when Node_Kind_Type | Entity_Kind_Type | Convention_Id => 8,
- when Mechanism_Type
- | List_Id
- | Elist_Id
- | Name_Id
- | String_Id
- | Uint
- | Ureal
- | Source_Ptr
- | Union_Id
- | Node_Id
- | Node_Or_Entity_Type => 32,
+ when Mechanism_Type
+ | List_Id
+ | Elist_Id
+ | Name_Id
+ | String_Id
+ | Uint
+ | Ureal
+ | Source_Ptr
+ | Union_Id
+ | Node_Id
+ | Node_Or_Entity_Type => 32,
when Between_Special_And_Abstract_Node_Types => -- can't happen
Bit_Offset'Last);
+ -- Size in bits of a a field of type T. It must be a power of 2, and
+ -- must match the size of the type in GNAT, which sometimes requires
+ -- a Size clause in GNAT.
+ --
-- Note that this is not the same as Type_Bit_Size of the field's
-- type. For one thing, Type_Bit_Size only covers concrete node and
-- entity types, which does not include most of the above. For
end loop;
-- Sort All_Fields based on how many concrete types have the field.
+ -- This is for efficiency; we want to choose the offsets of the most
+ -- common fields first, so they get low numbers.
Sorting.Sort (All_Fields);
-- Go through all the fields, and choose the lowest offset that is
- -- free in all types that have the field.
+ -- free in all types that have the field. This is basically a
+ -- graph-coloring algorithm on the interference graph. The
+ -- interference graph is an undirected graph with the fields being
+ -- nodes (not nodes in the compiler!) in the graph, and an edge
+ -- between a pair of fields if they appear in the same node in the
+ -- compiler. The "colors" are fields offsets, except that a
+ -- complication compared to standard graph coloring is that fields
+ -- are different sizes.
for F of All_Fields loop
Field_Table (F).Offset := Choose_Offset (F);