********************
Sparse's Type System
********************

struct symbol is used to represent symbols & types but
most parts pertaining to the types are in the field 'ctype'.
For the purpose of this document, things can be simplified into:

.. code-block:: c

	struct symbol {
		enum type type;	// SYM_...
		struct ctype {
			struct symbol *base_type;
			unsigned long modifiers;
			unsigned long alignment;
			struct context_list *contexts;
			struct indent *as;
		};
	};

Some bits, also related to the type, are in struct symbol itself:
  * type
  * size_bits
  * rank
  * variadic
  * string
  * designated_init
  * forced_arg
  * accessed
  * transparent_union

* ``base_type`` is used for the associated base type.
* ``modifiers`` is a bit mask for type specifiers (MOD_UNSIGNED, ...),
  type qualifiers (MOD_CONST, MOD_VOLATILE),
  storage classes (MOD_STATIC, MOD_EXTERN, ...), as well for various
  attributes. It's also used internally to keep track of some states
  (MOD_ACCESS or MOD_ADDRESSABLE).
* ``alignment`` is used for the alignment, in bytes.
* ``contexts`` is used to store the informations associated with the
  attribute ``context()``.
* ``as`` is used to hold the identifier of the attribute ``address_space()``.

Kind of types
=============

SYM_BASETYPE
------------
Used by integer, floating-point, void, 'type', 'incomplete' & bad types.

For integer types:
  * .ctype.base_type points to ``int_ctype``, the generic/abstract integer type
  * .ctype.modifiers has MOD_UNSIGNED/SIGNED/EXPLICITLY_SIGNED set accordingly.

For floating-point types:
  * .ctype.base_type points to ``fp_ctype``, the generic/abstract float type
  * .ctype.modifiers is zero.

For the other base types:
  * .ctype.base_type is NULL
  * .ctype.modifiers is zero.

SYM_NODE
--------
It's used to make variants of existing types. For example,
it's used as a top node for all declarations which can then
have their own modifiers, address_space, contexts or alignment
as well as the declaration's identifier.

Usage:
  * .ctype.base_type points to the unmodified type (which must not
    be a SYM_NODE itself)
  * .ctype.modifiers, .as, .alignment, .contexts will contains
    the 'variation' (MOD_CONST, the attributes, ...).

SYM_PTR
-------
For pointers:
  * .ctype.base_type points to the pointee type
  * .ctype.modifiers & .as are about the pointee too!

SYM_FN
------
For functions:
  * .ctype.base_type points to the return type
  * .ctype.modifiers & .as should be about the function itself
    but some return type's modifiers creep here (for example, in
    int foo(void), MOD_SIGNED will be set for the function).

SYM_ARRAY
---------
For arrays:
  * .ctype.base_type points to the underlying type
  * .ctype.modifiers & .as are a copy of the parent type (and unused)?
  * for literal strings, the modifier also contains MOD_STATIC
  * sym->array_size is *expression* for the array size.

SYM_STRUCT
----------
For structs:
  * .ctype.base_type is NULL
  * .ctype.modifiers & .as are not used?
  * .ident is the name tag.

SYM_UNION
---------
Same as for structs.

SYM_ENUM
--------
For enums:
  * .ctype.base_type points to the underlying type (integer)
  * .ctype.modifiers contains the enum signedness
  * .ident is the name tag.

SYM_BITFIELD
------------
For bitfields:
  * .ctype.base_type points to the underlying type (integer)
  * .ctype.modifiers & .as are a copy of the parent type (and unused)?
  * .bit_size is the size of the bitfield.

SYM_RESTRICT
------------
Used for bitwise types (aka 'restricted' types):
  * .ctype.base_type points to the underlying type (integer)
  * .ctype.modifiers & .as are like for SYM_NODE and the modifiers
    are inherited from the base type with MOD_SPECIFIER removed
  * .ident is the typedef name (if any).

SYM_FOULED
----------
Used for bitwise types when the negation op (~) is
used and the bit_size is smaller than an ``int``.
There is a 1-to-1 mapping between a fouled type and
its parent bitwise type.

Usage:
  * .ctype.base_type points to the parent type
  * .ctype.modifiers & .as are the same as for the parent type
  * .bit_size is bits_in_int.

SYM_TYPEOF
----------
Should not be present after evaluation:
  * .initializer points to the expression representing the type
  * .ctype is not used.

Typeofs with a type as argument are directly evaluated during parsing.

SYM_LABEL
---------
Used for labels only.

SYM_KEYWORD
-----------
Used for parsing only.

SYM_BAD
-------
Should not be used.

SYM_UNINTIALIZED
----------------
Should not be used.
