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C++ PATCH for c++/61687 (extra errors with -O2)


My earlier patch for 61659 caused more virtual functions to be instantiated when -fdevirtualize is on, leading to additional errors appearing at higher optimization levels. This patch shifts that instantiation to a new flag, -fuse-all-virtuals, which is on by default, and adds an explanatory note to help people understand why their code is breaking and how they can work around it if needed.

Tested x86_64-pc-linux-gnu, applying to trunk.
commit 82187fb06863161765a270f8ba00bdbf975b3af2
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Jul 7 03:15:02 2014 -0700

    	PR c++/61659
    	PR c++/61687
    gcc/c-family/
    	* c.opt (-fuse-all-virtuals): New.
    gcc/cp/
    	* decl2.c (mark_all_virtuals): New variable.
    	(maybe_emit_vtables): Check it instead of flag_devirtualize.
    	(cp_write_global_declarations): Set it and give helpful diagnostic
    	if it introduces errors.
    	* class.c (finish_struct_1): Check it.
    	* decl.c (grokdeclarator): Clear virtualp after 'virtual auto' error.

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index faef774..3a2084f 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1268,6 +1268,10 @@ funsigned-char
 C ObjC C++ ObjC++ LTO Var(flag_signed_char, 0)
 Make \"char\" unsigned by default
 
+fuse-all-virtuals
+C++ ObjC++ Var(flag_use_all_virtuals) Init(1)
+Treat all virtual functions as odr-used
+
 fuse-cxa-atexit
 C++ ObjC++ Var(flag_use_cxa_atexit) Init(DEFAULT_USE_CXA_ATEXIT)
 Use __cxa_atexit to register destructors
diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 3a44dba..d0eb103 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -6408,7 +6408,7 @@ finish_struct_1 (tree t)
 	 in every translation unit where the class definition appears.  If
 	 we're devirtualizing, we can look into the vtable even if we
 	 aren't emitting it.  */
-      if (CLASSTYPE_KEY_METHOD (t) == NULL_TREE || flag_devirtualize)
+      if (CLASSTYPE_KEY_METHOD (t) == NULL_TREE || flag_use_all_virtuals)
 	keyed_classes = tree_cons (NULL_TREE, t, keyed_classes);
     }
 
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 1ade586..01d74e3 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -9631,8 +9631,11 @@ grokdeclarator (const cp_declarator *declarator,
 				    "-std=gnu++1y");
 			  }
 			else if (virtualp)
-			  error ("virtual function cannot "
-				 "have deduced return type");
+			  {
+			    error ("virtual function cannot "
+				   "have deduced return type");
+			    virtualp = false;
+			  }
 		      }
 		    else if (!is_auto (type))
 		      {
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index 98897f4..0926dbc 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -106,6 +106,11 @@ static GTY(()) vec<tree, va_gc> *no_linkage_decls;
 /* Nonzero if we're done parsing and into end-of-file activities.  */
 
 int at_eof;
+
+/* Nonzero if we've instantiated everything used directly, and now want to
+   mark all virtual functions as used so that they are available for
+   devirtualization.  */
+static int mark_all_virtuals;
 
 
 /* Return a member function type (a METHOD_TYPE), given FNTYPE (a
@@ -2009,7 +2014,7 @@ maybe_emit_vtables (tree ctype)
       if (DECL_COMDAT (primary_vtbl)
 	  && CLASSTYPE_DEBUG_REQUESTED (ctype))
 	note_debug_info_needed (ctype);
-      if (flag_devirtualize)
+      if (mark_all_virtuals)
 	/* Make sure virtual functions get instantiated/synthesized so that
 	   they can be inlined after devirtualization even if the vtable is
 	   never emitted.  */
@@ -4340,6 +4345,8 @@ cp_write_global_declarations (void)
      instantiated, etc., etc.  */
 
   emit_support_tinfos ();
+  int errs = errorcount + sorrycount;
+  bool explained_devirt = false;
 
   do
     {
@@ -4572,6 +4579,27 @@ cp_write_global_declarations (void)
 					 pending_statics->length ()))
 	reconsider = true;
 
+      if (flag_use_all_virtuals)
+	{
+	  if (!reconsider && !mark_all_virtuals)
+	    {
+	      mark_all_virtuals = true;
+	      reconsider = true;
+	      errs = errorcount + sorrycount;
+	    }
+	  else if (mark_all_virtuals
+		   && !explained_devirt
+		   && (errorcount + sorrycount > errs))
+	    {
+	      inform (global_dc->last_location, "this error is seen due to "
+		      "instantiation of all virtual functions, which the C++ "
+		      "standard says are always considered used; this is done "
+		      "to support devirtualization optimizations, but can be "
+		      "disabled with -fno-use-all-virtuals");
+	      explained_devirt = true;
+	    }
+	}
+
       retries++;
     }
   while (reconsider);
diff --git a/gcc/testsuite/g++.dg/template/dtor9.C b/gcc/testsuite/g++.dg/template/dtor9.C
index fd71389..006a754 100644
--- a/gcc/testsuite/g++.dg/template/dtor9.C
+++ b/gcc/testsuite/g++.dg/template/dtor9.C
@@ -1,4 +1,5 @@
 // PR c++/60347
+// { dg-options "-fno-use-all-virtuals" }
 
 struct A;
 
diff --git a/gcc/testsuite/g++.dg/template/dtor9a.C b/gcc/testsuite/g++.dg/template/dtor9a.C
new file mode 100644
index 0000000..aaae8b6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/dtor9a.C
@@ -0,0 +1,13 @@
+// PR c++/60347
+// { dg-options "-fuse-all-virtuals" }
+
+struct A;
+
+template <class T>
+struct B
+{
+  T* p;
+  virtual ~B() { p->~T(); }	// { dg-error "incomplete" }
+};
+
+struct C: B<A> { };

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