Patchwork gengtype improvements for plugins, thirdround! patch 7/7 [doc]

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Submitter Basile Starynkevitch
Date Sept. 21, 2010, 7:41 p.m.
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Permalink /patch/65376/
State New
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Basile Starynkevitch - Sept. 21, 2010, 7:41 p.m.
Hello All

Please notice that I made a mail mistake in the previous patch chunk.
The 6th patch [wstate] is in and not in which got the
wrong attachments (no patches there!).  Sorry for the noise.


The last patch takes into account Lauryanas remarks in and is a
documentation patch. I added some more explanations so please check my
poor english language.

I still hope that my patch serie "thirdround" will be Ok, with perhaps
minor changes required.

#################### gcc/ChangeLog entry for documentation 
2010-09-21  Basile Starynkevitch  <>

	* gcc/doc/gty.texi:
	(Generating GGC code with gengtype): New node.
	(GTY Options): Documented that tag should be unique.
	Added ptr_alias documentation.
	(How to run the gengtype generator): New section.

The attached patch is relative to 

Ok for trunk? With what changes?

BTW, the dates in the patch series (and notably their ChangeLog entry)
might be wrong. If Ok-ed, I will put the date (as seen from Paris,
France) of the commit.

Laurynas Biveinis - Sept. 22, 2010, 3:10 a.m.
The patch7_doc-relto06.diff file size increased four times since the
last submission. Are you sure you sent the correct patch?


