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20. Allocation of Objects in XEmacs Lisp

20.1 Introduction to Allocation  
20.2 Garbage Collection  
20.3 GCPROing  
20.4 Garbage Collection - Step by Step  
20.5 Fixnums and Characters  
20.6 Allocation from Frob Blocks  
20.7 lrecords  
20.8 Low-level allocation  
20.9 Cons  
20.10 Vector  
20.11 Bit Vector  
20.12 Symbol  
20.13 Marker  
20.14 String  
20.15 Compiled Function  


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20.1 Introduction to Allocation

Emacs Lisp, like all Lisps, has garbage collection. This means that the programmer never has to explicitly free (destroy) an object; it happens automatically when the object becomes inaccessible. Most experts agree that garbage collection is a necessity in a modern, high-level language. Its omission from C stems from the fact that C was originally designed to be a nice abstract layer on top of assembly language, for writing kernels and basic system utilities rather than large applications.

Lisp objects can be created by any of a number of Lisp primitives. Most object types have one or a small number of basic primitives for creating objects. For conses, the basic primitive is cons; for vectors, the primitives are make-vector and vector; for symbols, the primitives are make-symbol and intern; etc. Some Lisp objects, especially those that are primarily used internally, have no corresponding Lisp primitives. Every Lisp object, though, has at least one C primitive for creating it.

Recall from section (VII) that a Lisp object, as stored in a 32-bit or 64-bit word, has a few tag bits, and a "value" that occupies the remainder of the bits. We can separate the different Lisp object types into three broad categories:

In the remaining two categories, the type is stored in the object itself. The tag for all such objects is the generic lrecord (Lisp_Type_Record) tag. The first bytes of the object's structure are an integer (actually a char) characterising the object's type and some flags, in particular the mark bit used for garbage collection. A structure describing the type is accessible thru the lrecord_implementation_table indexed with said integer. This structure includes the method pointers and a pointer to a string naming the type.

Note that bit vectors are a bit of a special case. They are simple lrecords as in category (b), but are individually malloc()ed like vectors. You can basically view them as exactly like vectors except that their type is stored in lrecord fashion rather than in directly-tagged fashion.


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20.2 Garbage Collection

Garbage collection is simple in theory but tricky to implement. Emacs Lisp uses the oldest garbage collection method, called mark and sweep. Garbage collection begins by starting with all accessible locations (i.e. all variables and other slots where Lisp objects might occur) and recursively traversing all objects accessible from those slots, marking each one that is found. We then go through all of memory and free each object that is not marked, and unmarking each object that is marked. Note that "all of memory" means all currently allocated objects. Traversing all these objects means traversing all frob blocks, all vectors (which are chained in one big list), and all lcrecords (which are likewise chained).

Garbage collection can be invoked explicitly by calling garbage-collect but is also called automatically by eval, once a certain amount of memory has been allocated since the last garbage collection (according to gc-cons-threshold).


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20.3 GCPROing

GCPROing is one of the ugliest and trickiest parts of Emacs internals. The basic idea is that whenever garbage collection occurs, all in-use objects must be reachable somehow or other from one of the roots of accessibility. The roots of accessibility are:

  1. All objects that have been staticpro()d or staticpro_nodump()ed. This is used for any global C variables that hold Lisp objects. A call to staticpro() happens implicitly as a result of any symbols declared with defsymbol() and any variables declared with DEFVAR_FOO(). You need to explicitly call staticpro() (in the vars_of_foo() method of a module) for other global C variables holding Lisp objects. (This typically includes internal lists and such things.). Use staticpro_nodump() only in the rare cases when you do not want the pointed variable to be saved at dump time but rather recompute it at startup.

    Note that obarray is one of the staticpro()d things. Therefore, all functions and variables get marked through this.

  2. Any shadowed bindings that are sitting on the specpdl stack.
  3. Any objects sitting in currently active (Lisp) stack frames, catches, and condition cases.
  4. A couple of special-case places where active objects are located.
  5. Anything currently marked with GCPRO.

Marking with GCPRO is necessary because some C functions (quite a lot, in fact), allocate objects during their operation. Quite frequently, there will be no other pointer to the object while the function is running, and if a garbage collection occurs and the object needs to be referenced again, bad things will happen. The solution is to mark those references with GCPRO. Note that it is a reference that is marked with GCPRO, not an object. If you declare a Lisp_Object variable, assign to it, GCPRO it, and then assign to it again, the first object assigned is not protected, while the second object is protected. Unfortunately GCPROing is easy to forget, and there is basically no way around this problem. Here are some rules, though:

  1. A garbage collection can occur whenever anything calls Feval, or whenever a QUIT can occur where execution can continue past this. (Remember, this is almost anywhere.) Note that Fsignal can GC, and it can return (even though it normally doesn't). This means that you must GCPRO before calling most of the error functions, including the `CONCHECK' family of macros, if references occur after the call.

  2. You must UNGCPRO anything that's GCPROed, and you must not UNGCPRO if you haven't GCPROed. Getting either of these wrong will lead to crashes, often in completely random places unrelated to where the problem lies. There are some functions (Fsignal is the canonical example) which may or may not return. In these cases, the function is responsible for cleaning up the GCPROs if it doesn't return, so you should treat it as an ordinary function.

