On Tue, 9 Jul 2019, Martin Liška wrote:
On 7/9/19 9:49 AM, Marc Glisse wrote:
On Tue, 9 Jul 2019, Marc Glisse wrote:
On Mon, 8 Jul 2019, Martin Liška wrote:
The patch apparently has DECL_IS_OPERATOR_DELETE only on the replaceable global
deallocation functions, not all delete operators, contrary to
DECL_IS_OPERATOR_NEW, so the name is misleading. On the other hand, those seem
to be the ones for which the optimization is legal (well, not quite, the rules
are in terms of operator new, and I am not sure how well operator delete has to
match, but close enough).
Are you talking about this location where we set OPERATOR_NEW:
https://github.com/gcc-mirror/gcc/blob/master/gcc/cp/decl.c#L13643
?
That's the only place where we set OPERATOR_NEW flag and not OPERATOR_DELETE.
Yes, I think that's the place.
Again, not setting DECL_IS_OPERATOR_DELETE on local operator delete
seems misleading, but setting it would let us optimize in cases where we
are not really allowed to. Maybe just rename your macro to
DECL_IS_GLOBAL_OPERATOR_DELETE?
Hmm, I replied too fast.
Global operator delete does not seem like a good terminology, the ones marked
in the patch would be the usual (=non-placement) replaceable deallocation
functions.
I cannot find a requirement that operator new and operator delete should match.
The rules to omit allocation are stated in terms of which operator new is
called, but do not seem to care which operator delete is used. So allocating
with the global operator new and deallocating with a class overload of operator
delete can be removed, but not the reverse (not sure how they came up with such
a rule...). Which means we would need:
Thank you Mark for digging deep in that.
keep DECL_IS_OPERATOR_NEW for the current uses
DECL_IS_REPLACEABLE_OPERATOR_NEW (equivalent to DECL_IS_OPERATOR_NEW &&
DECL_IS_MALLOC? not exactly but close I think) for DCE
DECL_IS_OPERATOR_DELETE (which also includes some class overloads) for DCE
Note that with the current version of the patch we are out of free bits in
struct GTY(()) tree_function_decl.
Would it be possible to tweak the current patch to cover what you described?
If you approximate DECL_IS_REPLACEABLE_OPERATOR_NEW with
DECL_IS_OPERATOR_NEW && DECL_IS_MALLOC, it shouldn't need more bits than
the current patch. I think the main difference is if a user adds attribute
malloc to his class-specific operator new, where it will enable DCE, but
since the attribute is non-standard, we can just document that behavior,
it might even be desirable.
I am not very confident about anything I said in this thread, for all I
know I may be misguiding you, please make sure someone else who
understands the C++ standard approves your final patch.
--
Marc Glisse
