@node internals@chapter Internal Architecture of the CompilerThis is meant to describe the C++ frontend for gcc in detail.Questions and comments to mrs@cygnus.com.@index delete, two argumentFor two argument delete, the second argument is always calculated by``virtual_size ='' in the source.  It currently has a problem, in thatobject size is not calculated by the virtual destructor and passed backfor the second parameter to delete.  Destructors need to return a valuejust like constructors.@index visibility checkingVisibility checking in general is unimplemented, there are a few caseswhere it is implemented.  grok_enum_decls should be used in more placesto do visibility checking, but this is only the tip of a bigger problem.@index volatilevolatile is not implemented in general.@index constconst is completely implemented except for function overload selection.@index protected base classesProtected base classes are not fully implemented.@item BLOCK_SUPERCONTEXTIn the outermost scope of each function, it points to the FUNCTION_DECLnode.  It aids in better DWARF support of inline functions.@item DECL_CLASS_CONTEXTIdentifys 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@item DECL_NESTED_TYPENAMEHolds the fully qualified type name.  Example, Base::Derived.Has values of:	IDENTIFIER_NODEWhat things can this be used on:	TYPE_DECLs@item TYPE_NAMENames 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_TYPEsHow 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.@item DECL_NAMEHas values of:	0 for things that don't have names.	IDENTIFIER_NODE for TYPE_DECLs.@item TYPE_DECLUsed 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 @code{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 @xref{DECL_SOURCE_FILE}.@item DECL_IGNORED_PA bit that can be set to inform the debug information output routines inthe backend that a certain _DECL node should be totally ignored.Used in cases where it is known that the debugging information will beoutput in another file, or where a sub-type is known not to be neededbecause the enclosing type is not needed.A compiler constructed virtual destructor in derived classes that do notdefine an exlicit destructor that was defined exlicit in a base classhas this bit set as well.  Also used on __FUNCTION__ and__PRETTY_FUNCTION__ to mark they are ``compiler generated.''  c-decl andc-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 thatthey are compiler generated, but unsure why.Currently, it is used in an absolute way in the C++ front-end, as anoptimization, to tell the debug information output routines to notgenerate debugging information that will be output by another separatelycompiled file.@findex DECL_SOURCE_FILE@item DECL_SOURCE_FILEIdentifies what source file a particular declaration was found in.Has values of:	"<built-in>" on TYPE_DECLs to mean the typedef is built in.@item DECL_SOURCE_LINEIdentifies what source line number in the source file the declarationwas 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.)@item DECL_VIRTUAL_PA flag used on FIELD_DECLs and VAR_DECLs.  (Documentation in tree.h iswrong.)  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.@item DECL_VINDEXUsed for FUNCTION_DECLs in two different ways.  Before the structurecontaining the FUNCTION_DECL is laid out, DECL_VINDEX may point to aFUNCTION_DECL in a base class which is the FUNCTION_DECL which thisFUNCTION_DECL will replace as a virtual function.  When the class islaid out, this pointer is changed to an INTEGER_CST node which issuitable for use as an index into the virtual function table.DECL_VINDEX may be a TREE_LIST, that would seem to be a list ofoverridden FUNCTION_DECLs.  add_virtual_function has code to deal withthis when it uses the variable base_fndecl_list, but it would seem thatsomehow, it is possible for the TREE_LIST to pursist until method_call,and it should not.@item TREE_USEDHas values of:	0 for unused labels.@item TREE_ADDRESSABLEA flag that is set for any type that has a constructor.@item CLASSTYPE_METHOD_VECThe following is true after finish_struct has been called (on theclass?) but not before.  Before finish_struct is called, things aredifferent to some extent.  Contains a TREE_VEC of methods of the class.The TREE_VEC_LENGTH is the number of differently named methods plus onefor the 0th entry.  The 0th entry is always allocated, and reserved forctors 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 agiven name, the DECL_CHAIN slot is NULL_TREE.  Otherwise it is the nextmethod that has the same name (but a different signature).  It wouldseem that it is not true that because the DECL_CHAIN slot is used inthis way, we cannot call pushdecl to put the method in the global scope(cause that would overwrite the TREE_CHAIN slot), because they usedifferent _CHAINs.friends are kept in TREE_LISTs, so that there's no need to use theirTREE_CHAIN slot for anything.Has values of:	TREE_VEC	@item TYPE_METHODRelated to CLASSTYPE_METHOD_VEC.  Chained together with TREE_CHAIN.dbxout.c uses this to get at the methods of a class.@item CLASSTYPE_TAGSCLASSTYPE_TAGS is a linked (via TREE_CHAIN) list of member classes of aclass. TREE_PURPOSE is the name, TREE_VALUE is the type (pushclass scansthese and calls pushtag on them.)finish_struct scans these to produce TYPE_DECLs to add to theTYPE_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 thisuniqueness.@item TYPE_FIELDSTYPE_FIELDS is a linked list (via TREE_CHAIN) of member types of aclass.  The list can contain TYPE_DECLs, but there can also be otherthings in the list apparently.  See also CLASSTYPE_TAGS.@item TREE_PRIVATESet 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@item TREE_PROTECTEDThe 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