  3. For every GCPROn, there have to be declarations of struct gcpro gcpro1, gcpro2, ..., gcpron.

  4. The way this actually works is that all currently active GCPROs are chained through the struct gcpro local variables, with the variable `gcprolist' pointing to the head of the list and the nth local gcpro variable pointing to the first gcpro variable in the next enclosing stack frame. Each GCPROed thing is an lvalue, and the struct gcpro local variable contains a pointer to this lvalue. This is why things will mess up badly if you don't pair up the GCPROs and UNGCPROs--you will end up with gcprolists containing pointers to struct gcpros or local Lisp_Object variables in no-longer-active stack frames.

  5. It is actually possible for a single struct gcpro to protect a contiguous array of any number of values, rather than just a single lvalue. To effect this, call GCPROn as usual on the first object in the array and then set gcpron.nvars.

  6. Strings are relocated. What this means in practice is that the pointer obtained using XSTRING_DATA() is liable to change at any time, and you should never keep it around past any function call, or pass it as an argument to any function that might cause a garbage collection. This is why a number of functions accept either a "non-relocatable" char * pointer or a relocatable Lisp string, and only access the Lisp string's data at the very last minute. In some cases, you may end up having to alloca() some space and copy the string's data into it.

  7. By convention, if you have to nest GCPRO's, use NGCPROn (along with struct gcpro ngcpro1, ngcpro2, etc.), NNGCPROn, etc. This avoids compiler warnings about shadowed locals.

  8. It is always better to err on the side of extra GCPROs rather than too few. The extra cycles spent on this are almost never going to make a whit of difference in the speed of anything.

  9. The general rule to follow is that caller, not callee, GCPROs. That is, you should not have to explicitly GCPRO any Lisp objects that are passed in as parameters.

    One exception from this rule is if you ever plan to change the parameter value, and store a new object in it. In that case, you must GCPRO the parameter, because otherwise the new object will not be protected.

    So, if you create any Lisp objects (remember, this happens in all sorts of circumstances, e.g. with Fcons(), etc.), you are responsible for GCPROing them, unless you are absolutely sure that there's no possibility that a garbage-collection can occur while you need to use the object. Even then, consider GCPROing.

  10. If you have the least smidgeon of doubt about whether you need to GCPRO, you should GCPRO.

  11. Beware of GCPROing something that is uninitialized. If you have any shade of doubt about this, initialize all your variables to Qnil.

  12. Be careful of traps, like calling Fcons() in the argument to another function. By the "caller protects" law, you should be GCPROing the newly-created cons, but you aren't. A certain number of functions that are commonly called on freshly created stuff (e.g. nconc2(), Fsignal()), break the "caller protects" law and go ahead and GCPRO their arguments so as to simplify things, but make sure and check if it's OK whenever doing something like this.

  13. Once again, remember to GCPRO! Bugs resulting from insufficient GCPROing are intermittent and extremely difficult to track down, often showing up in crashes inside of garbage-collect or in weirdly corrupted objects or even in incorrect values in a totally different section of code.

If you don't understand whether to GCPRO in a particular instance, ask on the mailing lists. A general hint is that prog1 is the canonical example.

Given the extremely error-prone nature of the GCPRO scheme, and the difficulties in tracking down, it should be considered a deficiency in the XEmacs code. A solution to this problem would involve implementing so-called conservative garbage collection for the C stack. That involves looking through all of stack memory and treating anything that looks like a reference to an object as a reference. This will result in a few objects not getting collected when they should, but it obviates the need for GCPROing, and allows garbage collection to happen at any point at all, such as during object allocation.


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20.4 Garbage Collection - Step by Step

20.4.1 Invocation  
20.4.2 garbage_collect_1  
20.4.3 mark_object  
20.4.4 gc_sweep  
20.4.5 sweep_lcrecords_1  
20.4.6 compact_string_chars  
20.4.7 sweep_strings  
20.4.8 sweep_bit_vectors_1  


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20.4.1 Invocation

The first thing that anyone should know about garbage collection is: when and how the garbage collector is invoked. One might think that this could happen every time new memory is allocated, e.g. new objects are created, but this is not the case. Instead, we have the following situation:

The entry point of any process of garbage collection is an invocation of the function garbage_collect_1 in file alloc.c. The invocation can occur explicitly by calling the function Fgarbage_collect (in addition this function provides information about the freed memory), or can occur implicitly in four different situations:

  1. In function main_1 in file emacs.c. This function is called at each startup of xemacs. The garbage collection is invoked after all initial creations are completed, but only if a special internal error checking-constant ERROR_CHECK_GC is defined.
  2. In function disksave_object_finalization in file alloc.c. The only purpose of this function is to clear the objects from memory which need not be stored with xemacs when we dump out an executable. This is only done by Fdump_emacs or by Fdump_emacs_data respectively (both in emacs.c). The actual clearing is accomplished by making these objects unreachable and starting a garbage collection. The function is only used while building xemacs.
  3. In function Feval / eval in file eval.c. Each time the well known and often used function eval is called to evaluate a form, one of the first things that could happen, is a potential call of garbage_collect_1. There exist three global variables, consing_since_gc (counts the created cons-cells since the last garbage collection), gc_cons_threshold (a specified threshold after which a garbage collection occurs) and always_gc. If always_gc is set or if the threshold is exceeded, the garbage collection will start.
  4. In function Ffuncall / funcall in file eval.c. This function evaluates calls of elisp functions and works according to Feval.

