On Tue, Mar 28, 2023 at 10:50 PM Jakub Jelinek <ja...@redhat.com> wrote:
>
> 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
>
Thanks, I didn't notice that there were lang specific data structures;
I'll take a look at using that instead.
>From my point of view I was expecting this would be too late for GCC13
(especially since it should only affect already invalid code, and the
risk of false positives), but I'm happy either way.
