Exposing concrete types of segments and addresses

[Quân Anh Mai]

Thank you very much for the detailed explanation, I agree that we need to
be patient as adding more types to the API is easier than removing those. I
can imagine that later on, we can expose only HeapMemorySegment<T>,
NativeMemorySegment and MappedMemorySegment if it is forced to do so.

Regarding a non-optimal circumstance, I discovered an interesting case
where I want to read a long value from a byte array given the read bytes
might be less than 8. The benchmark is as follow:

    @Benchmark
    public long read()  {
        int length = this.length;
        var segment = MemorySegment.ofArray(this.array);
        long result = 0;
        long offset = this.offset;
        if ((length & Byte.BYTES) != 0) {
            result = Byte.toUnsignedLong(segment.get(ValueLayout.JAVA_BYTE,
offset));
            offset += Byte.BYTES;
        }
        if ((length & Short.BYTES) != 0) {
            result = (result << Short.SIZE) |
Short.toUnsignedLong(segment.get(ValueLayout.JAVA_SHORT, offset));
            offset += Short.BYTES;
        }
        if ((length & Integer.BYTES) != 0) {
            result = (result << Integer.SIZE) |
Integer.toUnsignedLong(segment.get(ValueLayout.JAVA_INT, offset));
        }
        return result;
    }

Running with a fairly recent revision of openjdk/jdk (the difference is 12
commits as of right now, which means the running JVM contains the fix for
your mentioned bug already), the generated assembly seems to be not
optimal, with the segment failing to be scalarized.