The upshot is that garbage collection can basically occur everywhere Feval, respectively Ffuncall, is used - either directly or through another function. Since calls to these two functions are hidden in various other functions, many calls to garbage_collect_1 are not obviously foreseeable, and therefore unexpected. Instances where they are used that are worth remembering are various elisp commands, as for example or, and, if, cond, while, setq, etc., miscellaneous gui_item_... functions, everything related to eval (Feval_buffer, call0, ...) and inside Fsignal. The latter is used to handle signals, as for example the ones raised by every QUIT-macro triggered after pressing Ctrl-g.


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20.4.2 garbage_collect_1

We can now describe exactly what happens after the invocation takes place.

  1. There are several cases in which the garbage collector is left immediately: when we are already garbage collecting (gc_in_progress), when the garbage collection is somehow forbidden (gc_currently_forbidden), when we are currently displaying something (in_display) or when we are preparing for the armageddon of the whole system (preparing_for_armageddon).
  2. Next the correct frame in which to put all the output occurring during garbage collecting is determined. In order to be able to restore the old display's state after displaying the message, some data about the current cursor position has to be saved. The variables pre_gc_cursor and cursor_changed take care of that.
  3. The state of gc_currently_forbidden must be restored after the garbage collection, no matter what happens during the process. We accomplish this by record_unwind_protecting the suitable function restore_gc_inhibit together with the current value of gc_currently_forbidden.
  4. If we are concurrently running an interactive xemacs session, the next step is simply to show the garbage collector's cursor/message.
  5. The following steps are the intrinsic steps of the garbage collector, therefore gc_in_progress is set.
  6. For debugging purposes, it is possible to copy the current C stack frame. However, this seems to be a currently unused feature.
  7. Before actually starting to go over all live objects, references to objects that are no longer used are pruned. We only have to do this for events (clear_event_resource) and for specifiers (cleanup_specifiers).
  8. Now the mark phase begins and marks all accessible elements. In order to start from all slots that serve as roots of accessibility, the function mark_object is called for each root individually to go out from there to mark all reachable objects. All roots that are traversed are shown in their processed order:
  9. Hash tables in XEmacs belong to a kind of special objects that make use of a concept often called 'weak pointers'. To make a long story short, these kind of pointers are not followed during the estimation of the live objects during garbage collection. Any object referenced only by weak pointers is collected anyway, and the reference to it is cleared. In hash tables there are different usage patterns of them, manifesting in different types of hash tables, namely 'non-weak', 'weak', 'key-weak' and 'value-weak' (internally also 'key-car-weak' and 'value-car-weak') hash tables, each clearing entries depending on different conditions. More information can be found in the documentation to the function make-hash-table.

    Because there are complicated dependency rules about when and what to mark while processing weak hash tables, the standard marker method is only active if it is marking non-weak hash tables. As soon as a weak component is in the table, the hash table entries are ignored while marking. Instead their marking is done each separately by the function finish_marking_weak_hash_tables. This function iterates over each hash table entry hentries for each weak hash table in Vall_weak_hash_tables. Depending on the type of a table, the appropriate action is performed. If a table is acting as HASH_TABLE_KEY_WEAK, and a key already marked, everything reachable from the value component is marked. If it is acting as a HASH_TABLE_VALUE_WEAK and the value component is already marked, the marking starts beginning only from the key component. If it is a HASH_TABLE_KEY_CAR_WEAK and the car of the key entry is already marked, we mark both the key and value components. Finally, if the table is of the type HASH_TABLE_VALUE_CAR_WEAK and the car of the value components is already marked, again both the key and the value components get marked.

    Again, there are lists with comparable properties called weak lists. There exist different peculiarities of their types called simple, assoc, key-assoc and value-assoc. You can find further details about them in the description to the function make-weak-list. The scheme of their marking is similar: all weak lists are listed in Qall_weak_lists, therefore we iterate over them. The marking is advanced until we hit an already marked pair. Then we know that during a former run all the rest has been marked completely. Again, depending on the special type of the weak list, our jobs differ. If it is a WEAK_LIST_SIMPLE and the elem is marked, we mark the cons part. If it is a WEAK_LIST_ASSOC and not a pair or a pair with both marked car and cdr, we mark the cons and the elem. If it is a WEAK_LIST_KEY_ASSOC and not a pair or a pair with a marked car of the elem, we mark the cons and the elem. Finally, if it is a WEAK_LIST_VALUE_ASSOC and not a pair or a pair with a marked cdr of the elem, we mark both the cons and the elem.

    Since, by marking objects in reach from weak hash tables and weak lists, other objects could get marked, this perhaps implies further marking of other weak objects, both finishing functions are redone as long as yet unmarked objects get freshly marked.

  10. After completing the special marking for the weak hash tables and for the weak lists, all entries that point to objects that are going to be swept in the further process are useless, and therefore have to be removed from the table or the list.

    The function prune_weak_hash_tables does the job for weak hash tables. Totally unmarked hash tables are removed from the list Vall_weak_hash_tables. The other ones are treated more carefully by scanning over all entries and removing one as soon as one of the components key and value is unmarked.

    The same idea applies to the weak lists. It is accomplished by prune_weak_lists: An unmarked list is pruned from Vall_weak_lists immediately. A marked list is treated more carefully by going over it and removing just the unmarked pairs.

  11. The function prune_specifiers checks all listed specifiers held in Vall_specifiers and removes the ones from the lists that are unmarked.

