[PATCH v2] c++/modules: Differentiate extern templates and TYPE_DECL_SUPPRESS_DEBUG [PR112820]

[Nathaniel Shead <nathanieloshead@gmail.com>]

On Thu, Jan 04, 2024 at 03:39:15PM -0500, Patrick Palka wrote:
> On Sun, 3 Dec 2023, Nathaniel Shead wrote:
> 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
> > 
> > -- >8 --
> > 
> > The TYPE_DECL_SUPPRESS_DEBUG and DECL_EXTERNAL flags use the same
> > underlying bit. This is causing confusion when attempting to determine
> > the interface for a streamed-in class type, since the modules code
> > currently assumes that all DECL_EXTERNAL types are extern templates.
> > However, when -g is specified then TYPE_DECL_SUPPRESS_DEBUG (and hence
> > DECL_EXTERNAL) is marked on various other kinds of declarations, such as
> > vtables, which causes them to never be emitted.
> 
> Good catch.. Maybe we should use different bits for these flags?  I wouldn't be
> surprised if this bit sharing causes issues elsewhere in the compiler.  The
> documentation in tree.h / tree-core.h says DECL_EXTERNAL is only valid for
> VAR_DECL and FUNCTION_DECL, so at one point it was safe to share the same bit
> but that's not true anymore it seems.
> 
> Looking at tree-core.h:tree_decl_common luckily we have plenty of spare bits.
> We could also e.g. make TYPE_DECL_SUPPRESS_DEBUG use the decl_not_flexarray bit
> which is otherwise only used for FIELD_DECL.
> 

That seems like a good idea, thanks. How does this look?

Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?

-- >8 --

Currently, DECL_EXTERNAL and TYPE_DECL_SUPPRESS_DEBUG share a bit. This
causes issues with module code, which then incorrectly assumes that
anything with suppressed debug info (such as vtables when '-g' is
specified) is an extern template and thus prevents their emission.

This patch splits the two flags up; extern templates continue to use the
DECL_EXTERNAL flag (and the documentation is updated to indicate this),
but TYPE_DECL_SUPPRESS_DEBUG now uses the 'decl_not_flexarray' flag,
which currently is only used by FIELD_DECLs.

	PR c++/112820
	PR c++/102607

gcc/cp/ChangeLog:

	* pt.cc (mark_class_instantiated): Set DECL_EXTERNAL explicitly.

gcc/ChangeLog:

