In gcc/: 2011-05-21 Nicola Pero <nicola.pero@meta-innovation.com>

In gcc/:
2011-05-21  Nicola Pero  <nicola.pero@meta-innovation.com>

	* gengtype.c (walk_type): Implemented "atomic" GTY option.
	* doc/gty.texi (GTY Options): Document "atomic" GTY option.

From-SVN: r173996

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2011-05-21  Nicola Pero  <nicola.pero@meta-innovation.com>
+
+	* gengtype.c (walk_type): Implemented "atomic" GTY option.
+	* doc/gty.texi (GTY Options): Document "atomic" GTY option.
+
 2011-05-21  Joseph Myers  <joseph@codesourcery.com>
 
 	* opt-read.awk: New.  Split out of optc-gen.awk and opth-gen.awk.

gcc/doc/gty.texi

@@ -383,6 +383,51 @@ could be calculated as follows:
   size_t size = sizeof (struct sorted_fields_type) + n * sizeof (tree);
 @end smallexample
 
+@findex atomic
+@item atomic
+
+The @code{atomic} option can only be used with pointers.  It informs
+the GC machinery that the memory that the pointer points to does not
+contain any pointers, and hence it should be treated by the GC and PCH
+machinery as an ``atomic'' block of memory that does not need to be
+examined when scanning memory for pointers.  In particular, the
+machinery will not scan that memory for pointers to mark them as
+reachable (when marking pointers for GC) or to relocate them (when
+writing a PCH file).
+
+The @code{atomic} option differs from the @code{skip} option.
+@code{atomic} keeps the memory under Garbage Collection, but makes the
+GC ignore the contents of the memory.  @code{skip} is more drastic in
+that it causes the pointer and the memory to be completely ignored by
+the Garbage Collector.  So, memory marked as @code{atomic} is
+automatically freed when no longer reachable, while memory marked as
+@code{skip} is not.
+
+The @code{atomic} option must be used with great care, because all
+sorts of problem can occur if used incorrectly, that is, if the memory
+the pointer points to does actually contain a pointer.
+
+Here is an example of how to use it:
+@smallexample
+struct GTY(()) my_struct @{
+  int number_of_elements;
+  unsigned int GTY ((atomic)) * elements;
+@};
+@end smallexample
+In this case, @code{elements} is a pointer under GC, and the memory it
+points to needs to be allocated using the Garbage Collector, and will
+be freed automatically by the Garbage Collector when it is no longer
+referenced.  But the memory that the pointer points to is an array of
+@code{unsigned int} elements, and the GC must not try to scan it to
+find pointers to mark or relocate, which is why it is marked with the
+@code{atomic} option.
+
+Note that, currently, global variables can not be marked with
+@code{atomic}; only fields of a struct can.  This is a known
+limitation.  It would be useful to be able to mark global pointers
+with @code{atomic} to make the PCH machinery aware of them so that
+they are saved and restored correctly to PCH files.
+
 @findex special
 @item special ("@var{name}")
 

gcc/gengtype.c

@@ -2386,6 +2386,7 @@ walk_type (type_p t, struct walk_type_data *d)
   int maybe_undef_p = 0;
   int use_param_num = -1;
   int use_params_p = 0;
+  int atomic_p = 0;
   options_p oo;
   const struct nested_ptr_data *nested_ptr_d = NULL;
 
@@ -2415,6 +2416,8 @@ walk_type (type_p t, struct walk_type_data *d)
       ;
     else if (strcmp (oo->name, "skip") == 0)
       ;
+    else if (strcmp (oo->name, "atomic") == 0)
+      atomic_p = 1;
     else if (strcmp (oo->name, "default") == 0)
       ;
     else if (strcmp (oo->name, "param_is") == 0)
@@ -2480,6 +2483,12 @@ walk_type (type_p t, struct walk_type_data *d)
       return;
     }
 
+  if (atomic_p && (t->kind != TYPE_POINTER))
+    {
+      error_at_line (d->line, "field `%s' has invalid option `atomic'\n", d->val);
+      return;
+    }
+
   switch (t->kind)
     {
     case TYPE_SCALAR:
@@ -2495,6 +2504,25 @@ walk_type (type_p t, struct walk_type_data *d)
 	    break;
 	  }
 
+	/* If a pointer type is marked as "atomic", we process the
+	   field itself, but we don't walk the data that they point to.
+	   
+	   There are two main cases where we walk types: to mark
+	   pointers that are reachable, and to relocate pointers when
+	   writing a PCH file.  In both cases, an atomic pointer is
+	   itself marked or relocated, but the memory that it points
+	   to is left untouched.  In the case of PCH, that memory will
+	   be read/written unchanged to the PCH file.  */
+	if (atomic_p)
+	  {
+	    oprintf (d->of, "%*sif (%s != NULL) {\n", d->indent, "", d->val);
+	    d->indent += 2;
+	    d->process_field (t, d);
+	    d->indent -= 2;
+	    oprintf (d->of, "%*s}\n", d->indent, "");
+	    break;
+	  }
+
 	if (!length)
 	  {
 	    if (!UNION_OR_STRUCT_P (t->u.p)