On Wed, Feb 21, 2024 at 03:02:55PM +0400, Daniil Frolov wrote: > Hi. > > Following the recent introduction of more detailed CLOBBER types in GCC, a > minor > inconsistency has been identified in the description of > CLOBBER_OBJECT_BEGIN: > > /* Beginning of object lifetime, e.g. C++ constructor. */ > CLOBBER_OBJECT_BEGIN > > This comment appears somewhat contradictory, as according to the C++ > standard, > an object's lifetime commences upon completion of its initialization: > > > The lifetime of an object of type T begins when: > > -- storage with the proper alignment and size for type T is obtained, > > and > > -- its initialization (if any) is complete (including vacuous > > initialization) > etc. > > However, GCC emits CLOBBERs of this type at the beginning of a constructor. >
And similarly for CLOBBER_OBJECT_END; by the standard an object ends its lifetime at the start of the destructor, while we emit this at the end of the destructor call. However, both placements are useful. At least for constexpr, the current meaning of CLOBBER_OBJECT_END is important to know when we can still access members of a class undergoing construction or destruction (see [class.cdtor] p1) since this isn't directly tied to the lifetime of the containing object itself. But I'm still working for GCC 15 on correctly handling "partially constructed" objects in constexpr (e.g. PR109518), and having a version of CLOBBER_OBJECT_BEGIN at the /end/ of the constructor to signal this might be useful as due to splitting of non-constant initialisers this is otherwise a little awkward to determine from the IR currently. > Does anybody have any ideas how to make this description more precise? It'd > be > better to clearly define what an object's lifetime is at the GIMPLE IMHO. So, given the above, I think CLOBBER_OBJECT_BEGIN is still a pretty good name (though maybe not perfect?), but maybe the comment should say something like "Beginning of object construction"? Or otherwise mention that this is the start of the lifetime of either the object or its first subobject. Yours, Nathaniel. > --- > With best regards, > Daniil