  12. All syntax tables are stored in a list called Vall_syntax_tables. The function prune_syntax_tables walks through it and unlinks the tables that are unmarked.

  13. Next, we will attack the complete sweeping - the function gc_sweep which holds the predominance.
  14. First, all the variables with respect to garbage collection are reset. consing_since_gc - the counter of the created cells since the last garbage collection - is set back to 0, and gc_in_progress is not true anymore.
  15. In case the session is interactive, the displayed cursor and message are removed again.
  16. The state of gc_inhibit is restored to the former value by unwinding the stack.
  17. A small memory reserve is always held back that can be reached by breathing_space. If nothing more is left, we create a new reserve and exit.


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20.4.3 mark_object

The first thing that is checked while marking an object is whether the object is a real Lisp object Lisp_Type_Record or just a fixnum or a character. Fixnums and characters are the only two types that are stored directly - without another level of indirection, and therefore they don't have to be marked and collected. See section 19. How Lisp Objects Are Represented in C.

The second case is the one we have to handle. It is the one when we are dealing with a pointer to a Lisp object. But, there exist also three possibilities, that prevent us from doing anything while marking: The object is read only which prevents it from being garbage collected, i.e. marked (C_READONLY_RECORD_HEADER). The object in question is already marked, and need not be marked for the second time (checked by MARKED_RECORD_HEADER_P). If it is a special, unmarkable object (UNMARKABLE_RECORD_HEADER_P, apparently, these are objects that sit in some const space, and can therefore not be marked, see this_one_is_unmarkable in alloc.c).

Now, the actual marking is feasible. We do so by once using the macro MARK_RECORD_HEADER to mark the object itself (actually the special flag in the lrecord header), and calling its special marker "method" marker if available. The marker method marks every other object that is in reach from our current object. Note, that these marker methods should not call mark_object recursively, but instead should return the next object from where further marking has to be performed.

In case another object was returned, as mentioned before, we reiterate the whole mark_object process beginning with this next object.


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20.4.4 gc_sweep

The job of this function is to free all unmarked records from memory. As we know, there are different types of objects implemented and managed, and consequently different ways to free them from memory. See section 20.1 Introduction to Allocation.

We start with all objects stored through lcrecords. All bulkier objects are allocated and handled using that scheme of lcrecords. Each object is malloced separately instead of placing it in one of the contiguous frob blocks. All types that are currently stored using lcrecords's alloc_lcrecord and make_lcrecord_list are the types: vectors, buffers, char-table, char-table-entry, console, weak-list, database, device, ldap, hash-table, command-builder, extent-auxiliary, extent-info, face, coding-system, frame, image-instance, glyph, popup-data, gui-item, keymap, charset, color_instance, font_instance, opaque, opaque-list, process, range-table, specifier, symbol-value-buffer-local, symbol-value-lisp-magic, symbol-value-varalias, toolbar-button, tooltalk-message, tooltalk-pattern, window, and window-configuration. We take care of them in the fist place in order to be able to handle and to finalize items stored in them more easily. The function sweep_lcrecords_1 as described below is doing the whole job for us. For a description about the internals: See section 20.7 lrecords.

Our next candidates are the other objects that behave quite differently than everything else: the strings. They consists of two parts, a fixed-size portion (struct Lisp_String) holding the string's length, its property list and a pointer to the second part, and the actual string data, which is stored in string-chars blocks comparable to frob blocks. In this block, the data is not only freed, but also a compression of holes is made, i.e. all strings are relocated together. See section 20.14 String. This compacting phase is performed by the function compact_string_chars, the actual sweeping by the function sweep_strings is described below.

After that, the other types are swept step by step using functions sweep_conses, sweep_bit_vectors_1, sweep_compiled_functions, sweep_floats, sweep_symbols, sweep_extents, sweep_markers and sweep_extents. They are the fixed-size types cons, floats, compiled-functions, symbol, marker, extent, and event stored in so-called "frob blocks", and therefore we can basically do the same on every type objects, using the same macros, especially defined only to handle everything with respect to fixed-size blocks. The only fixed-size type that is not handled here are the fixed-size portion of strings, because we took special care of them earlier.

The only big exceptions are bit vectors stored differently and therefore treated differently by the function sweep_bit_vectors_1 described later.

At first, we need some brief information about how these fixed-size types are managed in general, in order to understand how the sweeping is done. They have all a fixed size, and are therefore stored in big blocks of memory - allocated at once - that can hold a certain amount of objects of one type. The macro DECLARE_FIXED_TYPE_ALLOC creates the suitable structures for every type. More precisely, we have the block struct (holding a pointer to the previous block prev and the objects in block[]), a pointer to current block (current_..._block)) and its last index (current_..._block_index), and a pointer to the free list that will be created. Also a macro FIXED_TYPE_FROM_BLOCK plus some related macros exists that are used to obtain a new object, either from the free list ALLOCATE_FIXED_TYPE_1 if there is an unused object of that type stored or by allocating a completely new block using ALLOCATE_FIXED_TYPE_FROM_BLOCK.

The rest works as follows: all of them define a macro UNMARK_... that is used to unmark the object. They define a macro ADDITIONAL_FREE_... that defines additional work that has to be done when converting an object from in use to not in use (so far, only markers use it in order to unchain them). Then, they all call the macro SWEEP_FIXED_TYPE_BLOCK instantiated with their type name and their struct name.

