[PATCH v2] c++: Redetermine whether to write vtables on stream-in [PR114229]

[Nathaniel Shead <nathanieloshead@gmail.com>]

On Wed, Mar 06, 2024 at 08:59:16AM -0500, Jason Merrill wrote:
> On 3/5/24 22:06, Nathaniel Shead wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
> > 
> > -- >8 --
> > 
> > Currently, reading a variable definition always marks that decl as
> > DECL_NOT_REALLY_EXTERN, with anything else imported still being
> > considered external. This is not sufficient for vtables, however; for an
> > extern template, a vtable may be generated (and its definition emitted)
> > but nonetheless the vtable should only be emitted in the TU where that
> > template is actually instantiated.
> 
> Does the vtable go through import_export_decl?  I've been thinking that that
> function (and import_export_class) need to be more module-aware. Would it
> make sense to do that rather than stream DECL_NOT_REALLY_EXTERN?
> 
> Jason
> 

Right. It doesn't go through 'import_export_decl' because when it's
imported, DECL_INTERFACE_KNOWN is already set. So it seems an obvious
fix here is to just ensure that we clear that flag on stream-in for
vtables (we can't do it generally as it seems to be needed to be kept on
various other kinds of declarations).

Linaro complained about the last version of this patch too on ARM;
hopefully this version is friendlier.

I might also spend some time messing around to see if I can implement
https://github.com/itanium-cxx-abi/cxx-abi/issues/170 later, but that
will probably have to be a GCC 15 change.

Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk if
Linaro doesn't complain about this patch?

-- >8 --

We currently always stream DECL_INTERFACE_KNOWN, which is needed since
many kinds of declarations already have their interface determined at
parse time.  But for vtables and type-info declarations we need to
re-evaluate on stream-in, as whether they need to be emitted or not
changes in each TU, so this patch clears DECL_INTERFACE_KNOWN on these
kinds of declarations so that they can go through 'import_export_decl'
again.

Note that the precise details of the virt-2 tests will need to change
when we implement the resolution of [1]; for now I just updated the test
to not fail with the new (current) semantics.

[1]: https://github.com/itanium-cxx-abi/cxx-abi/pull/171

	PR c++/114229

gcc/cp/ChangeLog:

	* module.cc (trees_out::core_bools): Redetermine
	DECL_INTERFACE_KNOWN on stream-in for vtables and tinfo.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/virt-2_b.C: Update test to acknowledge that we
	now emit vtables here too.
	* g++.dg/modules/virt-3_a.C: New test.
	* g++.dg/modules/virt-3_b.C: New test.
	* g++.dg/modules/virt-3_c.C: New test.
	* g++.dg/modules/virt-3_d.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 gcc/cp/module.cc                        | 12 +++++++++++-
 gcc/testsuite/g++.dg/modules/virt-2_b.C |  5 ++---
 gcc/testsuite/g++.dg/modules/virt-3_a.C |  9 +++++++++
 gcc/testsuite/g++.dg/modules/virt-3_b.C |  6 ++++++
 gcc/testsuite/g++.dg/modules/virt-3_c.C |  3 +++
 gcc/testsuite/g++.dg/modules/virt-3_d.C |  7 +++++++
 6 files changed, 38 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/modules/virt-3_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/virt-3_b.C
 create mode 100644 gcc/testsuite/g++.dg/modules/virt-3_c.C
 create mode 100644 gcc/testsuite/g++.dg/modules/virt-3_d.C

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index f7e8b357fc2..d77286328f5 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -5390,7 +5390,17 @@ trees_out::core_bools (tree t)
       WB (t->decl_common.lang_flag_2);
       WB (t->decl_common.lang_flag_3);
       WB (t->decl_common.lang_flag_4);
-      WB (t->decl_common.lang_flag_5);
+
+      {
+	/* This is DECL_INTERFACE_KNOWN: We should redetermine whether
+	   we need to import or export any vtables or typeinfo objects
+	   on stream-in.  */
+	bool interface_known = t->decl_common.lang_flag_5;
+	if (VAR_P (t) && (DECL_VTABLE_OR_VTT_P (t) || DECL_TINFO_P (t)))
+	  interface_known = false;
+	WB (interface_known);
+      }
+
       WB (t->decl_common.lang_flag_6);
       WB (t->decl_common.lang_flag_7);
       WB (t->decl_common.lang_flag_8);
diff --git a/gcc/testsuite/g++.dg/modules/virt-2_b.C b/gcc/testsuite/g++.dg/modules/virt-2_b.C
index e041f0721f9..2bc5eced013 100644
--- a/gcc/testsuite/g++.dg/modules/virt-2_b.C
+++ b/gcc/testsuite/g++.dg/modules/virt-2_b.C
@@ -21,8 +21,7 @@ int main ()
   return !(Visit (&me) == 1);
 }
 
-// We do not emit Visitor vtable
-// but we do emit rtti here
-// { dg-final { scan-assembler-not {_ZTVW3foo7Visitor:} } }
+// Again, we emit Visitor vtable and rtti here
+// { dg-final { scan-assembler {_ZTVW3foo7Visitor:} } }
 // { dg-final { scan-assembler {_ZTIW3foo7Visitor:} } }
 // { dg-final { scan-assembler {_ZTSW3foo7Visitor:} } }
diff --git a/gcc/testsuite/g++.dg/modules/virt-3_a.C b/gcc/testsuite/g++.dg/modules/virt-3_a.C
new file mode 100644
index 00000000000..a7eae7f9d35
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/virt-3_a.C
@@ -0,0 +1,9 @@
+// PR c++/114229
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
+// { dg-module-cmi modA }
+
+module;
+template<class> struct basic_streambuf { virtual void overflow() { } };
+extern template struct basic_streambuf<long>;
+export module modA;
+export basic_streambuf<long> *p;
diff --git a/gcc/testsuite/g++.dg/modules/virt-3_b.C b/gcc/testsuite/g++.dg/modules/virt-3_b.C
new file mode 100644
index 00000000000..4d87b965bbf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/virt-3_b.C
@@ -0,0 +1,6 @@
+// PR c++/114229
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+// { dg-module-cmi modB }
+
+export module modB;
+import modA;
diff --git a/gcc/testsuite/g++.dg/modules/virt-3_c.C b/gcc/testsuite/g++.dg/modules/virt-3_c.C
new file mode 100644
index 00000000000..224bb4aed49
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/virt-3_c.C
@@ -0,0 +1,3 @@
+// PR c++/114229
+template<class> struct basic_streambuf { virtual void overflow() { } };
+template struct basic_streambuf<long>;
diff --git a/gcc/testsuite/g++.dg/modules/virt-3_d.C b/gcc/testsuite/g++.dg/modules/virt-3_d.C
new file mode 100644
index 00000000000..b3a0aad7abe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/virt-3_d.C
@@ -0,0 +1,7 @@
+// PR c++/114229
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+
+import modA;
+import modB;
+int main() { }
-- 
2.43.2