[Bug c/8081] ICE with variably sized types returned from nested functions
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- Subject: [Bug c/8081] ICE with variably sized types returned from nested functions
- From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
- Date: Fri, 13 Jan 2012 09:08:30 +0000
- Message-id: <bug-8081-5724-z3n43N1X2g@http.gcc.gnu.org/bugzilla/>
- In-reply-to: <bug-8081-5724@http.gcc.gnu.org/bugzilla/>
- References: <bug-8081-5724@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8081
--- Comment #26 from rguenther at suse dot de <rguenther at suse dot de> 2012-01-13 09:08:30 UTC ---
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.
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