c++: Retry the aliasing of base/complete cdtor optimization at import_export_decl...

When expand_or_defer_fn is called at_eof time, it calls import_export_decl
and then maybe_clone_body, which uses DECL_ONE_ONLY and comdat name in a
couple of places to try to optimize cdtors which are known to have the
same body by making the complete cdtor an alias to base cdtor (and in
that case also uses *[CD]5* as comdat group name instead of the normal
comdat group names specific to each mangled name).
Now, this optimization depends on DECL_ONE_ONLY and DECL_INTERFACE_KNOWN,
maybe_clone_body and can_alias_cdtor use:
      if (DECL_ONE_ONLY (fn))
        cgraph_node::get_create (clone)->set_comdat_group (cxx_comdat_group (clone));
...
  bool can_alias = can_alias_cdtor (fn);
...
      /* Tell cgraph if both ctors or both dtors are known to have
         the same body.  */
      if (can_alias
          && fns[0]
          && idx == 1
          && cgraph_node::get_create (fns[0])->create_same_body_alias
               (clone, fns[0]))
        {
          alias = true;
          if (DECL_ONE_ONLY (fns[0]))
            {
              /* For comdat base and complete cdtors put them
                 into the same, *[CD]5* comdat group instead of
                 *[CD][12]*.  */
              comdat_group = cdtor_comdat_group (fns[1], fns[0]);
              cgraph_node::get_create (fns[0])->set_comdat_group (comdat_group);
              if (symtab_node::get (clone)->same_comdat_group)
                symtab_node::get (clone)->remove_from_same_comdat_group ();
              symtab_node::get (clone)->add_to_same_comdat_group
                (symtab_node::get (fns[0]));
            }
        }
and
  /* Don't use aliases for weak/linkonce definitions unless we can put both
     symbols in the same COMDAT group.  */
  return (DECL_INTERFACE_KNOWN (fn)
          && (SUPPORTS_ONE_ONLY || !DECL_WEAK (fn))
          && (!DECL_ONE_ONLY (fn)
              || (HAVE_COMDAT_GROUP && DECL_WEAK (fn))));
The following testcase regressed with Marek's r14-5979 change,
when pr113208_0.C is compiled where the ctor is marked constexpr,
we no longer perform this optimization, where
_ZN6vectorI12QualityValueEC2ERKS1_ was emitted in the
_ZN6vectorI12QualityValueEC5ERKS1_ comdat group and
_ZN6vectorI12QualityValueEC1ERKS1_ was made an alias to it,
instead we emit _ZN6vectorI12QualityValueEC2ERKS1_ in
_ZN6vectorI12QualityValueEC2ERKS1_ comdat group and the same
content _ZN6vectorI12QualityValueEC1ERKS1_ as separate symbol in
_ZN6vectorI12QualityValueEC1ERKS1_ comdat group.
Now, the linker seems to somehow cope with that, eventhough it
probably keeps both copies of the ctor, but seems LTO can't cope
with that and Honza doesn't know what it should do in that case
(linker decides that the prevailing symbol is
_ZN6vectorI12QualityValueEC2ERKS1_ (from the
_ZN6vectorI12QualityValueEC2ERKS1_ comdat group) and
_ZN6vectorI12QualityValueEC1ERKS1_ alias (from the other TU,
from _ZN6vectorI12QualityValueEC5ERKS1_ comdat group)).

Note, the case where some constructor is marked constexpr in one
TU and not in another one happens pretty often in libstdc++ when
one mixes -std= flags used to compile different compilation units.

The reason the optimization doesn't trigger when the constructor is
constexpr is that expand_or_defer_fn is called in that case much earlier
than when it is not constexpr; in the former case it is called when we
try to constant evaluate that constructor.  But DECL_INTERFACE_KNOWN
is false in that case and comdat_linkage hasn't been called either
(I think it is desirable, because comdat group is stored in the cgraph
node and am not sure it is a good idea to create cgraph nodes for
something that might not be needed later on at all), so maybe_clone_body
clones the bodies, but doesn't make them as aliases.

The following patch is an attempt to redo this optimization when
import_export_decl is called at_eof time on the base/complete cdtor
(or deleting dtor).  It will not do anything if maybe_clone_body
hasn't been called uyet (the TREE_ASM_WRITTEN check on the
DECL_MAYBE_IN_CHARGE_CDTOR_P), or when one or both of the base/complete
cdtors have been lowered already, or when maybe_clone_body called
maybe_thunk_body and it was successful.  Otherwise retries the
can_alias_cdtor check and makes the complete cdtor alias to the
base cdtor with adjustments to the comdat group.