This call in particular does the following: we go over all blocks starting with the current moving towards the oldest. For each block, we look at every object in it. If the object already freed (checked with FREE_STRUCT_P using the first pointer of the object), or if it is set to read only (C_READONLY_RECORD_HEADER_P, nothing must be done. If it is unmarked (checked with MARKED_RECORD_HEADER_P), it is put in the free list and set free (using the macro FREE_FIXED_TYPE, otherwise it stays in the block, but is unmarked (by UNMARK_...). While going through one block, we note if the whole block is empty. If so, the whole block is freed (using xfree) and the free list state is set to the state it had before handling this block.


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20.4.5 sweep_lcrecords_1

After nullifying the complete lcrecord statistics, we go over all lcrecords two separate times. They are all chained together in a list with a head called all_lcrecords.

The first loop calls for each object its finalizer method, but only in the case that it is not read only (C_READONLY_RECORD_HEADER_P), it is not already marked (MARKED_RECORD_HEADER_P), it is not already in a free list (list of freed objects, field free) and finally it owns a finalizer method.

The second loop actually frees the appropriate objects again by iterating through the whole list. In case an object is read only or marked, it has to persist, otherwise it is manually freed by calling xfree. During this loop, the lcrecord statistics are kept up to date by calling tick_lcrecord_stats with the right arguments,


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20.4.6 compact_string_chars

The purpose of this function is to compact all the data parts of the strings that are held in so-called string_chars_block, i.e. the strings that do not exceed a certain maximal length.

The procedure with which this is done is as follows. We are keeping two positions in the string_chars_blocks using two pointer/integer pairs, namely from_sb/from_pos and to_sb/to_pos. They stand for the actual positions, from where to where, to copy the actually handled string.

While going over all chained string_char_blocks and their held strings, staring at first_string_chars_block, both pointers are advanced and eventually a string is copied from from_sb to to_sb, depending on the status of the pointed at strings.

More precisely, we can distinguish between the following actions.

After compacting, the pointer to the current string_chars_block, sitting in current_string_chars_block, is reset on the last block to which we moved a string, i.e. to_block, and all remaining blocks (we know that they just carry garbage) are explicitly xfreed.


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20.4.7 sweep_strings

The sweeping for the fixed sized string objects is essentially exactly the same as it is for all other fixed size types. As before, the freeing into the suitable free list is done by using the macro SWEEP_FIXED_SIZE_BLOCK after defining the right macros UNMARK_string and ADDITIONAL_FREE_string. These two definitions are a little bit special compared to the ones used for the other fixed size types.

UNMARK_string is defined the same way except some additional code used for updating the bookkeeping information.

For strings, ADDITIONAL_FREE_string has to do something in addition: in case, the string was not allocated in a string_chars_block because it exceeded the maximal length, and therefore it was malloced separately, we know also xfree it explicitly.


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20.4.8 sweep_bit_vectors_1

Bit vectors are also one of the rare types that are malloced individually. Consequently, while sweeping, all further needless bit vectors must be freed by hand. This is done, as one might imagine, the expected way: since they are all registered in a list called all_bit_vectors, all elements of that list are traversed, all unmarked bit vectors are unlinked by calling xfree and all of them become unmarked. In addition, the bookkeeping information used for garbage collector's output purposes is updated.


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20.5 Fixnums and Characters

Fixnum and character Lisp objects are created from C integers using the functions make_fixnum() and make_char(). (These are actually macros on most systems.) These functions basically just do some moving of bits around, since the integral value of the object is stored directly in the Lisp_Object.


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20.6 Allocation from Frob Blocks

The uninitialized memory required by a Lisp_Object of a particular type is allocated using ALLOCATE_FIXED_TYPE(). This only occurs inside of the lowest-level object-creating functions in `alloc.c': Fcons(), make_float(), Fmake_byte_code(), Fmake_symbol(), allocate_extent(), allocate_event(), Fmake_marker(), and make_uninit_string(). The idea is that, for each type, there are a number of frob blocks (each 2K in size); each frob block is divided up into object-sized chunks. Each frob block will have some of these chunks that are currently assigned to objects, and perhaps some that are free. (If a frob block has nothing but free chunks, it is freed at the end of the garbage collection cycle.) The free chunks are stored in a free list, which is chained by storing a pointer in the first four bytes of the chunk. (Except for the free chunks at the end of the last frob block, which are handled using an index which points past the end of the last-allocated chunk in the last frob block.) ALLOCATE_FIXED_TYPE() first tries to retrieve a chunk from the free list; if that fails, it calls ALLOCATE_FIXED_TYPE_FROM_BLOCK(), which looks at the end of the last frob block for space, and creates a new frob block if there is none. (There are actually two versions of these macros, one of which is more defensive but less efficient and is used for error-checking.)


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20.7 lrecords

[see `lrecord.h']

This node needs updating for the "new garbage collection algorithms" (KKCC) and the "incremental" collector.

All lrecords have at the beginning of their structure a struct lrecord_header. This just contains a type number and some flags, including the mark bit. All builtin type numbers are defined as constants in enum lrecord_type, to allow the compiler to generate more efficient code for typeP. The type number, thru the lrecord_implementation_table, gives access to a struct lrecord_implementation, which is a structure containing method pointers and such. There is one of these for each type, and it is a global, constant, statically-declared structure that is declared in the DEFINE_*_LISP_OBJECT() macro.

