X-Git-Url: https://git.distorted.org.uk/~mdw/sod/blobdiff_plain/a07d8d005f69c0f9f5da2e09c6ee39cb1e1801aa..dea4d05507e59ab779ed4bb209e05971d87e260c:/layout.org?ds=sidebyside

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-* Instance layout
-
-This is fairly easy.  The superclasses are partitioned into chains.
-Each chain is named after its head class (i.e., the class with no
-successor).
-
-** Things in instance layouts
-
-An instance layout contains a chunk for each component chain.
-
-	struct CLASS__ilayout {
-	  struct CLASS__ichain_CHAINn NICKn;
-	  /* ... */
-	};
-
-An ilayout is a C structure consisting of an ichain for each of the
-class's chains, with the primary chain first.  The others are in
-direct-superclass order.
-
-** Instance slots
-
-An islots structure is a C structure consisting of a class's instance
-slots, in order.
-
-	struct CLASS__islots {
-	  TYPEn SLOTn;
-	  /* ... */
-	};
-
-If a class defines no slots then it has no islots structure.
-
-** Instance chains
-
-	struct CLASS__ichain_CHAIN {
-	  const struct CLASS__vt_CHAIN *_vt;
-	  struct SUPERn__islots NICKn;
-	  /* ... */
-	};
-
-A ichain is a C structure consisting of:
-
-  * A pointer `_vt' to the chain's vtable structure.
-
-  * An islots substructure, named after the class's nick for each class
-    on the chain, least-specific first.
-
-Because of the chain invariant, all of a class's ichains are prefixes of
-the corresponding ichains of any of its subclasses.
-
-The type CLASS is an alias for the class's primary ichain
-CLASS__ichain_CHAIN.  One needs to do a cross-chain upcast to find slots
-in non-primary chains.
-
-* Vtable layout
-
-This is more complicated.  The vtable for a chain doesn't just contain
-things directly relevant to the classes on the chain: because a vtable
-is (assumed) immutable, we can have copies of values from other chains
-where this is convenient.
-
-Note that effective methods are customized for particular classes: they
-can assume that their argument points to a specific ichain of a an
-instance of a specific class.  This makes conversions in effective
-methods very cheap.  By including apparently effective-method pointers
-for messages defined in other chains, we can speed up dispatch.
-
-** Things in a vtable chain
-
-There are three kinds of items to store in a vtable chain.
-
-  * Class pointers
-  * The base offset
-  * Chain offsets
-  * Effective method pointers
-
-	struct CLASS__vt_CHAIN {
-	  struct METACLASS__ichain_sod_object *_class;
-	  size_t _base;
-	  struct METACLASS__ichain_METACHAINn *_cls_NICKn;
-	  ptrdiff_t _off_CHAINn;
-	  struct SUPERn__vtmsgs NICKn;
-	};
-
-A class has a separate vtable chain for each of its chains.
-
-** The base offset
-
-There is a single member _base which is the offset of the chain's ichain
-in the overall ilayout structure.  This lets you find the bottom of the
-ilayout given a pointer to any ichain as
-
-	(CLASS__ilayout *)((char *)p - p->_vt._base)
-
-** Class pointers
-
-The class's metaclass may have multiple chains.  For each chain of the
-metaclass, there is a separate pointer to that metaclass's ichain, named
-_cls_NICKn after the metaclass's chain head.  Exception: _cls_cls is
-called _class instead.
-
-** Chain offsets
-
-For each other chain, there is a member _off_NICKn named after the
-chain's head giving the offset of that ichain from the current chain's
-ichain.  (There's a long way around, exploring the class's layout
-information, but this provides a much easier way of doing cross-chain
-upcasts.)
-
-** Effective method pointers
-
-For each class, there may be a structure
-
-	struct CLASS__vtmsgs {
-	  TYPEn (*MSGn)(ARGnn *, ...);
-	  /* ... */
-	};
-
-of pointers to effective methods for the messages defined by the class.
-If a class defines no messages then it won't have a vtmsgs structure.
-
-** Layout order
-
-The first two items are always _class and _base.  After that:
-
-  * for each class in the chain, from least to most specific,
-
-  * for each of that class's superclasses, in reverse class-precedence-
-    list order, which has not yet been processed:
-
-  * if the class is in a chain which hasn't been seen before (it must be
-    the chain head!), emit a chain offset for it;
-
-  * if the class has a metaclass chain which hasn't been seen before,
-    emit a class pointer for it;
-
-  * if the class has a vtmsgs structure, emit it.
-
-* Questions
-
-Are class-slot initializers inherited?  No.  We have instance
-initializers on metaclasses for that.