	* tree-core.h (struct tree_decl_common): Update comments.
	* tree.h (DECL_EXTERNAL): Update comments.
	(TYPE_DECL_SUPPRESS_DEBUG): Use 'decl_not_flexarray' instead.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/debug-2_a.C: New test.
	* g++.dg/modules/debug-2_b.C: New test.
	* g++.dg/modules/debug-2_c.C: New test.
	* g++.dg/modules/debug-3_a.C: New test.
	* g++.dg/modules/debug-3_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 gcc/cp/pt.cc                             | 1 +
 gcc/testsuite/g++.dg/modules/debug-2_a.C | 9 +++++++++
 gcc/testsuite/g++.dg/modules/debug-2_b.C | 8 ++++++++
 gcc/testsuite/g++.dg/modules/debug-2_c.C | 9 +++++++++
 gcc/testsuite/g++.dg/modules/debug-3_a.C | 8 ++++++++
 gcc/testsuite/g++.dg/modules/debug-3_b.C | 9 +++++++++
 gcc/tree-core.h                          | 6 +++---
 gcc/tree.h                               | 8 ++++----
 8 files changed, 51 insertions(+), 7 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/modules/debug-2_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/debug-2_b.C
 create mode 100644 gcc/testsuite/g++.dg/modules/debug-2_c.C
 create mode 100644 gcc/testsuite/g++.dg/modules/debug-3_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/debug-3_b.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index e38e7a773f0..7839745035b 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -26256,6 +26256,7 @@ mark_class_instantiated (tree t, int extern_p)
   SET_CLASSTYPE_EXPLICIT_INSTANTIATION (t);
   SET_CLASSTYPE_INTERFACE_KNOWN (t);
   CLASSTYPE_INTERFACE_ONLY (t) = extern_p;
+  DECL_EXTERNAL (TYPE_NAME (t)) = extern_p;
   TYPE_DECL_SUPPRESS_DEBUG (TYPE_NAME (t)) = extern_p;
   if (! extern_p)
     {
diff --git a/gcc/testsuite/g++.dg/modules/debug-2_a.C b/gcc/testsuite/g++.dg/modules/debug-2_a.C
new file mode 100644
index 00000000000..eed0905542b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/debug-2_a.C
@@ -0,0 +1,9 @@
+// PR c++/112820
+// { dg-additional-options "-fmodules-ts -g" }
+// { dg-module-cmi io }
+
+export module io;
+
+export struct error {
+  virtual const char* what() const noexcept;
+};
diff --git a/gcc/testsuite/g++.dg/modules/debug-2_b.C b/gcc/testsuite/g++.dg/modules/debug-2_b.C
new file mode 100644
index 00000000000..fc9afbc02e0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/debug-2_b.C
@@ -0,0 +1,8 @@
+// PR c++/112820
+// { dg-additional-options "-fmodules-ts -g" }
+
+module io;
+
+const char* error::what() const noexcept {
+  return "bla";
+}
diff --git a/gcc/testsuite/g++.dg/modules/debug-2_c.C b/gcc/testsuite/g++.dg/modules/debug-2_c.C
new file mode 100644
index 00000000000..37117f69dcd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/debug-2_c.C
@@ -0,0 +1,9 @@
+// PR c++/112820
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts -g" }
+
+import io;
+
+int main() {
+  error{};
+}
diff --git a/gcc/testsuite/g++.dg/modules/debug-3_a.C b/gcc/testsuite/g++.dg/modules/debug-3_a.C
new file mode 100644
index 00000000000..9e33d8260fd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/debug-3_a.C
@@ -0,0 +1,8 @@
+// PR c++/102607
+// { dg-additional-options "-fmodules-ts -g" }
+// { dg-module-cmi mod }
+
+export module mod;
+export struct B {
+  virtual ~B() = default;
+};
diff --git a/gcc/testsuite/g++.dg/modules/debug-3_b.C b/gcc/testsuite/g++.dg/modules/debug-3_b.C
new file mode 100644
index 00000000000..03c78b71b5d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/debug-3_b.C
@@ -0,0 +1,9 @@
+// PR c++/102607
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts -g" }
+
+import mod;
+int main() {
+  struct D : B {};
+  (void)D{};
+}
diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index 8a89462bd7e..1ca4d4c07bd 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -1814,8 +1814,7 @@ struct GTY(()) tree_decl_common {
      In FIELD_DECL, this is DECL_FIELD_ABI_IGNORED.  */
   unsigned decl_flag_0 : 1;
   /* In FIELD_DECL, this is DECL_BIT_FIELD
-     In VAR_DECL and FUNCTION_DECL, this is DECL_EXTERNAL.
-     In TYPE_DECL, this is TYPE_DECL_SUPPRESS_DEBUG.  */
+     In VAR_DECL, FUNCTION_DECL, and TYPE_DECL, this is DECL_EXTERNAL.  */
   unsigned decl_flag_1 : 1;
   /* In FIELD_DECL, this is DECL_NONADDRESSABLE_P
      In VAR_DECL, PARM_DECL and RESULT_DECL, this is
@@ -1845,7 +1844,8 @@ struct GTY(()) tree_decl_common {
      TYPE_WARN_IF_NOT_ALIGN.  */
   unsigned int warn_if_not_align : 6;
 
-  /* In FIELD_DECL, this is DECL_NOT_FLEXARRAY.  */
+  /* In FIELD_DECL, this is DECL_NOT_FLEXARRAY.
+     In TYPE_DECL, this is TYPE_DECL_SUPPRESS_DEBUG.  */
   unsigned int decl_not_flexarray : 1;
 
   /* 5 bits unused.  */
diff --git a/gcc/tree.h b/gcc/tree.h
index 972a067a1f7..8bdc471ad4e 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -2848,9 +2848,9 @@ extern tree vector_element_bits_tree (const_tree);
 #define DECL_LANG_SPECIFIC(NODE) \
   (DECL_COMMON_CHECK (NODE)->decl_common.lang_specific)
 
-/* In a VAR_DECL or FUNCTION_DECL, nonzero means external reference:
-   do not allocate storage, and refer to a definition elsewhere.  Note that
-   this does not necessarily imply the entity represented by NODE
+/* In a VAR_DECL, FUNCTION_DECL, or TYPE_DECL, nonzero means external
+   reference: do not allocate storage, and refer to a definition elsewhere.
+   Note that this does not necessarily imply the entity represented by NODE
    has no program source-level definition in this translation unit.  For
    example, for a FUNCTION_DECL, DECL_SAVED_TREE may be non-NULL and
    DECL_EXTERNAL may be true simultaneously; that can be the case for
@@ -3553,7 +3553,7 @@ extern vec<tree, va_gc> **decl_debug_args_insert (tree);
    into stabs.  Instead it will generate cross reference ('x') of names.
    This uses the same flag as DECL_EXTERNAL.  */
 #define TYPE_DECL_SUPPRESS_DEBUG(NODE) \
-  (TYPE_DECL_CHECK (NODE)->decl_common.decl_flag_1)
+  (TYPE_DECL_CHECK (NODE)->decl_common.decl_not_flexarray)
 
 /* Getter of the imported declaration associated to the
    IMPORTED_DECL node.  */
-- 
2.43.0