On 9/25/20 2:30 AM, Richard Biener wrote:
On Thu, 24 Sep 2020, Jason Merrill wrote:
On 9/24/20 3:43 AM, Richard Biener wrote:
On Wed, 23 Sep 2020, Jason Merrill wrote:
On 9/23/20 2:42 PM, Richard Biener wrote:
On September 23, 2020 7:53:18 PM GMT+02:00, Jason Merrill
<ja...@redhat.com>
wrote:
On 9/23/20 4:14 AM, Richard Biener wrote:
C++ operator delete, when DECL_IS_REPLACEABLE_OPERATOR_DELETE_P,
does not cause the deleted object to be escaped. It also has no
other interesting side-effects for PTA so skip it like we do
for BUILT_IN_FREE.
Hmm, this is true of the default implementation, but since the function
is replaceable, we don't know what a user definition might do with the
pointer.
But can the object still be 'used' after delete? Can delete fail /
throw?
What guarantee does the predicate give us?
The deallocation function is called as part of a delete expression in
order
to
release the storage for an object, ending its lifetime (if it was not
ended
by
a destructor), so no, the object can't be used afterward.
OK, but the delete operator can access the object contents if there
wasn't a destructor ...
A deallocation function that throws has undefined behavior.
OK, so it seems the 'replaceable' operators are the global ones
(for user-defined/class-specific placement variants I see arbitrary
extra arguments that we'd possibly need to handle).
I'm happy to revert but I'd like to have a testcase that FAILs
with the patch ;)
Now, the following aborts:
struct X {
static struct X saved;
int *p;
X() { __builtin_memcpy (this, &saved, sizeof (X)); }
};
void operator delete (void *p)
{
__builtin_memcpy (&X::saved, p, sizeof (X));
}
int main()
{
int y = 1;
X *p = new X;
p->p = &y;
delete p;
X *q = new X;
*(q->p) = 2;
if (y != 2)
__builtin_abort ();
}
and I could fix this by not making *p but what *p points to escape.
The testcase is of course maximally awkward, but hey ... ;)
Now this would all be moot if operator delete may not access
the object (or if the object contents are undefined at that point).
Oh, and the testcase segfaults when compiled with GCC 10 because
there we elide the new X / delete p pair ... which is invalid then?
Hmm, we emit
MEM[(struct X *)_8] ={v} {CLOBBER};
operator delete (_8, 8);
so the object contents are undefined _before_ calling delete
even when I do not have a DTOR? That is, the above,
w/o -fno-lifetime-dse, makes the PTA patch OK for the testcase.
Yes, all classes have a destructor, even if it's trivial, so the object's
lifetime definitely ends before the call to operator delete. This is less
clear for scalar objects, but treating them similarly would be consistent
with
other recent changes, so I think it's fine for us to assume that scalar
objects are also invalidated before the call to operator delete. But of
course this doesn't apply to explicit calls to operator delete outside of a
delete expression.
OK, so change the testcase main slightly to
int main()
{
int y = 1;
X *p = new X;
p->p = &y;
::operator delete(p);
X *q = new X;
*(q->p) = 2;
if (y != 2)
__builtin_abort ();
}
in this case the lifetime of *p does not end before calling
::operator delete() and delete can stash the object contents
somewhere before ending its lifetime. For the very same reason
we may not elide a new/delete pair like in
int main()
{
int *p = new int;
*p = 1;
::operator delete (p);
}
Correct; the permission to elide new/delete pairs are for the expressions, not
the functions.
which we before the change did not do only because calling
operator delete made p escape. Unfortunately points-to analysis
cannot really reconstruct whether delete was called as part of
a delete expression or directly (and thus whether object lifetime
ended already), neither can DCE. So I guess we need to mark
the operator delete call in some way to make those transforms
safe. At least currently any operator delete call makes the
alias guarantee of a operator new call moot by forcing the object
to be aliased with all global and escaped memory ...
Looks like there are some unallocated flags for CALL_EXPR we could
pick but I wonder if we can recycle protected_flag which is
CALL_FROM_THUNK_P and
CALL_ALLOCA_FOR_VAR_P in
CALL_EXPR
for calls to DECL_IS_OPERATOR_{NEW,DELETE}_P, thus whether
we have CALL_FROM_THUNK_P for those operators. Guess picking
a new flag is safer.
We won't ever call those operators from a thunk, so it should be OK to reuse
it.
But, does it seem correct that we need to distinguish
delete expressions from plain calls to operator delete?
A reason for that distinction came up in the context of omitting new/delete
pairs: we want to consider the operator first called by the new or delete
expression, not a call from that first operator to another operator new/delete
and exposed by inlining.
https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543404.html
In this context I also wonder about non-replaceable operator delete,
specifically operator delete in classes - are there any semantic
differences between those or why did we choose to only mark
the replaceable ones?
The standard says that for omitting a 'new' allocation, the operator new has
to be a replaceable one, but does not say the same about 'delete'; it just
says that if the allocation was omitted, the delete-expression does not call a
deallocation function. It may not be necessary to make this distinction for
delete. And this distinction could be local to the front end.
In the front end, we currently have cxx_replaceable_global_alloc_fn that
already ignores the replaceability of operator delete. And we have
CALL_FROM_NEW_OR_DELETE_P, that would just need to move into the middle end.
And perhaps get renamed to CALL_OMITTABLE_NEW_OR_DELETE_P, and not get set for
calls to non-replaceable operator new.
