> 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 <ja...@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.
Given my experience about numbers of functions that become reachable when you
stream all virtuals into LTO,
I wonder if we don't want to use possible_polymorphic_call_targets within the
front-end to avoid instantiating
those that can't be called?
I think it should not be too hard - all we need is to populate the type
inheritance graph from FE and then
for each polymorphic call produce the list to mark possible targets are
reachable.
Honza
>
> 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> { };