--- ../thirdround_06_wstate//gty.texi	2010-09-21 20:21:19.000000000 +0200
+++ gcc/doc/gty.texi	2010-09-21 20:20:20.000000000 +0200
@@ -0,0 +1,552 @@ 
+@c Copyright (C) 2002, 2003, 2004, 2007, 2008, 2009
+@c Free Software Foundation, Inc.
+@c This is part of the GCC manual.
+@c For copying conditions, see the file gcc.texi.
+@node Type Information
+@chapter Memory Management and Type Information
+@cindex GGC
+@findex GTY
+GCC uses some fairly sophisticated memory management techniques, which
+involve determining information about GCC's data structures from GCC's
+source code and using this information to perform garbage collection and
+implement precompiled headers.
+A full C parser would be too complicated for this task, so a limited
+subset of C is interpreted and special markers are used to determine
+what parts of the source to look at.  All @code{struct} and
+@code{union} declarations that define data structures that are
+allocated under control of the garbage collector must be marked.  All
+global variables that hold pointers to garbage-collected memory must
+also be marked.  Finally, all global variables that need to be saved
+and restored by a precompiled header must be marked.  (The precompiled
+header mechanism can only save static variables if they're scalar.
+Complex data structures must be allocated in garbage-collected memory
+to be saved in a precompiled header.)
+The full format of a marker is
+GTY (([@var{option}] [(@var{param})], [@var{option}] [(@var{param})] @dots{}))
+@end smallexample
+but in most cases no options are needed.  The outer double parentheses
+are still necessary, though: @code{GTY(())}.  Markers can appear:
+@itemize @bullet
+In a structure definition, before the open brace;
+In a global variable declaration, after the keyword @code{static} or
+@code{extern}; and
+In a structure field definition, before the name of the field.
+@end itemize
+Here are some examples of marking simple data structures and globals.
+struct GTY(()) @var{tag}
+  @var{fields}@dots{}
+typedef struct GTY(()) @var{tag}
+  @var{fields}@dots{}
+@} *@var{typename};
+static GTY(()) struct @var{tag} *@var{list};   /* @r{points to GC memory} */
+static GTY(()) int @var{counter};        /* @r{save counter in a PCH} */
+@end smallexample
+The parser understands simple typedefs such as
+@code{typedef struct @var{tag} *@var{name};} and
+@code{typedef int @var{name};}.
+These don't need to be marked.
+* GTY Options::         What goes inside a @code{GTY(())}.
+* GGC Roots::           Making global variables GGC roots.
+* Files::               How the generated files work.
+* Generating GGC code with gengtype::    How to run the gengtype generator.
+* Invoking the garbage collector::   How to invoke the garbage collector.
+@end menu
+@node GTY Options
+@section The Inside of a @code{GTY(())}
+Sometimes the C code is not enough to fully describe the type
+structure.  Extra information can be provided with @code{GTY} options
+and additional markers.  Some options take a parameter, which may be
+either a string or a type name, depending on the parameter.  If an
+option takes no parameter, it is acceptable either to omit the
+parameter entirely, or to provide an empty string as a parameter.  For
+example, @code{@w{GTY ((skip))}} and @code{@w{GTY ((skip ("")))}} are
+When the parameter is a string, often it is a fragment of C code.  Four
+special escapes may be used in these strings, to refer to pieces of
+the data structure being marked:
+@cindex % in GTY option
+@table @code
+@item %h
+The current structure.
+@item %1
+The structure that immediately contains the current structure.
+@item %0
+The outermost structure that contains the current structure.
+@item %a
+A partial expression of the form @code{[i1][i2]@dots{}} that indexes
+the array item currently being marked.
+@end table
+For instance, suppose that you have a structure of the form
+struct A @{
+  @dots{}
+struct B @{
+  struct A foo[12];
+@end smallexample
+and @code{b} is a variable of type @code{struct B}.  When marking
+@samp{[11]}, @code{%h} would expand to @samp{[11]},
+@code{%0} and @code{%1} would both expand to @samp{b}, and @code{%a}
+would expand to @samp{[11]}.
+As in ordinary C, adjacent strings will be concatenated; this is
+helpful when you have a complicated expression.
+GTY ((chain_next ("TREE_CODE (&%h.generic) == INTEGER_TYPE"
+                  " ? TYPE_NEXT_VARIANT (&%h.generic)"
+                  " : TREE_CHAIN (&%h.generic)")))
+@end group
+@end smallexample
+The available options are:
+@table @code
+@findex length
+@item length ("@var{expression}")
+There are two places the type machinery will need to be explicitly told
+the length of an array.  The first case is when a structure ends in a
+variable-length array, like this:
+struct GTY(()) rtvec_def @{
+  int num_elem;         /* @r{number of elements} */
+  rtx GTY ((length ("%h.num_elem"))) elem[1];
+@end smallexample
+In this case, the @code{length} option is used to override the specified
+array length (which should usually be @code{1}).  The parameter of the
+option is a fragment of C code that calculates the length.
+The second case is when a structure or a global variable contains a
+pointer to an array, like this:
+struct gimple_omp_for_iter * GTY((length ("%h.collapse"))) iter;
+@end smallexample
+In this case, @code{iter} has been allocated by writing something like