2024-04-25  Jakub Jelinek  <jakub@redhat.com>

	PR lto/113208
	* cp-tree.h (maybe_optimize_cdtor): Declare.
	* decl2.cc (import_export_decl): Call it for cloned cdtors.
	* optimize.cc (maybe_optimize_cdtor): New function.

	* g++.dg/abi/comdat2.C: New test.
	* g++.dg/abi/comdat5.C: New test.
	* g++.dg/lto/pr113208_0.C: New test.
	* g++.dg/lto/pr113208_1.C: New file.
	* g++.dg/lto/pr113208.h: New file.

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 1dbb577a38d7dac2c5dd91e3220cf165a66682c5..bafdf63dc63b6fbdd841b1db6ea5dbae4c7af51e 100644 (file)
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7447,6 +7447,7 @@ extern bool handle_module_option (unsigned opt, const char *arg, int value);
 /* In optimize.cc */
 extern tree clone_attrs                                (tree);
 extern bool maybe_clone_body                   (tree);
+extern void maybe_optimize_cdtor               (tree);
 
 /* In parser.cc */
 extern tree cp_convert_range_for (tree, tree, tree, cp_decomp *, bool,

diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 1339f210dde52460b72d377e64e38719e1cb9a42..806a2a4bc69d0131825d1c7fd668a4e073219812 100644 (file)
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -3568,6 +3568,9 @@ import_export_decl (tree decl)
     }
 
   DECL_INTERFACE_KNOWN (decl) = 1;
+
+  if (DECL_CLONED_FUNCTION_P (decl))
+    maybe_optimize_cdtor (decl);
 }
 
 /* Return an expression that performs the destruction of DECL, which

diff --git a/gcc/cp/optimize.cc b/gcc/cp/optimize.cc
index ff4fb5be5bd9daa51fa62c696a8f2e2cec90b045..7147f5e00f1331a17a06918e1c034d0723df43ea 100644 (file)
--- a/gcc/cp/optimize.cc
+++ b/gcc/cp/optimize.cc
@@ -723,3 +723,58 @@ maybe_clone_body (tree fn)
   /* We don't need to process the original function any further.  */
   return 1;
 }
+
+/* If maybe_clone_body is called while the cdtor is still tentative,
+   DECL_ONE_ONLY will be false and so will be can_alias_cdtor (fn).
+   In that case we wouldn't try to optimize using an alias and instead
+   would emit separate base and complete cdtor.  The following function
+   attempts to still optimize that case when we import_export_decl
+   is called first time on one of the clones.  */
+
+void
+maybe_optimize_cdtor (tree orig_decl)
+{
+  tree fns[3];
+  tree fn = DECL_CLONED_FUNCTION (orig_decl);
+  gcc_checking_assert (DECL_MAYBE_IN_CHARGE_CDTOR_P (fn));
+  if (DECL_INTERFACE_KNOWN (fn)
+      || !TREE_ASM_WRITTEN (fn)
+      || !DECL_ONE_ONLY (orig_decl)
+      || symtab->global_info_ready)
+    return;
+
+  populate_clone_array (fn, fns);
+
+  if (!fns[0] || !fns[1])
+    return;
+  for (int i = 2 - !fns[2]; i >= 0; --i)
+    if (fns[i] != orig_decl && DECL_INTERFACE_KNOWN (fns[i]))
+      return;
+  DECL_INTERFACE_KNOWN (fn) = 1;
+  comdat_linkage (fn);
+  if (!can_alias_cdtor (fn))
+    return;
+  /* For comdat base and complete cdtors put them into the same,
+     *[CD]5* comdat group instead of *[CD][12]*.  */
+  auto n0 = cgraph_node::get_create (fns[0]);
+  auto n1 = cgraph_node::get_create (fns[1]);
+  auto n2 = fns[2] ? cgraph_node::get_create (fns[1]) : NULL;
+  if (n0->lowered || n1->lowered || (n2 && n2->lowered))
+    return;
+  import_export_decl (fns[0]);
+  n1->definition = false;
+  if (!n0->create_same_body_alias (fns[1], fns[0]))
+    return;
+
+  tree comdat_group = cdtor_comdat_group (fns[1], fns[0]);
+  n1 = cgraph_node::get (fns[1]);
+  n0->set_comdat_group (comdat_group);
+  if (n1->same_comdat_group)
+    n1->remove_from_same_comdat_group ();
+  n1->add_to_same_comdat_group (n0);
+  if (fns[2])
+    n2->add_to_same_comdat_group (n0);
+  import_export_decl (fns[1]);
+  /* Remove the body now that it is an alias.  */
+  release_function_body (fns[1]);
+}

