Internal Architecture of the Compiler
*************************************

This is meant to describe the C++ frontend for gcc in detail. Questions and comments to mrs.com.

For two argument delete, the second argument is always calculated by ``virtual_size ='' in the source.  It currently has a problem, in that object size is not calculated by the virtual destructor and passed back for the second parameter to delete.  Destructors need to return a value just like constructors.

Visibility checking in general is unimplemented, there are a few cases where it is implemented.  grok_enum_decls should be used in more places to do visibility checking, but this is only the tip of a bigger problem.

volatile is not implemented in general.

const is completely implemented except for function overload selection.

Protected base classes are not fully implemented.

BLOCK_SUPERCONTEXT
In the outermost scope of each function, it points to the FUNCTION_DECL node.  It aids in better DWARF support of inline functions.

DECL_CLASS_CONTEXT
Identifys the context that the _DECL was found in.

Has values of:

RECORD_TYPE, or UNION_TYPE.

What things can this be used on:

TYPE_DECLs, *_DECLs

DECL_NESTED_TYPENAME
Holds the fully qualified type name.  Example, Base::Derived.

Has values of:

IDENTIFIER_NODE

What things can this be used on:

TYPE_DECLs

TYPE_NAME
Names the type.

Has values of:

0 for things that don't have names. should be IDENTIFIER_NODE for RECORD_TYPEs UNION_TYPEs and ENUM_TYPEs. TYPE_DECL for RECORD_TYPEs, UNION_TYPEs and ENUM_TYPEs, but shouldn't be. TYPE_DECL for typedefs, unsure why.

What things can one use this on:

TYPE_DECLs RECORD_TYPEs UNION_TYPEs ENUM_TYPEs

How is it used:

Used by dwarfout.c to fetch the name of structs, unoins and enums to create AT_name fields.

History:

It currently points to the TYPE_DECL for RECORD_TYPEs, UNION_TYPEs and ENUM_TYPEs, but it should be history soon.

DECL_NAME

Has values of:

0 for things that don't have names. IDENTIFIER_NODE for TYPE_DECLs.

TYPE_DECL
Used to represent typedefs, and used to represent bindings layers.

Components:

DECL_NAME is the name of the typedef.  For example, foo would be found in the DECL_NAME slot when typedef int foo; is seen.

DECL_SOURCE_LINE identifies what source line number in the source file the declaration was found at.  A value of 0 indicates that this TYPE_DECL is just an internal binding layer marker, and does not correspond to a user suppiled typedef.

DECL_SOURCE_FILE see DECL_SOURCE_FILE.

DECL_IGNORED_P
A bit that can be set to inform the debug information output routines in the backend that a certain _DECL node should be totally ignored.

Used in cases where it is known that the debugging information will be output in another file, or where a sub-type is known not to be needed because the enclosing type is not needed.

A compiler constructed virtual destructor in derived classes that do not define an exlicit destructor that was defined exlicit in a base class has this bit set as well.  Also used on __FUNCTION__ and __PRETTY_FUNCTION__ to mark they are ``compiler generated.''  c-decl and c-lex.c both want DECL_IGNORED_P set for ``internally generated vars,'' and ``user-invisible variable.''

Functions built by the C++ front-end such as default destructors, virtual desctructors and default constructors want to be marked that they are compiler generated, but unsure why.

Currently, it is used in an absolute way in the C++ front-end, as an optimization, to tell the debug information output routines to not generate debugging information that will be output by another separately compiled file.

DECL_SOURCE_FILE
Identifies what source file a particular declaration was found in.

Has values of:

"<built-in>" on TYPE_DECLs to mean the typedef is built in.

DECL_SOURCE_LINE
Identifies what source line number in the source file the declaration was found at.

Has values of:

0 for an undefined label.

0 for TYPE_DECLs that are internally generated.

0 for FUNCTION_DECLs for functions generated by the compiler. (not yet, but should be.)