+  x->iter = ggc_alloc_cleared_vec_gimple_omp_for_iter (collapse);
+@end smallexample
+and the @code{collapse} provides the length of the field.
+This second use of @code{length} also works on global variables, like:
+static GTY((length("reg_known_value_size"))) rtx *reg_known_value;
+@end verbatim
+@findex skip
+@item skip
+If @code{skip} is applied to a field, the type machinery will ignore it.
+This is somewhat dangerous; the only safe use is in a union when one
+field really isn't ever used.
+@findex desc
+@findex tag
+@findex default
+@item desc ("@var{expression}")
+@itemx tag ("@var{constant}")
+@itemx default
+The type machinery needs to be told which field of a @code{union} is
+currently active.  This is done by giving each field a constant
+@code{tag} value, and then specifying a discriminator using @code{desc}.
+The value of the expression given by @code{desc} is compared against
+each @code{tag} value, each of which should be different.  If no
+@code{tag} is matched, the field marked with @code{default} is used if
+there is one, otherwise no field in the union will be marked.
+A union field can have at most one @code{tag}.
+In the @code{desc} option, the ``current structure'' is the union that
+it discriminates.  Use @code{%1} to mean the structure containing it.
+There are no escapes available to the @code{tag} option, since it is a
+For example,
+struct GTY(()) tree_binding
+  struct tree_common common;
+  union tree_binding_u @{
+    tree GTY ((tag ("0"))) scope;
+    struct cp_binding_level * GTY ((tag ("1"))) level;
+  @} GTY ((desc ("BINDING_HAS_LEVEL_P ((tree)&%0)"))) xscope;
+  tree value;
+@end smallexample
+In this example, the value of BINDING_HAS_LEVEL_P when applied to a
+@code{struct tree_binding *} is presumed to be 0 or 1.  If 1, the type
+mechanism will treat the field @code{level} as being present and if 0,
+will treat the field @code{scope} as being present.
+@findex param_is
+@findex use_param
+@item param_is (@var{type})
+@itemx use_param
+Sometimes it's convenient to define some data structure to work on
+generic pointers (that is, @code{PTR}) and then use it with a specific
+type.  @code{param_is} specifies the real type pointed to, and
+@code{use_param} says where in the generic data structure that type
+should be put.
+For instance, to have a @code{htab_t} that points to trees, one would
+write the definition of @code{htab_t} like this:
+typedef struct GTY(()) @{
+  @dots{}
+  void ** GTY ((use_param, @dots{})) entries;
+  @dots{}
+@} htab_t;
+@end smallexample
+and then declare variables like this:
+  static htab_t GTY ((param_is (union tree_node))) ict;
+@end smallexample
+@findex param@var{n}_is
+@findex use_param@var{n}
+@item param@var{n}_is (@var{type})
+@itemx use_param@var{n}
+In more complicated cases, the data structure might need to work on
+several different types, which might not necessarily all be pointers.
+For this, @code{param1_is} through @code{param9_is} may be used to
+specify the real type of a field identified by @code{use_param1} through
+@findex use_params
+@item use_params
+When a structure contains another structure that is parameterized,
+there's no need to do anything special, the inner structure inherits the
+parameters of the outer one.  When a structure contains a pointer to a
+parameterized structure, the type machinery won't automatically detect
+this (it could, it just doesn't yet), so it's necessary to tell it that
+the pointed-to structure should use the same parameters as the outer
+structure.  This is done by marking the pointer with the
+@code{use_params} option.
+@findex deletable
+@item deletable
+@code{deletable}, when applied to a global variable, indicates that when
+garbage collection runs, there's no need to mark anything pointed to
+by this variable, it can just be set to @code{NULL} instead.  This is used
+to keep a list of free structures around for re-use.
+@findex if_marked
+@item if_marked ("@var{expression}")
+Suppose you want some kinds of object to be unique, and so you put them
+in a hash table.  If garbage collection marks the hash table, these
+objects will never be freed, even if the last other reference to them
+goes away.  GGC has special handling to deal with this: if you use the
+@code{if_marked} option on a global hash table, GGC will call the
+routine whose name is the parameter to the option on each hash table
+entry.  If the routine returns nonzero, the hash table entry will
+be marked as usual.  If the routine returns zero, the hash table entry
+will be deleted.
+The routine @code{ggc_marked_p} can be used to determine if an element
+has been marked already; in fact, the usual case is to use
+@code{if_marked ("ggc_marked_p")}.
+@findex mark_hook
+@item mark_hook ("@var{hook-routine-name}")
+If provided for a structure or union type, the given
+@var{hook-routine-name} (between double-quotes) is the name of a
+routine called when the garbage collector has just marked the data as
+reachable. This routine should not change the data, or call any ggc
+routine. Its only argument is a pointer to the just marked (const)
+structure or union.
+@findex maybe_undef
+@item maybe_undef
+When applied to a field, @code{maybe_undef} indicates that it's OK if
+the structure that this fields points to is never defined, so long as
+this field is always @code{NULL}.  This is used to avoid requiring