Frob-block lrecords just have a struct lrecord_header at their beginning. lcrecords, however, actually have a struct old_lcrecord_header. This, in turn, has a struct lrecord_header at its beginning, so sanity is preserved; but it also has a pointer used to chain all lcrecords together.

lcrecords are now obsolete when using the write-barrier-based collector.

Frob-block objects are created using ALLOC_FROB_BLOCK_LISP_OBJECT(). All this does is call ALLOCATE_FIXED_TYPE() to allocate an object, and set_lheader_implementation() to initialize the header.

Normal objects (i.e. lcrecords) are created using ALLOC_NORMAL_LISP_OBJECT(), which takes a type name (resolved internally to a structure named lrecord_foo for type foo). If they are of variable size, however, they are created with ALLOC_SIZED_LISP_OBJECT(), which takes a size to allocate in addition to a type. This basically just malloc()s the storage, initializes the struct lcrecord_header, and chains the lcrecord onto the head of the list of all lcrecords, which is stored in the variable all_lcrecords. The calls to the above allocation macros generally occur in the lowest-level allocation function for each lrecord type.

Whenever you create a normal object, you need to call one of the DEFINE_*_LISP_OBJECT() macros. This needs to be specified in a `.c' file, at the top level. What this actually does is define and initialize the implementation structure for the lrecord. (And possibly declares a function error_check_foo() that implements the XFOO() macro when error-checking is enabled.) The arguments to the macros are the actual type name (this is used to construct the C variable name of the lrecord implementation structure and related structures using the `##' macro concatenation operator), a string that names the type on the Lisp level (this may not be the same as the C type name; typically, the C type name has underscores, while the Lisp string has dashes), various method pointers, and the name of the C structure that contains the object. The methods are used to encapsulate type-specific information about the object, such as how to print it or mark it for garbage collection, so that it's easy to add new object types without having to add a specific case for each new type in a bunch of different places.

The various macros for defining Lisp objects are as follows:

MAKE_LISP_OBJECT and MAKE_MODULE_LISP_OBJECT are what underlies all of these; they define a structure containing pointers to object methods and other info such as the size of the structure containing the object.

For the purpose of keeping allocation statistics, the allocation engine keeps a list of all the different types that exist. Note that, since DEFINE_*_LISP_OBJECT() is a macro that is specified at top-level, there is no way for it to initialize the global data structures containing type information, like lrecord_implementations_table. For this reason a call to INIT_LISP_OBJECT() must be added to the same source file containing DEFINE_*_LISP_OBJECT(), but instead of to the top level, to one of the init functions, typically syms_of_foo.c. INIT_LISP_OBJECT() must be called before an object of this type is used.

The type number is also used to index into an array holding the number of objects of each type and the total memory allocated for objects of that type. The statistics in this array are computed during the sweep stage. These statistics are returned by the call to garbage-collect.

Note that for every type defined with a DEFINE_*_LISP_OBJECT() macro, there needs to be a DECLARE_LISP_OBJECT() somewhere in a `.h' file, and this `.h' file needs to be included by `inline.c'.

Furthermore, there should generally be a set of XFOOBAR(), FOOBARP(), etc. macros in a `.h' (or occasionally `.c') file. To create one of these, copy an existing model and modify as necessary.

Please note: If you define an lrecord in an external dynamically-loaded module, you must use DECLARE_MODULE_LISP_OBJECT(), DEFINE_MODULE_*_LISP_OBJECT(), and INIT_MODULE_LISP_OBJECT() instead of the non-MODULE forms. These macros will dynamically add new type numbers to the global enum that records them, whereas the non-MODULE forms assume that the programmer has already inserted the correct type numbers into the enum's code at compile-time.

The various methods in the lrecord implementation structure are:

  1. A mark method. This is called during the marking stage and passed a function pointer (usually the mark_object() function), which is used to mark an object. All Lisp objects that are contained within the object need to be marked by applying this function to them. The mark method should also return a Lisp object, which should be either nil or an object to mark. (This can be used in lieu of calling mark_object() on the object, to reduce the recursion depth, and consequently should be the most heavily nested sub-object, such as a long list.)

    Please note: When the mark method is called, garbage collection is in progress, and special precautions need to be taken when accessing objects; see section (B) above.

    If your mark method does not need to do anything, it can be NULL.

  2. A print method. This is called to create a printed representation of the object, whenever princ, prin1, or the like is called. It is passed the object, a stream to which the output is to be directed, and an escapeflag which indicates whether the object's printed representation should be escaped so that it is readable. (This corresponds to the difference between princ and prin1.) Basically, escaped means that strings will have quotes around them and confusing characters in the strings such as quotes, backslashes, and newlines will be backslashed; and that special care will be taken to make symbols print in a readable fashion (e.g. symbols that look like numbers will be backslashed). Other readable objects should perhaps pass escapeflag on when sub-objects are printed, so that readability is preserved when necessary (or if not, always pass in a 1 for escapeflag). Non-readable objects should in general ignore escapeflag, except that some use it as an indication that more verbose output should be given.

    Sub-objects are printed using print_internal(), which takes exactly the same arguments as are passed to the print method.

    Literal C strings should be printed using write_cistring(), or write_string_1() for non-null-terminated strings.

    Functions that do not have a readable representation should check the print_readably flag and signal an error if it is set.

    If you specify NULL for the print method, the default_object_printer() will be used.

