diff options
| -rw-r--r-- | gcc/ChangeLog | 5 | ||||
| -rw-r--r-- | gcc/doc/gty.texi | 45 | ||||
| -rw-r--r-- | gcc/gengtype.c | 28 |
3 files changed, 78 insertions, 0 deletions
diff --git a/gcc/ChangeLog b/gcc/ChangeLog index fcb71b7..21204a0 100644 --- a/gcc/ChangeLog +++ b/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. diff --git a/gcc/doc/gty.texi b/gcc/doc/gty.texi index 55986a1..03bcbf8 100644 --- a/gcc/doc/gty.texi +++ b/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}") diff --git a/gcc/gengtype.c b/gcc/gengtype.c index 95e2a92..85832d3 100644 --- a/gcc/gengtype.c +++ b/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) |