On Mar 9, 2015, at 5:04 PM, Andrew Haley <a...@redhat.com> wrote:
> On 03/09/2015 03:10 PM, Paul Sandoz wrote:
>> Do you want to tackle the single-address access methods as a follow up issue?
>
> It's not clear to me that we need any single-address access methods
> because we can pass in a null object. Personally I would prefer that.
> Thoughts?
>
I agree that would be desirable.
I know that people rely on that behaviour for off-heap CAS etc, but i was
wondering about such behaviour given that the base and offset should, as
documented, be derived from an object/staticFieldBase and
objectFieldOffset/staticFieldOffset respectfully. Are there subtle differences?
I looked at the C2 code and it seems the results might be equivalent but i
cannot say the same for C1 from quickly eyeballing the code.
e.g. LibraryCallKit::inline_unsafe_access:
if (!is_native_ptr) {
// The base is either a Java object or a value produced by
Unsafe.staticFieldBase
Node* base = argument(1); // type: oop
// The offset is a value produced by Unsafe.staticFieldOffset or
Unsafe.objectFieldOffset
offset = argument(2); // type: long
// We currently rely on the cookies produced by Unsafe.xxxFieldOffset
// to be plain byte offsets, which are also the same as those accepted
// by oopDesc::field_base.
assert(Unsafe_field_offset_to_byte_offset(11) == 11,
"fieldOffset must be byte-scaled"); <----- weird assertion
// 32-bit machines ignore the high half!
offset = ConvL2X(offset);
adr = make_unsafe_address(base, offset);
heap_base_oop = base;
val = is_store ? argument(4) : NULL;
} else {
Node* ptr = argument(1); // type: long
ptr = ConvL2X(ptr); // adjust Java long to machine word
adr = make_unsafe_address(NULL, ptr);
val = is_store ? argument(3) : NULL;
}
Paul.
