Jim Blandy wrote:
>
>
>Is this optimization really legit?
>
>I've never come across an ABI which actually requires functions that
>return structures by value to leave the return value buffer untouched
>until just before they return.
>
The optimization checks first that the address is not exposed:
else if (!is_gimple_non_addressable (*to_p))
/* Don't use the original target if it's already addressable;
if its address escapes, and the called function uses the
NRV optimization, a conforming program could see *to_p
change before the called function returns; see c++/19317.
When optimizing, the return_slot pass marks more functions
as safe after we have escape info. */
use_target = false;
is_gimple_non_addressable checks needs_to_live_in_memory:
bool
needs_to_live_in_memory (tree t)
{
return (TREE_ADDRESSABLE (t)
|| is_global_var (t)
|| (TREE_CODE (t) == RESULT_DECL
&& aggregate_value_p (t, current_function_decl)));
}
> Consider code like this:
>
>struct f { int a, b };
>struct f x;
>
>struct f foo (void) { struct f y; y.a = 42; y.b = x.a; return y; }
>
>struct f bar () { x = foo (); return x; }
>
>The ABI's I've seen permit foo to actually store the 42 directly into
>the 'a' slot of their return buffer, before fetching x.a. If bar then
>passes x itself to foo as its return value buffer, then you wipe out
>x.a before you have a chance to reference it.
>
>
This would be caught by the is_global_var check:
bool
is_global_var (tree t)
{
return (TREE_STATIC (t) || DECL_EXTERNAL (t));
}