.type_name)))
+
+
+def _reorder_fields(fields):
+    """
+    Returns a list of fields ordered to minimise padding.
+    """
+    # Separate out bit fields from non bit fields
+    bit_fields = [field for field in fields if field.is_bit_field]
+    non_bit_fields = [field for field in fields if not field.is_bit_field]
+
     # Non bit fields go first, then the bit fields.
-    return list(non_bit_fields) + _flatten_list(field_buckets)
+    return _reorder_non_bit_fields(non_bit_fields) + _reorder_bit_fields(bit_fields)
 
 
 class ComputedStyleBaseWriter(make_style_builder.StyleBuilderWriter):