On Tue, Mar 28, 2023 at 10:32:28PM +1100, Nathaniel Shead via Gcc-patches wrote:
> * tree-core.h (struct tree_decl_common): New flag to check if
> value lifetime has expired.
> * tree.h (DECL_EXPIRED): Access the new flag.
> * print-tree.cc (print_node): Print the new flag.
> --- a/gcc/tree-core.h
> +++ b/gcc/tree-core.h
> @@ -1834,7 +1834,10 @@ struct GTY(()) tree_decl_common {
> /* In FIELD_DECL, this is DECL_NOT_FLEXARRAY. */
> unsigned int decl_not_flexarray : 1;
>
> - /* 13 bits unused. */
> + /* In VAR_DECL, PARM_DECL, or RESULT_DECL, this is DECL_EXPIRED. */
> + unsigned int decl_expired_flag : 1;
> +
> + /* 12 bits unused. */
>
> /* UID for points-to sets, stable over copying from inlining. */
> unsigned int pt_uid;
> --- a/gcc/tree.h
> +++ b/gcc/tree.h
> @@ -2697,6 +2697,12 @@ extern tree vector_element_bits_tree (const_tree);
> ??? Need to figure out some way to check this isn't a PARM_DECL. */
> #define DECL_INITIAL(NODE) (DECL_COMMON_CHECK (NODE)->decl_common.initial)
>
> +/* Used in a VAR_DECL, PARM_DECL, or RESULT_DECL to indicate whether
> + this declaration is currently in lifetime for constant evaluation
> + purposes. */
> +#define DECL_EXPIRED(NODE) \
> + (DECL_COMMON_CHECK(NODE)->decl_common.decl_expired_flag)
> +
> /* Holds the size of the datum, in bits, as a tree expression.
> Need not be constant and may be null. May be less than TYPE_SIZE
> for a C++ FIELD_DECL representing a base class subobject with its
While it is true that tree_decl_common has some spare bits, this support
is for an implementation detail of the C++ FE and as such, I think it is
highly preferred to use some bit in the lang specific data structures
rather than wasting bits that we could need later on for things that will
be needed in the middle-end.
struct lang_decl_base I think also has 12 spare bits...
Rest I'll defer to C++ maintainers (also whether this is appropriate now
or should be deferred for GCC 14 stage1).
Jakub
