[Bug c/8081] ICE with variably sized types returned from nested functions

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8081

--- Comment #27 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-13 10:11:43 UTC ---
(In reply to comment #26)
> On Fri, 13 Jan 2012, ebotcazou at gcc dot gnu.org wrote:
> 
> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8081
> > 
> > Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:
> > 
> >            What    |Removed                     |Added
> > ----------------------------------------------------------------------------
> >                  CC|                            |ebotcazou at gcc dot
> >                    |                            |gnu.org
> > 
> > --- Comment #25 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-01-13 08:36:56 UTC ---
> > > It's of course not that easy as we gimplify before un-nesting.  The frontend
> > > would be responsible to arrange things that way, similar to how we pass
> > > a return slot in the C++ frontend (DECL_BY_REFERENCE on the DECL_RESULT
> > > variable).  Like for the attached patch.  Passes
> > > 
> > > extern void abort (void);
> > > int
> > > main (int argc, char **argv)
> > > {
> > >   int size = 10;
> > >   typedef struct
> > >     {
> > >       char val[size];
> > >     }
> > >   block;
> > >   block a, b;
> > >   block __attribute__((noinline))
> > >   retframe_block ()
> > >     {
> > >       return *(block *) &b;
> > >     }
> > >   b.val[0] = -1;
> > >   b.val[1] = -2;
> > >   a=retframe_block ();
> > >   if (a.val[0] != -1
> > >       || a.val[1] != -2)
> > >     abort ();
> > >   return 0;
> > > }
> > > 
> > > I'm not sure if one can construct a testcase where using return-slot
> > > optimization causes wrong-code generation.  Alternatively checking
> > > DECL_BY_REFERENCE on the callees DECL_RESULT instead of applying it to
> > > all VLA types could work (though not for indirect calls).
> > 
> > You should ask specialists. :-)  In Ada, we do this routinely and the strategy
> > used is that of the "forced RSO": we generate INIT_EXPR instead of MODIFY_EXPR
> > and we create an explicit temporary if we detect potential overlap.
> > 
> > Btw, I don't understand why you're mixing DECL_BY_REFERENCE and RSO here, just
> > 
> > Index: gimplify.c
> > ===================================================================
> > --- gimplify.c  (revision 183104)
> > +++ gimplify.c  (working copy)
> > @@ -4417,6 +4417,9 @@ gimplify_modify_expr_rhs (tree *expr_p,
> >                 /* It's OK to use the target directly if it's being
> >                    initialized. */
> >                 use_target = true;
> > +             else if (variably_modified_type_p (TREE_TYPE (*to_p), NULL_TREE))
> > +               /* Always use the target for variable-sized types.  */
> > +               use_target = true;
> >               else if (TREE_CODE (*to_p) != SSA_NAME
> >                       && (!is_gimple_variable (*to_p)
> >                           || needs_to_live_in_memory (*to_p)))
> > 
> > works for me on the testcase.
> 
> Ah, ok.  So I suppose the Frontend could force RSO here as well by
> just setting CALL_EXPR_RETURN_SLOT_OPT on the CALL_EXPR.  Not sure
> which approach is better.

OTOH as GIMPLE cannot deal with VLAs on the LHS of a CALL when not applying RSO
the above is correct anyway.

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