Re: C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions (v4)

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Sep 20, 2018 at 09:12:53AM +0200, Andreas Schwab wrote:
> On Sep 19 2018, Jason Merrill <jason@redhat.com> wrote:
> 
> > Andreas, do the new testcases pass?  That would surprise me, but OK if so.
> 
> No, they don't.
> 
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:29:26: error: non-constant condition for static assertion
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:29:23: error: expression '((& X2::_ZTV2X2) + 16)' does not designate a 'constexpr' function
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:33:26: error: non-constant condition for static assertion
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:33:23: error: expression '((& X2::_ZTV2X2) + 16)' does not designate a 'constexpr' function
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:37:27: error: non-constant condition for static assertion
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:37:24: error: expression '((& X2::_ZTV2X2) + 16)' does not designate a 'constexpr' function
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:41:26: error: non-constant condition for static assertion
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:41:23: error: expression '((& X4::_ZTV2X4) + 16)' does not designate a 'constexpr' function
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:45:26: error: non-constant condition for static assertion
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:45:23: error: expression '((& X4::_ZTV2X4) + 16)' does not designate a 'constexpr' function
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:49:27: error: non-constant condition for static assertion
> /usr/local/gcc/gcc-20180920/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual2.C:49:24: error: expression '((& X4::_ZTV2X4) + 16)' does not designate a 'constexpr' function
> compiler exited with status 1
> FAIL: g++.dg/cpp2a/constexpr-virtual2.C   (test for excess errors)

I think the primary problem here is:
      /* When using function descriptors, the address of the
         vtable entry is treated as a function pointer.  */
      if (TARGET_VTABLE_USES_DESCRIPTORS)
        e2 = build1 (NOP_EXPR, TREE_TYPE (e2),
                     cp_build_addr_expr (e2, complain));
in typeck.c, on non-descriptor targets we have an INDIRECT_REF where we
read the vtable function pointer.  On ia64, the above optimizes the
INDIRECT_REF away, so what the cxx_eval_call_expression actually gets
after constexpr evaluating the CALL_FN is not ADDR_EXPR of a function,
but the address of the function descriptor (e.g. &_ZTV2X2 + 16 ).

So, perhaps in cxx_eval_call_expression we need:
       if (TREE_CODE (fun) == ADDR_EXPR)
 	fun = TREE_OPERAND (fun, 0);
+      else if (TARGET_VTABLE_USES_DESCRIPTORS
+	       && TREE_CODE (fun) == POINTER_PLUS_EXPR
+	       && ...)
where we verify that p+ first argument is ADDR_EXPR of a virtual table,
second arg is INTEGER_CST and just walk the DECL_INITIAL of that, finding
the FDESC_EXPR at the right offset (therefore, I believe you need following
rather than the patch you've posted, so that you can actually find it) and
finally pick the function from the FDESC_EXPR entry.
Makes me wonder what happens with indirect calls in constexpr evaluation,
e.g. if I do:
constexpr int bar () { return 42; }
constexpr int foo () { int (*fn) () = bar; return fn (); }
static_assert (foo () == 42);
but apparently this works.

--- gcc/cp/class.c.jj	2018-09-20 09:56:59.229751895 +0200
+++ gcc/cp/class.c	2018-09-20 10:12:17.447370890 +0200
@@ -9266,7 +9266,6 @@ build_vtbl_initializer (tree binfo,
       tree vcall_index;
       tree fn, fn_original;
       tree init = NULL_TREE;
-      tree idx = size_int (jx++);
 
       fn = BV_FN (v);
       fn_original = fn;
@@ -9370,7 +9369,7 @@ build_vtbl_initializer (tree binfo,
 	  int i;
 	  if (init == size_zero_node)
 	    for (i = 0; i < TARGET_VTABLE_USES_DESCRIPTORS; ++i)
-	      CONSTRUCTOR_APPEND_ELT (*inits, idx, init);
+	      CONSTRUCTOR_APPEND_ELT (*inits, size_int (jx++), init);
 	  else
 	    for (i = 0; i < TARGET_VTABLE_USES_DESCRIPTORS; ++i)
 	      {
@@ -9378,11 +9377,11 @@ build_vtbl_initializer (tree binfo,
 				     fn, build_int_cst (NULL_TREE, i));
 		TREE_CONSTANT (fdesc) = 1;
 
-		CONSTRUCTOR_APPEND_ELT (*inits, idx, fdesc);
+		CONSTRUCTOR_APPEND_ELT (*inits, size_int (jx++), fdesc);
 	      }
 	}
       else
-	CONSTRUCTOR_APPEND_ELT (*inits, idx, init);
+	CONSTRUCTOR_APPEND_ELT (*inits, size_int (jx++), init);
     }
 }
 


	Jakub