Regards,
Quan Anh

On Tue, 21 Dec 2021 at 05:29, Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> Hi,
> thanks for your email. This is a really tricky area, where no optimal
> solution exists yet.
>
> First, we have recently spotted an issue with escape analysis not
> working correctly with memory segments - for this I filed the issue:
>
> https://bugs.openjdk.java.net/browse/JDK-8278429
>
> Which has been closed as a duplicate of another VM bug which is being
> worked on. I believe that fix should generally improve all scenario
> where there is a bottleneck due to failure of scalarization when
> creating new segments (e.g. slicing).
>
> That said, this does not address your fundamental point that, at the end
> of the day, some of these optimizations depend on the ability of C2 to
> inline through code (but this is also true for the ByteBuffer API).
>
> The ultimate solution would be IMHO to make memory segment _less_
> polymorphic, by having a single implementation class which then
> delegates its memory access behavior to a secondary abstraction (which
> could be a constant, based on the access type: on-heap, or off-heap).
>
> If we did that, a memory segment would become a dumb wrapper around a
> base object, a length and some (constant) access object helper.
>
> Unfortunately this solution (which we have tried) doesn't work - because
> Unsafe memory access needs to know whether access is going to be on- or
> off-heap (in order to remove important memory barriers). Currently this
> is done with the help of type profiling: if we are accessing memory on a
> type that C2 can prove to be "NativeMemorySegmentImpl", then C2 also
> knows that access is going to be off-heap - and unsafe access is fast.
> To have profiling working correctly we need one concrete segment type
> for each possible access type (native, mapped, and one for each
> primitive on-heap array). But if there's only one concrete type, there's
> no type profiling to go on, so we gain monomorphism, but we lose (very
> badly) when it comes to profile pollution exposure. To fix this, we need
> better ways to do type profiling (based not only on receiver/parameter
> types, but maybe the type of some fields in an instance).
>
> Now, in the current implementation we can hide the polymorphism, pretty
> much like ByteBuffer does, under a common interface. Exposing concrete
> types as you suggest is going to be painful - as users will see another
> 9 more segment types (7 primitive arrays, + mapped + native), which
> would increase the size of the API quite considerably. Maybe some
> intermediate point might also be useful to consider (e.g. perhaps only
> two types - for native segments and heap segments, but do not
> differentiate between mapped/native or between byte[] and long[] in the
> public API). But we need to conder any such moves very carefully: while
> we can add these types very easily in the future, if it proves to be the
> only possible path (e.g. even after Valhalla) in order to use memory
> segments sanely, the reverse is not true: if we add these new types now,
> and later on we discover these new types to be superseded by some new VM
> optimization, or better support thanks to Valhalla, we'd be stuck with
> these types for a long time.
>
> I think at this point in time we'd like to know where the performance
> potholes are - so if you happen to have a benchmark which shows the
> problem you discussed, we'd be very happy to take a look. Our experience
> so far seems to suggest that performance is acceptable - even in cases
> where segments are created in very hot paths (we do have a spliterator
> test which indundates the system with slices - and that doesn't seem to
> perform too bad). At the same time, I can believe you when you say that
> some of the optimizations we might rely upon are fraglie (I've been
> there when using the API on my own, so the mileage of certain idioms can
> vary).
>
> Unfortunately this is a bigger problem IMHO than just MemorySegments:
> currently writing immutable APIs in Java can lead to spotty performance.
> The hope is that Valhalla will give us tools to help us manage that kind
> of complexity - but even then, some of the optimizations (e.g.
> scalarization) might be gated by excessive polymorphism and/or lack of
> inlining. If we can improve the VM enough to do the type profiling we
> need to keep unsafe access sharp even in the face of a "monomorphic"
> implementation, then I believe the current API could take advantage of
> Valhalla in a more straightfoward fashion (and we could, in the future,
> add Valhalla optimizations to special case treatment for sealed
> interfaces whose only implementation is a primitive class).
>
> [Btw, this discussion is really about MemorySegment - for MemoryAddress,
> in my own experiments I could already see Valhalla making quick work of
> all the address instantiations - as MemoryAddressImpl is the only
> implementation of MemoryAddress].
>
> Maurizio
>
>
> On 20/12/2021 07:21, Quân Anh Mai wrote:
> > Hi,
> >
> > Currently, we can only access MemorySegments and MemoryAddresses through
> > the respective interface. While this provides a nice interface for all
> > kinds of memory segments, the lack of ability to use the concrete types
> > leads to a lot of performance caveats.
> >
> > Firstly, polymorphism disables scalarization. While a non-escaped object
> > can be scalarized in most cases, there are still circumstances that
> scalar
> > replacement fails (e.g when we want to continuously slice a segment in a
> > loop). Furthermore, this makes us become dependent on the inlining
> ability
> > of the compiler, which is unpredictable and limits the use of segments
> and
> > addresses for desired performance. On the other hand, scalarization of
> > polymorphic types in fields and calling convention seems to be really
> > really complicated. With primitive classes, we could make the performance
> > of foreign API become much more predictable with the elimination of
> > allocations as well as pointer chasings where we can and want to limit
> the
> > kind of segment we operate on.
> >
> > The above caveats lead to possible usage of foreign API to pass around
> the
> > naked addresses as long values and only construct segments where it is
> > needed. This approach, while being an ugly hack, is still not ideal cause
> > multiple methods may fail to be inlined.
> >
> > Secondly, polymorphism limits specialisation. With JEP 218, we may have
> > multiple specialisations of the same methods operating on different kinds
> > of segments. While it is still possible, to some extent, to have
> > specialisation with a polymorphic type MemorySegment, it would likely be
> a
> > fragile optimisation that relies on inlining and a lot of type checks.
> >
> > Furthermore, while having common aspects, MemorySegments expose different
> > behaviours on the others. E.g. HeapMemorySegment is not Addressable,
> > MappedMemorySegment has various additional specific methods. While this
> is
> > not an argument for the design of foreign API, it is a small bonus point
> > over those above.
> >
> > Overall, the current status of foreign API seems to put us in a position
> > that relies too much on the compiler to get the desired performance.
> > Exposing the concrete types would enable us to write more predictable
> codes
> > where it needs to and flexible code (i.e using polymorphic MemotySegment,
> > MemoryAddress, etc) where it is more desirable.
> >
> > My apologies if this question has been addressed before. Thank you very
> > much.
> > