On Tue, Nov 17, 2015 at 4:22 AM, Richard Biener
<[email protected]> wrote:
> On Tue, Nov 17, 2015 at 12:01 PM, H.J. Lu <[email protected]> wrote:
>> Empty record should be returned and passed the same way in C and C++.
>> This patch adds LANG_HOOKS_EMPTY_RECORD_P for C++ empty class, which
>> defaults to return false. For C++, LANG_HOOKS_EMPTY_RECORD_P is defined
>> to is_really_empty_class, which returns true for C++ empty classes. For
>> LTO, we stream out a bit to indicate if a record is empty and we store
>> it in TYPE_LANG_FLAG_0 when streaming in. get_ref_base_and_extent is
>> changed to set bitsize to 0 for empty records. Middle-end and x86
>> backend are updated to ignore empty records for parameter passing and
>> function value return. Other targets may need similar changes.
>
> Please avoid a new langhook for this and instead claim a bit in
> tree_type_common
> like for example restrict_flag (double-check it is unused for non-pointers).
There is no bit in tree_type_common I can overload. restrict_flag is
checked for non-pointers to issue an error when it is used on non-pointers:
/export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/template/qualttp20.C:19:38:
error: ‘__restrict__’ qualifiers cannot be applied to ‘AS::L’
typedef typename T::L __restrict__ r;// { dg-error "'__restrict__'
qualifiers cannot" "" }
Should I add a bit to tree_type_common? It may crease the size
of tree_type_common by 4 bytes since there is unused bits in
tree_type_common.
> I don't like that you need to modify targets - those checks should be done
> in the caller (which may just use a new wrapper with the logic and then
> dispatching to the actual hook).
I can do that.
> Why do you need do adjust get_ref_base_and_extent?
get_ref_base_and_extent is changed to set bitsize to 0 for empty records
so that when ref_maybe_used_by_call_p_1 calls get_ref_base_and_extent to
get 0 as the maximum size on empty record. Otherwise, find_tail_calls
won't perform tail call optimization for functions with empty record
parameters, as in
struct dummy { };
struct true_type { struct dummy i; };
extern true_type y;
extern void xxx (true_type c);
void
yyy (void)
{
xxx (y);
}
--
H.J.
