Re: [PATCH] c++: conversion to base of vbase in NSDMI [PR80431]

[Jason Merrill via Gcc-patches]

On 6/18/21 4:39 PM, Patrick Palka wrote:
The delayed processing of conversions to a virtual base inside an NSDMI
assumes the target base type is a (possibly indirect) virtual base of
the current class, but the target base type could also be an indirect
non-virtual base inherited from a virtual base, as in the testcase below.
Since such a base isn't a part of CLASSTYPE_VBASECLASSES, we end up
miscompiling the testcase due to build_base_path (called with
binfo=NULL_TREE) silently returning error_mark_node.  Fix this by
using convert_to_base to build the conversion.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

        PR c++/80431

gcc/cp/ChangeLog:

        * tree.c (bot_replace): Use convert_to_base instead of
        only looking through CLASSTYPE_VBASECLASSES of the current class
        type.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/nsdmi-virtual1a.C: New test.
---
  gcc/cp/tree.c                                | 10 ++-----
  gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C | 29 ++++++++++++++++++++
  2 files changed, 32 insertions(+), 7 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index fec5afaa2be..3537f395960 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -3244,13 +3244,9 @@ bot_replace (tree* t, int* /*walk_subtrees*/, void* 
data_)
      {
        /* In an NSDMI build_base_path defers building conversions to virtual

So this should be "morally virtual" 
(https://itanium-cxx-abi.github.io/cxx-abi/abi.html#definitions)

OK with that change.

         bases, and we handle it here.  */
-      tree basetype = TYPE_MAIN_VARIANT (TREE_TYPE (TREE_TYPE (*t)));
-      vec<tree, va_gc> *vbases = CLASSTYPE_VBASECLASSES (current_class_type);
-      int i; tree binfo;
-      FOR_EACH_VEC_SAFE_ELT (vbases, i, binfo)
-       if (BINFO_TYPE (binfo) == basetype)
-         break;
-      *t = build_base_path (PLUS_EXPR, TREE_OPERAND (*t, 0), binfo, true,
+      tree basetype = TREE_TYPE (*t);
+      *t = convert_to_base (TREE_OPERAND (*t, 0), basetype,
+                           /*check_access=*/false, /*nonnull=*/true,
                            tf_warning_or_error);
      }
  
diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C
new file mode 100644
index 00000000000..fe647fe3cf7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-virtual1a.C
@@ -0,0 +1,29 @@
+// PR c++/80431
+// { dg-do run { target c++11 } }
+
+// A variant of nsdmi-virtual1.C that turns A from a virtual base of B to a 
base
+// of a virtual base of B, using the intermediate class D.
+
+struct A
+{
+  A(): i(42) { }
+  int i;
+  int f() { return i; }
+};
+
+struct D : A { int pad; };
+
+struct B : virtual D
+{
+  int j = i + f();
+  int k = A::i + A::f();
+};
+
+struct C: B { int pad; };
+
+int main()
+{
+  C c;
+  if (c.j != 84 || c.k != 84)
+    __builtin_abort();
+}