On Thu, 2 May 2024, Nathaniel Shead wrote:
> On Wed, May 01, 2024 at 10:11:20AM -0400, Patrick Palka wrote:
> > On Wed, 1 May 2024, Nathaniel Shead wrote:
> >
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
> > >
> > > -- >8 --
> > >
> > > When calling instantiate_pending_templates at end of parsing, any new
> > > functions that are instantiated from this point have their module
> > > purview set based on the current value of module_kind.
> > >
> > > This is unideal, however, as the modules code will then treat these
> > > instantiations as reachable and cause large swathes of the GMF to be
> > > emitted into the module CMI, despite no code in the actual module
> > > purview referencing it.
> > >
> > > This patch fixes this by also remembering the value of module_kind when
> > > the instantiation was deferred, and restoring it when doing this
> > > deferred instantiation. That way newly instantiated declarations
> > > appropriately get a DECL_MODULE_PURVIEW_P appropriate for where the
> > > instantiation was required, meaning that GMF entities won't be counted
> > > as reachable unless referenced by an actually reachable entity.
> > >
> > > Note that purviewness and attachment etc. is generally only determined
> > > by the base template: this is purely for determining whether a
> > > specialisation was declared in the module purview and hence whether it
> > > should be streamed out. See the comment on 'set_instantiating_module'.
> > >
> > > PR c++/114630
> > > PR c++/114795
> > >
> > > gcc/cp/ChangeLog:
> > >
> > > * cp-tree.h (struct tinst_level): Add field for tracking
> > > module_kind.
> > > * pt.cc (push_tinst_level_loc): Cache module_kind in new_level.
> > > (reopen_tinst_level): Restore module_kind from level.
> > > (instantiate_pending_templates): Save and restore module_kind so
> > > it isn't affected by reopen_tinst_level.
> >
> > LGTM. Another approach is to instantiate all so-far deferred
> > instantiations and vtables once we reach the start of the module
> > purview, but your approach is much cleaner.
> >
> > Note that deferred instantiation can induce more deferred instantiation,
> > but your approach will do the right thing here (module_kind will be
> > inherited from the point of instantiation).
> >
> > What if an instantiation is needed from both the GMF and the module
> > purview? Then IIUC it'll be instantiated as if from the GMF, which
> > seems right to me.
> >
>
> Hm..., so I believe it'll be marked according to whichever instantiates
> it "first", so if e.g. deferred from GMF and then instantiated
> non-deferred from purview it'll be marked purview (and the deferred
> instantiation will be skipped as it's already instantiated). This could
> mean it gets unnecessarily emitted if it only got instantiated because
> it got used in e.g. a non-inline function body, I suppose; but that's
> true in general actually, at the moment.
FWIW I think the most relevant situation where we need to instantiate
immediately and can't defer until EOF is during constexpr evaluation of
a variable initializer. (We also immediately instantiate variable
template specializations, and function template specializations with
deduced return type, but in these cases we'd always instantiate from the
first use, i.e. from the GMF in this scenario.)
So I guess a testcase for this situation could be something like:
module;
template<class T> constexpr int f(T t) { return t; }
template int f(int);
export module foo;
constexpr int w = f(0);
// f<int> ideally shouldn't be emitted in the CMI?
>
> Maybe then a better approach would be to instead always use the
> DECL_MODULE_PURVIEW_P of the instantiating template instead of the
> global 'module_purview_p' function in 'set_instantiating_module'?
> I think that should still make sure that instantiations are emitted
> when they need to be (because they'll be reachable from a declaration
> in the purview): I might try to experiment with something like this.
The approach of instantiating all so-far deferred instantiations/vtables
at the start of the module purview should handle this properly too.
>
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > > * g++.dg/modules/gmf-3.C: New test.
> > >
> > > Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
> > > ---
> > > gcc/cp/cp-tree.h | 3 +++
> > > gcc/cp/pt.cc | 4 ++++
> > > gcc/testsuite/g++.dg/modules/gmf-3.C | 13 +++++++++++++
> > > 3 files changed, 20 insertions(+)
> > > create mode 100644 gcc/testsuite/g++.dg/modules/gmf-3.C
> > >
> > > diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> > > index 1938ada0268..0e619120ccc 100644
> > > --- a/gcc/cp/cp-tree.h
> > > +++ b/gcc/cp/cp-tree.h
> > > @@ -6626,6 +6626,9 @@ struct GTY((chain_next ("%h.next"))) tinst_level {
> > > /* The location where the template is instantiated. */
> > > location_t locus;
> > >
> > > + /* The module kind where the template is instantiated. */
> > > + unsigned module_kind;
> > > +
> > > /* errorcount + sorrycount when we pushed this level. */
> > > unsigned short errors;
> > >
> > > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > > index 1c3eef60c06..401aa92bc3e 100644
> > > --- a/gcc/cp/pt.cc
> > > +++ b/gcc/cp/pt.cc
> > > @@ -11277,6 +11277,7 @@ push_tinst_level_loc (tree tldcl, tree targs,
> > > location_t loc)
> > > new_level->tldcl = tldcl;
> > > new_level->targs = targs;
> > > new_level->locus = loc;
> > > + new_level->module_kind = module_kind;
> > > new_level->errors = errorcount + sorrycount;
> > > new_level->next = NULL;
> > > new_level->refcount = 0;
> > > @@ -11345,6 +11346,7 @@ reopen_tinst_level (struct tinst_level *level)
> > > for (t = level; t; t = t->next)
> > > ++tinst_depth;
> > >
> > > + module_kind = level->module_kind;
> > > set_refcount_ptr (current_tinst_level, level);
> > > pop_tinst_level ();
> > > if (current_tinst_level)
> > > @@ -27442,6 +27444,7 @@ instantiate_pending_templates (int retries)
> > > {
> > > int reconsider;
> > > location_t saved_loc = input_location;
> > > + unsigned saved_module_kind = module_kind;
> > >
> > > /* Instantiating templates may trigger vtable generation. This in turn
> > > may require further template instantiations. We place a limit here
> > > @@ -27532,6 +27535,7 @@ instantiate_pending_templates (int retries)
> > > while (reconsider);
> > >
> > > input_location = saved_loc;
> > > + module_kind = saved_module_kind;
> > > }
> > >
> > > /* Substitute ARGVEC into T, which is a list of initializers for
> > > diff --git a/gcc/testsuite/g++.dg/modules/gmf-3.C
> > > b/gcc/testsuite/g++.dg/modules/gmf-3.C
> > > new file mode 100644
> > > index 00000000000..e52ae904ea9
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/modules/gmf-3.C
> > > @@ -0,0 +1,13 @@
> > > +// PR c++/114630
> > > +// { dg-additional-options "-fmodules-ts -Wno-global-module
> > > -fdump-lang-module" }
> > > +// { dg-module-cmi M }
> > > +
> > > +module;
> > > +template <typename> struct allocator {
> > > + allocator() {}
> > > +};
> > > +template class allocator<wchar_t>;
> > > +export module M;
> > > +
> > > +// The whole GMF should be discarded here
> > > +// { dg-final { scan-lang-dump "Wrote 0 clusters" module } }
> > > --
> > > 2.43.2
> > >
> > >
> >
>
>
