Hello,
On Wed, 21 Feb 2024, Daniil Frolov wrote:
> >> 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
The "e.g." comment mixes concepts of the C++ language with a
middle-end/GIMPLE concept, and hence is a bit misleading. What the
clobbers are intended to convey to the middle-end is the low-level notion
of "storage associated with this and that object is now accessible
(BEGIN)/not accessible anymore (END), for purposes related to that very
object". The underlying motivation, _why_ that knowledge is interesting
to the middle-end, is to be able to share storage between different
objects.
"purposes related to that object" are any operations on the object:
construction, destruction, access, frobnification. It's not tied to a
particular frontend language (although it's the language semantics that
dictate when emitting the begin/end clobbers is appropriate). For the
middle-end the C++ concept of construction/deconstruction are simply
modifications (conceptual or real) of the storage associated with an
object ("object" not in the C++ sense, but from a low-level perspective;
i.e. an "object" doesn't only exist after c++-construction, it comes into
existence before that, even if perhaps in an indeterminate/invalid state
from the frontends perspective).
Initially these clobbers were only emitted when decls went ouf of
scope, and so did have some relation to frontend language semantics
(although a fairly universal one, namely "scope"). The
C++ frontend then found further uses (e.g. after a dtor for an
object _ends_ it's storage can also be reused), and eventually we also
needed the BEGIN clobbers to convey the meaning of "now storage use for
object potentially starts, don't share it with any other object".
If certain frontends find use for more fine-grained definitions of
life-times, then further note kinds need to be invented for those
frontends use. They likely won't have influence on the middle-end though
(perhaps for some sanitizers such new kinds might be useful?). But the
current BEGIN/END clobbers need to continue to mark the outermost borders
of storage validity for an object.
Ciao,
Michael.