  3. A finalize method. This is called at the beginning of the sweep stage on lcrecords that are about to be freed, and should be used to perform any extra object cleanup. This typically involves freeing any extra malloc()ed memory associated with the object, releasing any operating-system and window-system resources associated with the object (e.g. pixmaps, fonts), etc.

    The finalize method can be NULL if nothing needs to be done.

    Finalize methods should, as a rule, set to zero any pointers after they've been freed, and check to make sure pointers are not zero before freeing. Although I'm pretty sure that finalize methods are not called twice on the same object, we've gotten nastily burned in some cases by not doing this.

    WARNING #1: The finalize method is only called for normal objects, not for frob-block objects. If you need a finalize method for frob-block objects, you have to stick it in the ADDITIONAL_FREE_foo() macro in `alloc.c'.

    WARNING #2: Things are in an extremely bizarre state when ADDITIONAL_FREE_foo() is called, so you have to be incredibly careful when writing one of these functions. See the comment in gc_sweep(). If you ever have to add one of these, consider using an lcrecord or dealing with the problem in a different fashion.

  4. An equal method. This compares the two objects for similarity, when equal is called. It should compare the contents of the objects in some reasonable fashion. It is passed the two objects and a depth value, which is used to catch circular objects. To compare sub-Lisp-objects, call internal_equal() and bump the depth value by one. If this value gets too high, a circular-object error will be signaled.

    If this is NULL, objects are equal only when they are eq, i.e. identical.

  5. A hash method. This is used to hash objects when they are to be compared with equal. The rule here is that if two objects are equal, they must hash to the same value; i.e. your hash function should use some subset of the sub-fields of the object that are compared in the "equal" method. If you specify this method as NULL, the object's pointer will be used as the hash, which will fail if the object has an equal method, so don't do this.

    To hash a sub-Lisp-object, call internal_hash(). Bump the depth by one, just like in the "equal" method.

    To convert a Lisp object directly into a hash value (using its pointer), use LISP_HASH(). This is what happens when the hash method is NULL.

    To hash two or more values together into a single value, use HASH2(), HASH3(), HASH4(), etc.

  6. getprop, putprop, remprop, and plist methods. These are used for object types that have properties, and are called when get, put, remprop, and object-plist, respectively are called on the object. If you create one of these objects, you have to use a different macro to define them, i.e. DEFINE_*_GENERAL_LISP_OBJECT().

  7. A size_in_bytes method, when the object is of variable-size. (i.e. declared with a DEFINE_*_SIZABLE_*_LISP_OBJECT macro.) This should simply return the object's size in bytes, exactly as you might expect. For an example, see the methods for lstreams and opaques.

  8. A disksave method. This is called at the end of the dump phase. It is used for objects that contain pointers or handles to objects created in external libraries, such as window-system windows or file handles. Such external objects cannot be dumped, so it is necessary to release them at dump time and arrange somehow or other for them to be resurrected if necessary later on.

    It seems that even non-dumpable objects may be around at dump time, and a disksaver may be provided. (In fact, the only object currently with a disksaver, lstream, is non-dumpable.)

    Objects rarely need to provide this method; most of the time it will be NULL. If you want to provide this method, you have to use the DEFINE_*_GENERAL_LISP_OBJECT() macro to define your object.


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20.8 Low-level allocation

Memory that you want to allocate directly should be allocated using xmalloc() rather than malloc(). This implements error-checking on the return value, and once upon a time did some more vital stuff (i.e. BLOCK_INPUT, which is no longer necessary). Free using xfree(), and realloc using xrealloc(). Note that xmalloc() will do a non-local exit if the memory can't be allocated. (Many functions, however, do not expect this, and thus XEmacs will likely crash if this happens. This is a bug. If you can, you should strive to make your function handle this OK. However, it's difficult in the general circumstance, perhaps requiring extra unwind-protects and such.)

Note that XEmacs provides two separate replacements for the standard malloc() library function. These are called old GNU malloc (`malloc.c') and new GNU malloc (`gmalloc.c'), respectively. New GNU malloc is better in pretty much every way than old GNU malloc, and should be used if possible. (It used to be that on some systems, the old one worked but the new one didn't. I think this was due specifically to a bug in SunOS, which the new one now works around; so I don't think the old one ever has to be used any more.) The primary difference between both of these mallocs and the standard system malloc is that they are much faster, at the expense of increased space. The basic idea is that memory is allocated in fixed chunks of powers of two. This allows for basically constant malloc time, since the various chunks can just be kept on a number of free lists. (The standard system malloc typically allocates arbitrary-sized chunks and has to spend some time, sometimes a significant amount of time, walking the heap looking for a free block to use and cleaning things up.) The new GNU malloc improves on things by allocating large objects in chunks of 4096 bytes rather than in ever larger powers of two, which results in ever larger wastage. There is a slight speed loss here, but it's of doubtful significance.

NOTE: Apparently there is a third-generation GNU malloc that is significantly better than the new GNU malloc, and should probably be included in XEmacs.

There is also the relocating allocator, `ralloc.c'. This actually moves blocks of memory around so that the sbrk() pointer shrunk and virtual memory released back to the system. On some systems, this is a big win. On all systems, it causes a noticeable (and sometimes huge) speed penalty, so I turn it off by default. `ralloc.c' only works with the new GNU malloc in `gmalloc.c'. There are also two versions of `ralloc.c', one that uses mmap() rather than block copies to move data around. This purports to be faster, although that depends on the amount of data that would have had to be block copied and the system-call overhead for mmap(). I don't know exactly how this works, except that the relocating-allocation routines are pretty much used only for the memory allocated for a buffer, which is the biggest consumer of space, esp. of space that may get freed later.