diff --git a/gcc/testsuite/g++.dg/abi/comdat2.C b/gcc/testsuite/g++.dg/abi/comdat2.C
new file mode 100644 (file)
index 0000000..502d00c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/abi/comdat2.C
@@ -0,0 +1,26 @@
+// PR lto/113208
+// { dg-do compile { target { c++11 && { *-*-*gnu* } } } }
+// { dg-additional-options "-O2 -fkeep-inline-functions" }
+// { dg-final { scan-assembler "_ZN1BI1CEC5ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC1ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC2ERKS1_,comdat" } }
+
+template <typename T>
+struct A {
+  int foo () const;
+  A (int, int);
+};
+template <typename T>
+struct B : A<T> {
+  constexpr B (const B &x) : A<T> (1, x.foo ()) {}
+  B () : A<T> (1, 2) {}
+};
+struct C;
+struct D : B<C> {};
+void bar (D);
+
+void
+baz (D x)
+{
+  bar (x);
+}

diff --git a/gcc/testsuite/g++.dg/abi/comdat5.C b/gcc/testsuite/g++.dg/abi/comdat5.C
new file mode 100644 (file)
index 0000000..f6542aa
--- /dev/null
+++ b/gcc/testsuite/g++.dg/abi/comdat5.C
@@ -0,0 +1,28 @@
+// PR lto/113208
+// { dg-do compile { target { c++11 && { *-*-*gnu* } } } }
+// { dg-additional-options "-O2" }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC5ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC1ERKS1_,comdat" } }
+// { dg-final { scan-assembler-not "_ZN1BI1CEC2ERKS1_,comdat" } }
+
+template <typename T>
+struct A {
+  int foo () const;
+  A (int, int);
+};
+template <typename T>
+struct B : A<T> {
+  constexpr B (const B &x) : A<T> (1, x.foo ()) {}
+  B () : A<T> (1, 2) {}
+};
+struct C;
+struct D : B<C> {};
+void bar (D);
+
+void
+baz (D x)
+{
+  bar (x);
+}
+
+extern template struct B<C>;

diff --git a/gcc/testsuite/g++.dg/lto/pr113208.h b/gcc/testsuite/g++.dg/lto/pr113208.h
new file mode 100644 (file)
index 0000000..7901ede
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lto/pr113208.h
@@ -0,0 +1,10 @@
+template <typename _Tp> struct _Vector_base {
+  int g() const;
+  _Vector_base(int, int);
+};
+template <typename _Tp>
+struct vector : _Vector_base<_Tp> {
+  CONSTEXPR vector(const vector &__x)
+      : _Vector_base<_Tp>(1, __x.g()) {}
+ vector() : _Vector_base<_Tp>(1, 2) {}
+};

diff --git a/gcc/testsuite/g++.dg/lto/pr113208_0.C b/gcc/testsuite/g++.dg/lto/pr113208_0.C
new file mode 100644 (file)
index 0000000..dbaca55
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lto/pr113208_0.C
@@ -0,0 +1,13 @@
+// { dg-lto-do link }
+// { dg-lto-options { {-O1 -std=c++20 -flto}} }
+// { dg-extra-ld-options "-r -nostdlib -flinker-output=nolto-rel" }
+// { dg-require-linker-plugin "" }
+
+#define CONSTEXPR constexpr
+#include "pr113208.h"
+
+struct QualityValue;
+struct k : vector<QualityValue> {};
+
+void m(k);
+void n(k i) { m(i); }

diff --git a/gcc/testsuite/g++.dg/lto/pr113208_1.C b/gcc/testsuite/g++.dg/lto/pr113208_1.C
new file mode 100644 (file)
index 0000000..f925086
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lto/pr113208_1.C
@@ -0,0 +1,6 @@
+#define CONSTEXPR 
+#include "pr113208.h"
+
+struct QualityValue;
+vector<QualityValue> values1;
+vector<QualityValue> values{values1};