+backends to define certain optional structures.  It doesn't work with
+language frontends.
+@findex nested_ptr
+@item nested_ptr (@var{type}, "@var{to expression}", "@var{from expression}")
+The type machinery expects all pointers to point to the start of an
+object.  Sometimes for abstraction purposes it's convenient to have
+a pointer which points inside an object.  So long as it's possible to
+convert the original object to and from the pointer, such pointers
+can still be used.  @var{type} is the type of the original object,
+the @var{to expression} returns the pointer given the original object,
+and the @var{from expression} returns the original object given
+the pointer.  The pointer will be available using the @code{%h}
+@findex ptr_alias
+@item ptr_alias (@var{type})
+When applied to a type, @code{ptr_alias} indicates that the GTY-ed
+type is an alias or synonym of the given @var{type}.
+@findex chain_next
+@findex chain_prev
+@findex chain_circular
+@item chain_next ("@var{expression}")
+@itemx chain_prev ("@var{expression}")
+@itemx chain_circular ("@var{expression}")
+It's helpful for the type machinery to know if objects are often
+chained together in long lists; this lets it generate code that uses
+less stack space by iterating along the list instead of recursing down
+it.  @code{chain_next} is an expression for the next item in the list,
+@code{chain_prev} is an expression for the previous item.  For singly
+linked lists, use only @code{chain_next}; for doubly linked lists, use
+both.  The machinery requires that taking the next item of the
+previous item gives the original item.  @code{chain_circular} is similar
+to @code{chain_next}, but can be used for circular single linked lists.
+@findex reorder
+@item reorder ("@var{function name}")
+Some data structures depend on the relative ordering of pointers.  If
+the precompiled header machinery needs to change that ordering, it
+will call the function referenced by the @code{reorder} option, before
+changing the pointers in the object that's pointed to by the field the
+option applies to.  The function must take four arguments, with the
+signature @samp{@w{void *, void *, gt_pointer_operator, void *}}.
+The first parameter is a pointer to the structure that contains the
+object being updated, or the object itself if there is no containing
+structure.  The second parameter is a cookie that should be ignored.
+The third parameter is a routine that, given a pointer, will update it
+to its correct new value.  The fourth parameter is a cookie that must
+be passed to the second parameter.
+PCH cannot handle data structures that depend on the absolute values
+of pointers.  @code{reorder} functions can be expensive.  When
+possible, it is better to depend on properties of the data, like an ID
+number or the hash of a string instead.
+@findex variable_size
+@item variable_size
+The type machinery expects the types to be of constant size.  When this
+is not true, for example, with structs that have array fields or unions,
+the type machinery cannot tell how many bytes need to be allocated at 
+each allocation.  The @code{variable_size} is used to mark such types.
+The type machinery then provides allocators that take a parameter 
+indicating an exact size of object being allocated.  
+For example,
+struct GTY((variable_size)) sorted_fields_type @{
+  int len;
+  tree GTY((length ("%h.len"))) elts[1];
+@end smallexample
+Then the objects of @code{struct sorted_fields_type} are allocated in GC 
+memory as follows:
+  field_vec = ggc_alloc_sorted_fields_type (size);
+@end smallexample
+@findex special
+@item special ("@var{name}")
+The @code{special} option is used to mark types that have to be dealt
+with by special case machinery.  The parameter is the name of the
+special case.  See @file{gengtype.c} for further details.  Avoid
+adding new special cases unless there is no other alternative.
+@end table
+@node GGC Roots
+@section Marking Roots for the Garbage Collector
+@cindex roots, marking
+@cindex marking roots
+In addition to keeping track of types, the type machinery also locates
+the global variables (@dfn{roots}) that the garbage collector starts
+at.  Roots must be declared using one of the following syntaxes:
+@itemize @bullet
+@code{extern GTY(([@var{options}])) @var{type} @var{name};}
+@code{static GTY(([@var{options}])) @var{type} @var{name};}
+@end itemize
+The syntax
+@itemize @bullet
+@code{GTY(([@var{options}])) @var{type} @var{name};}
+@end itemize
+is @emph{not} accepted.  There should be an @code{extern} declaration
+of such a variable in a header somewhere---mark that, not the
+definition.  Or, if the variable is only used in one file, make it
+@node Files
+@section Source Files Containing Type Information
+@cindex generated files
+@cindex files, generated
+Whenever you add @code{GTY} markers to a source file that previously
+had none, or create a new source file containing @code{GTY} markers,
+there are three things you need to do:
+You need to add the file to the list of source files the type
+machinery scans.  There are four cases:
+@enumerate a
+For a back-end file, this is usually done
+automatically; if not, you should add it to @code{target_gtfiles} in
+the appropriate port's entries in @file{config.gcc}.
+For files shared by all front ends, add the filename to the
+@code{GTFILES} variable in @file{}.
+For files that are part of one front end, add the filename to the
