Re: [PATCH] c++ modules: lazy loading from within template [PR99377]

[Patrick Palka <ppalka@redhat.com>]

On Mon, 10 Oct 2022, Nathan Sidwell wrote:

> On 10/4/22 13:36, Patrick Palka wrote:
> > Here when lazily loading the binding for f at parse time from the
> > template g, processing_template_decl is set and thus the call to
> > note_vague_linkage_fn from module_state::read_cluster has no effect,
> > and we never push f onto deferred_fns and end up never emitting its
> > definition.
> > 
> > ISTM the behavior of the lazy loading machinery shouldn't be sensitive
> > to whether we're inside a template, and therefore we should probably be
> > clearing processing_template_decl somewhere e.g in lazy_load_binding.
> > This is sufficient to fix the testcase.
> 
> yeah, I remember hitting issues with this, but thought I'd got rid of the need
> to override processing_template_decl.  Do you also need to override it in
> lazy_load_pendings though? that's a lazy loader and my suspicion is it might
> be susceptible to the same issues.

Hmm yeah, looks like we need to override it in lazy_load_pendings too:
I ran the testsuite with gcc_assert (!processing_template_decl) added to
module_state::read_cluster and if we don't also override it in
lazy_load_binding then the assert triggers for pr99425-2_b.X.

> 
> > 
> > But it also seems the processing_template_decl test in
> > note_vague_linkage_fn, added by r8-7539-g977bc3ee11383e for PR84973, is
> > perhaps too strong: if the intent is to avoid deferring output for
> > uninstantiated templates, we should make sure that DECL in question is
> > an uninstantiated template by checking e.g. value_dependent_expression_p.
> > This too is sufficient to fix the testcase (since f isn't a template)
> > and survives bootstrap and regtest.
> 
> I think this is an orthogonal issue -- can we remove it from this patch?

Done.

-- >8 --

Subject: [PATCH] c++ modules: lazy loading from within template [PR99377]

Here when lazily loading the binding for f at parse time from the
template g, processing_template_decl is set and thus the call to
note_vague_linkage_fn from module_state::read_cluster has no effect,
and we never push f onto deferred_fns and end up never emitting its
definition.

ISTM the behavior of the lazy loading machinery shouldn't be sensitive
to whether we're inside a template, and therefore we should be clearing
processing_template_decl in the entrypoints lazy_load_binding and
lazy_load_pendings.

	PR c++/99377

gcc/cp/ChangeLog:

	* module.cc (lazy_load_binding): Clear processing_template_decl.
	(lazy_load_pendings): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr99377-2_a.C: New test.
	* g++.dg/modules/pr99377-2_b.C: New test.
---
 gcc/cp/module.cc                           | 8 ++++++++
 gcc/testsuite/g++.dg/modules/pr99377-2_a.C | 5 +++++
 gcc/testsuite/g++.dg/modules/pr99377-2_b.C | 6 ++++++
 3 files changed, 19 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/modules/pr99377-2_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/pr99377-2_b.C

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 94fbee85225..7c48602136c 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -19081,6 +19081,10 @@ lazy_load_binding (unsigned mod, tree ns, tree id, binding_slot *mslot)
 
   timevar_start (TV_MODULE_IMPORT);
 
+  /* Make sure lazy loading from a template context behaves as if
+     from a non-template context.  */
+  processing_template_decl_sentinel ptds;
+
   /* Stop GC happening, even in outermost loads (because our caller
      could well be building up a lookup set).  */
   function_depth++;
@@ -19129,6 +19133,10 @@ lazy_load_binding (unsigned mod, tree ns, tree id, binding_slot *mslot)
 void
 lazy_load_pendings (tree decl)
 {
+  /* Make sure lazy loading from a template context behaves as if
+     from a non-template context.  */
+  processing_template_decl_sentinel ptds;
+
   tree key_decl;
   pending_key key;
   key.ns = find_pending_key (decl, &key_decl);
diff --git a/gcc/testsuite/g++.dg/modules/pr99377-2_a.C b/gcc/testsuite/g++.dg/modules/pr99377-2_a.C
new file mode 100644
index 00000000000..26e2bccbbbe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pr99377-2_a.C
@@ -0,0 +1,5 @@
+// PR c++/99377
+// { dg-additional-options -fmodules-ts }
+// { dg-module-cmi pr99377 }
+export module pr99377;
+export inline void f() { }
diff --git a/gcc/testsuite/g++.dg/modules/pr99377-2_b.C b/gcc/testsuite/g++.dg/modules/pr99377-2_b.C
new file mode 100644
index 00000000000..69571952c8a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pr99377-2_b.C
@@ -0,0 +1,6 @@
+// PR c++/99377
+// { dg-additional-options -fmodules-ts }
+// { dg-do link }
+import pr99377;
+template<class> void g() { f(); }
+int main() { g<int>(); }
-- 
2.38.0.15.gbbe21b64a0