DECL_VIRTUAL_P
A flag used on FIELD_DECLs and VAR_DECLs.  (Documentation in tree.h is wrong.)  Used in VAR_DECLs to indicate that the variable is a vtable. It is also used in FIELD_DECLs for vtable pointers.

What things can this be used on:

FIELD_DECLs and VAR_DECLs.

DECL_VINDEX
Used for FUNCTION_DECLs in two different ways.  Before the structure containing the FUNCTION_DECL is laid out, DECL_VINDEX may point to a FUNCTION_DECL in a base class which is the FUNCTION_DECL which this FUNCTION_DECL will replace as a virtual function.  When the class is laid out, this pointer is changed to an INTEGER_CST node which is suitable for use as an index into the virtual function table.

DECL_VINDEX may be a TREE_LIST, that would seem to be a list of overridden FUNCTION_DECLs.  add_virtual_function has code to deal with this when it uses the variable base_fndecl_list, but it would seem that somehow, it is possible for the TREE_LIST to pursist until method_call, and it should not.

TREE_USED

Has values of:

0 for unused labels.

TREE_ADDRESSABLE
A flag that is set for any type that has a constructor.

CLASSTYPE_METHOD_VEC
The following is true after finish_struct has been called (on the class?) but not before.  Before finish_struct is called, things are different to some extent.  Contains a TREE_VEC of methods of the class. The TREE_VEC_LENGTH is the number of differently named methods plus one for the 0th entry.  The 0th entry is always allocated, and reserved for ctors and dtors.  If there are none, TREE_VEC_ELT(N,0) == NULL_TREE. Each entry of the TREE_VEC is a FUNCTION_DECL.  For each FUNCTION_DECL, there is a DECL_CHAIN slot.  If the FUNCTION_DECL is the last one with a given name, the DECL_CHAIN slot is NULL_TREE.  Otherwise it is the next method that has the same name (but a different signature).  It would seem that it is not true that because the DECL_CHAIN slot is used in this way, we cannot call pushdecl to put the method in the global scope (cause that would overwrite the TREE_CHAIN slot), because they use different _CHAINs.

friends are kept in TREE_LISTs, so that there's no need to use their TREE_CHAIN slot for anything.

Has values of:

TREE_VEC

TYPE_METHOD
Related to CLASSTYPE_METHOD_VEC.  Chained together with TREE_CHAIN. dbxout.c uses this to get at the methods of a class.

CLASSTYPE_TAGS
CLASSTYPE_TAGS is a linked (via TREE_CHAIN) list of member classes of a class. TREE_PURPOSE is the name, TREE_VALUE is the type (pushclass scans these and calls pushtag on them.)

finish_struct scans these to produce TYPE_DECLs to add to the TYPE_FIELDS of the type.

It is expected that name found in the TREE_PURPOSE slot is unique, resolve_scope_to_name is one such place that depends upon this uniqueness.

TYPE_FIELDS
TYPE_FIELDS is a linked list (via TREE_CHAIN) of member types of a class.  The list can contain TYPE_DECLs, but there can also be other things in the list apparently.  See also CLASSTYPE_TAGS.

TREE_PRIVATE
Set for FIELD_DECLs by finish_struct.  But not uniformly set.

The following routines do something with PRIVATE visibility: build_method_call, alter_visibility, finish_struct_methods, finish_struct, convert_to_aggr, CWriteLanguageDecl, CWriteLanguageType, CWriteUseObject, compute_visibility, lookup_field, dfs_pushdecl, GNU_xref_member, dbxout_type_fields, dbxout_type_method_1

TREE_PROTECTED
The following routines do something with PROTECTED visibility: build_method_call, alter_visibility, finish_struct, convert_to_aggr, CWriteLanguageDecl, CWriteLanguageType, CWriteUseObject, compute_visibility, lookup_field, GNU_xref_member, dbxout_type_fields, dbxout_type_method_1