+@code{gtfiles} variable defined in the appropriate
+@file{}.  For C, the file is @file{}.
+Headers should appear before non-headers in this list.
+For files that are part of some but not all front ends, add the
+filename to the @code{gtfiles} variable of @emph{all} the front ends
+that use it.
+@end enumerate
+If the file was a header file, you'll need to check that it's included
+in the right place to be visible to the generated files.  For a back-end
+header file, this should be done automatically.  For a front-end header
+file, it needs to be included by the same file that includes
+@file{gtype-@var{lang}.h}.  For other header files, it needs to be
+included in @file{gtype-desc.c}, which is a generated file, so add it to
+@code{ifiles} in @code{open_base_file} in @file{gengtype.c}.
+For source files that aren't header files, the machinery will generate a
+header file that should be included in the source file you just changed.
+The file will be called @file{gt-@var{path}.h} where @var{path} is the
+pathname relative to the @file{gcc} directory with slashes replaced by
+@verb{|-|}, so for example the header file to be included in
+@file{cp/parser.c} is called @file{gt-cp-parser.c}.  The
+generated header file should be included after everything else in the
+source file.  Don't forget to mention this file as a dependency in the
+@end enumerate
+For language frontends, there is another file that needs to be included
+somewhere.  It will be called @file{gtype-@var{lang}.h}, where
+@var{lang} is the name of the subdirectory the language is contained in.
+Plugins can add additional root tables.  Run the @code{gengtype}
+utility in plugin mode as @code{gengtype -P @var{gt-pluginout.h} -r
+@var{gtype.state} @var{plugin*.c}} with your plugin files
+@var{plugin*.c} using @code{GTY} to generate the @var{gt-pluginout.h}
+file.  The @var{gtype.state} is usually in the plugin directory.
+@node Generating GGC code with gengtype
+@section How to run the gengtype generator.
+@cindex garbage collector, generation, gengtype
+@findex gengtype
+The @code{gengtype} utility (which may be installed as
+@code{gcc-gengtype} or some other name on your system) is a C code
+generator program.  It generates suitable marking and type-walking C
+routines for GGC.  Generated GC marking routines ensure that every
+live data is ultimately marked, so that GGC can delete, at the end of
+its @code{ggc_collect} function, every non-marked data (which is
+unreachable and dead).  Generated PCH type-walking routines are used
+for ``compilation'' of header files @var{*.h} into a @var{*.gch}
+file.  The @code{gengtype} utility is used when building GCC itself
+from its source tree, and also for plugins dealing with their own
+GTY-ed data.
+The @code{gengtype} utility is run once specially when compiling GCC
+itself.  It parses many GCC source files (mostly internal GCC headers
+declaring @code{GTY}-ed types and internal GCC files declaring
+@code{GTY}-ed variables). It saves its state in a persistent textual
+file @file{gtype.state}.  This file has a format tied to a particular
+GCC release and configuration.  This @file{gtype.state} file should be
+parsed only by @code{gengtype} (of the same version which has
+generated it).  Following GNU usage, @code{gengtype} accepts the usual
+@code{--version} or @code{-V} and @code{--help} arguments.  It also
+accepts @code{--verbose} or @code{-v} to show what it is doing, and
+@code{--debug} or @code{-D} for debugging purposes.  It may dump every
+internal data in human readable form with @code{--dump} or @code{-d}.
+The @code{--backupdir} @var{directory} or @code{-B} @var{directory}
+specifies a back-up directory used to store, as e.g. @file{gt-*.h~}
+files, the previous version of generated C files.
+During GCC build process, a file @file{gtyp-input.list} is built and
+contains a long list of files to be parsed.  Then, @code{gengtype} is
+run once to generate its state with @code{gengtype -S
+@file{source-directory} -I @file{gtyp-input.list} -w
+@file{gtype.state}}.  Immediately after, it is re-run to read its just
+generated state and generate @file{gt*.[ch]} files with @code{gengtype
+-r @file{gtype.state}}.
+Plugin developers using @code{GTY} in their plugin source code should
+run @code{gengtype -P @var{gt-pluginout.h} -r @file{gtype.state}
+@node Invoking the garbage collector
+@section How to invoke the garbage collector
+@cindex garbage collector, invocation
+@findex ggc_collect
+The GCC garbage collector GGC is only invoked explicitly. In contrast
+with many other garbage collectors, it is not implicitly invoked by
+allocation routines when a lot of memory has been consumed. So the
+only way to have GGC reclaim storage it to call the @code{ggc_collect}
+function explicitly. This call is an expensive operation, as it may
+have to scan the entire heap. Beware that local variables (on the GCC
+call stack) are not followed by such an invocation (as many other
+garbage collectors do): you should reference all your data from static
+or external @code{GTY}-ed variables, and it is advised to call
+@code{ggc_collect} with a shallow call stack. The GGC is an exact mark
+and sweep garbage collector (so it does not scan the call stack for
+pointers). In practice GCC passes don't often call @code{ggc_collect}
+themselves, because it is called by the pass manager between passes.