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

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

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.

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