Note that the GNU mallocs have some "memory warning" facilities. XEmacs taps into them and issues a warning through the standard warning system, when memory gets to 75%, 85%, and 95% full. (On some systems, the memory warnings are not functional.)

Allocated memory that is going to be used to make a Lisp object is created using allocate_lisp_storage(). This just calls xmalloc(). It used to verify that the pointer to the memory can fit into a Lisp word, before the current Lisp object representation was introduced. allocate_lisp_storage() is called by alloc_lcrecord(), ALLOCATE_FIXED_TYPE(), and the vector and bit-vector creation routines. These routines also call INCREMENT_CONS_COUNTER() at the appropriate times; this keeps statistics on how much memory is allocated, so that garbage-collection can be invoked when the threshold is reached.


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20.9 Cons

Conses are allocated in standard frob blocks. The only thing to note is that conses can be explicitly freed using free_cons() and associated functions free_list() and free_alist(). This immediately puts the conses onto the cons free list, and decrements the statistics on memory allocation appropriately. This is used to good effect by some extremely commonly-used code, to avoid generating extra objects and thereby triggering GC sooner. However, you have to be extremely careful when doing this. If you mess this up, you will get BADLY BURNED, and it has happened before.


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20.10 Vector

As mentioned above, each vector is malloc()ed individually, and all are threaded through the variable all_vectors. Vectors are marked strangely during garbage collection, by kludging the size field. Note that the struct Lisp_Vector is declared with its contents field being a stretchy array of one element. It is actually malloc()ed with the right size, however, and access to any element through the contents array works fine.


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20.11 Bit Vector

Bit vectors work exactly like vectors, except for more complicated code to access an individual bit, and except for the fact that bit vectors are lrecords while vectors are not. (The only difference here is that there's an lrecord implementation pointer at the beginning and the tag field in bit vector Lisp words is "lrecord" rather than "vector".)


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20.12 Symbol

Symbols are also allocated in frob blocks. Symbols in the awful horrible obarray structure are chained through their next field.

Remember that intern looks up a symbol in an obarray, creating one if necessary.


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20.13 Marker

Markers are allocated in frob blocks, as usual. They are kept in a buffer unordered, but in a doubly-linked list so that they can easily be removed. (Formerly this was a singly-linked list, but in some cases garbage collection took an extraordinarily long time due to the O(N^2) time required to remove lots of markers from a buffer.) Markers are removed from a buffer in the finalize stage, in ADDITIONAL_FREE_marker().


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20.14 String

As mentioned above, strings are a special case. A string is logically two parts, a fixed-size object (containing the length, property list, and a pointer to the actual data), and the actual data in the string. The fixed-size object is a struct Lisp_String and is allocated in frob blocks, as usual. The actual data is stored in special string-chars blocks, which are 8K blocks of memory. Currently-allocated strings are simply laid end to end in these string-chars blocks, with a pointer back to the struct Lisp_String stored before each string in the string-chars block. When a new string needs to be allocated, the remaining space at the end of the last string-chars block is used if there's enough, and a new string-chars block is created otherwise.

There are never any holes in the string-chars blocks due to the string compaction and relocation that happens at the end of garbage collection. During the sweep stage of garbage collection, when objects are reclaimed, the garbage collector goes through all string-chars blocks, looking for unused strings. Each chunk of string data is preceded by a pointer to the corresponding struct Lisp_String, which indicates both whether the string is used and how big the string is, i.e. how to get to the next chunk of string data. Holes are compressed by block-copying the next string into the empty space and relocating the pointer stored in the corresponding struct Lisp_String. This means you have to be careful with strings in your code. See the section above on GCPROing.

Note that there is one situation not handled: a string that is too big to fit into a string-chars block. Such strings, called big strings, are all malloc()ed as their own block. (#### Although it would make more sense for the threshold for big strings to be somewhat lower, e.g. 1/2 or 1/4 the size of a string-chars block. It seems that this was indeed the case formerly--indeed, the threshold was set at 1/8--but Mly forgot about this when rewriting things for 19.8.)

Note also that the string data in string-chars blocks is padded as necessary so that proper alignment constraints on the struct Lisp_String back pointers are maintained.

Finally, strings can be resized. This happens in Mule when a character is substituted with a different-length character, or during modeline frobbing. (You could also export this to Lisp, but it's not done so currently.) Resizing a string is a potentially tricky process. If the change is small enough that the padding can absorb it, nothing other than a simple memory move needs to be done. Keep in mind, however, that the string can't shrink too much because the offset to the next string in the string-chars block is computed by looking at the length and rounding to the nearest multiple of four or eight. If the string would shrink or expand beyond the correct padding, new string data needs to be allocated at the end of the last string-chars block and the data moved appropriately. This leaves some dead string data, which is marked by putting a special marker of 0xFFFFFFFF in the struct Lisp_String pointer before the data (there's no real struct Lisp_String to point to and relocate), and storing the size of the dead string data (which would normally be obtained from the now-non-existent struct Lisp_String) at the beginning of the dead string data gap. The string compactor recognizes this special 0xFFFFFFFF marker and handles it correctly.


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20.15 Compiled Function

Not yet documented.


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