On Tue, Jan 07, 2014 at 02:27:04PM +0100, Florian Weimer wrote:
> gimplify_modify_expr_rhs, in the CALL_EXPR case:
>
> if (use_target)
> {
> CALL_EXPR_RETURN_SLOT_OPT (*from_p) = 1;
> mark_addressable (*to_p);
> }
Yeah, that sets it in some cases too, not in other testcases.
Just look at how the flag is used when actually expanding it:
if (target && MEM_P (target) && CALL_EXPR_RETURN_SLOT_OPT (exp))
structure_value_addr = XEXP (target, 0);
else
{
/* For variable-sized objects, we must be called with a target
specified. If we were to allocate space on the stack here,
we would have no way of knowing when to free it. */
rtx d = assign_temp (rettype, 1, 1);
structure_value_addr = XEXP (d, 0);
target = 0;
}
so, if it is set, the address of the var on the LHS is passed to the
function as hidden argument, if it is not set, we pass address of
a stack temporary instead. Both the automatic var and the stack temporary
can overflow, if the callee does something wrong.
Jakub
