Notes:
This patch optimizes out calls to __dynamic_cast when the type being cast
to is final (or its destructor), replacing them with a simple comparison of the
types' vtable addresses. This is already implemented and done by default in
clang (https://review
s.llvm.org/D154658), but can be turned off with -fno-assume-unique-vtables,
due to the problems it can cause with unmerged vtables when using shared
libraries. With the optimization not really fitting well under any existing
flag (that I know of), I believe an -fassume-unique-vtables flag (or something
similar) would be appropriate to add to gcc, which besides enabling this
optimization, should probably also define __GXX_MERGED_TYPEINFO_NAMES as 1. I
believe instead of adding this flag and the vtable addr comparison logic, just
replacing __dynamic_cast with type_info::operator== (whose behaviour depends on
__GXX_MERGED_TYPEINFO_NAMES) is insufficient as it performs additional
dereferences compared to a simple vtable addr comparison, still leaving room
for further optimization.
gcc/c-family/c.opt | 4 ++
gcc/cp/rtti.cc | 55 +++++++++++++++++++++++++
gcc/testsuite/g++.dg/rtti/dyncast10.C | 58 +++++++++++++++++++++++++++
gcc/testsuite/g++.dg/rtti/dyncast9.C | 58 +++++++++++++++++++++++++++
4 files changed, 175 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/rtti/dyncast10.C
create mode 100644 gcc/testsuite/g++.dg/rtti/dyncast9.C
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 12877eb0e17..b597df70bfe 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1719,6 +1719,10 @@ fassume-sane-operators-new-delete
C++ ObjC++ Optimization Var(flag_assume_sane_operators_new_delete) Init(1)
Assume C++ replaceable global operators new, new[], delete, delete[] don't
read or write visible global state.
+fassume-unique-vtables
+C++ ObjC++ Optimization Var(flag_assume_unique_vtables) Init(0)
+Assume that, throughout the program's lifetime, each pair of class types
representing the same class shares a common vtable.
+
; Define extra predefined macros for use in libgcc.
fbuilding-libgcc
C ObjC C++ ObjC++ Undocumented Var(flag_building_libgcc)
diff --git a/gcc/cp/rtti.cc b/gcc/cp/rtti.cc
index c06a18b3ff1..7227386ddbf 100644
--- a/gcc/cp/rtti.cc
+++ b/gcc/cp/rtti.cc
@@ -761,6 +761,61 @@ build_dynamic_cast_1 (location_t loc, tree type, tree expr,
if (tc == REFERENCE_TYPE)
expr1 = cp_build_addr_expr (expr1, complain);
+
+
+ /* If type is final, don't call dynamic_cast.
+ * Instead just check vtable equivalence at runtime.
+ * TYPE_FINAL_P does not return true for non-final class with
+ * final destructor overriding virtual though,
+ * so look through virtual functions for final destructor */
+
+ bool can_inherit = true;
+ if(flag_assume_unique_vtables)
+ {
+ can_inherit = !TYPE_FINAL_P (target_type);
+ tree vchain;
+ for (vchain = BINFO_VIRTUALS (TYPE_BINFO (target_type));
+ vchain && can_inherit;
+ vchain = TREE_CHAIN (vchain))
+ {
+ if (!DECL_DESTRUCTOR_P (BV_FN (vchain)))
+ continue;
+ if (!DECL_FINAL_P (BV_FN (vchain)))
+ continue;
+ can_inherit = false;
+ }
+ }
+
+ if (!can_inherit)
+ {
+ tree binfo = lookup_base (target_type, static_type, ba_check, NULL,
complain);
+
+ if (!binfo || binfo == error_mark_node)
+ return error_mark_node;
+
+ /* Retrieve vtable declaration and address.
+ * The offset-to-top field is adjusted for here. */
+ tree trgt_vptr = build_vtbl_address (
+ BINFO_VTABLE (binfo) ? binfo : TYPE_BINFO (target_type));
+
+ tree src_obj = cp_build_fold_indirect_ref (expr);
+ tree src_vptr = build_vfield_ref (src_obj, static_type);
+
+ tree fail_result = tc == REFERENCE_TYPE ? throw_bad_cast () :
nullptr_node;
+ tree succ_result = build_base_path (MINUS_EXPR, expr1, binfo, true,
complain);
+
+ /* Check vtable equivalence by vptr address */
+ tree cond = build2 (NE_EXPR, boolean_type_node, trgt_vptr, src_vptr);
+ tree result = build3 (COND_EXPR, type, cond, fail_result, succ_result);
+
+ SET_EXPR_LOCATION (result, loc);
+
+ if (tc == REFERENCE_TYPE)
+ return result;
+
+ return build_if_nonnull (expr, result, complain);
+ }
+
elems[0] = expr1;
elems[1] = td3;
elems[2] = td2;
diff --git a/gcc/testsuite/g++.dg/rtti/dyncast10.C
b/gcc/testsuite/g++.dg/rtti/dyncast10.C
new file mode 100644
index 00000000000..e4c9b303cef
--- /dev/null
+++ b/gcc/testsuite/g++.dg/rtti/dyncast10.C
@@ -0,0 +1,58 @@
+// { dg-options "-fassume-unique-vtables" }
+// { dg-final { scan-tree-dump-not "__dynamic_cast" "original" } }
+
+extern "C" void abort ();
+
+struct A
+{
+ virtual ~A () {}
+};
+
+struct B final : public A
+{
+ ~B() {}
+ void p () { c++; }
+ int c;
+};
+struct C final : public A
+{
+ ~C() {}
+ void f () { c++; }
+ int c;
+};
+
+
+bool f (A* a) {
+ return dynamic_cast<B*>(a) != nullptr;
+}
+bool g (A& a) {
+ try
+ {
+ dynamic_cast<B&>(a).p();
+ return true;
+ }
+ catch (...)
+ {
+ return false;
+ }
+}
+
+int
+main (void)
+{
+ A a;
+ B b;
+ C c;
+ if (f (&a))
+ abort ();
+ if (g (a))
+ abort ();
+ if (!f (&b))
+ abort ();
+ if (!g (b))
+ abort ();
+ if (f (&c))
+ abort ();
+ if (g (c))
+ abort ();
+}
diff --git a/gcc/testsuite/g++.dg/rtti/dyncast9.C
b/gcc/testsuite/g++.dg/rtti/dyncast9.C
new file mode 100644
index 00000000000..4f3b2348d13
--- /dev/null
+++ b/gcc/testsuite/g++.dg/rtti/dyncast9.C
@@ -0,0 +1,58 @@
+// { dg-options "-fassume-unique-vtables" }
+// { dg-final { scan-tree-dump-not "__dynamic_cast" "original" } }
+
+extern "C" void abort ();
+
+struct A
+{
+ virtual ~A () {}
+};
+
+struct B : public A
+{
+ ~B() final {}
+ void p () { c++; }
+ int c;
+};
+struct C : public A
+{
+ ~C() final {}
+ void f () { c++; }
+ int c;
+};
+
+
+bool f (A* a) {
+ return dynamic_cast<B*>(a) != nullptr;
+}
+bool g (A& a) {
+ try
+ {
+ dynamic_cast<B&>(a).p();
+ return true;
+ }
+ catch (...)
+ {
+ return false;
+ }
+}
+
+int
+main (void)
+{
+ A a;
+ B b;
+ C c;
+ if (f (&a))
+ abort ();
+ if (g (a))
+ abort ();
+ if (!f (&b))
+ abort ();
+ if (!g (b))
+ abort ();
+ if (f (&c))
+ abort ();
+ if (g (c))
+ abort ();
+}
--
2.34.1
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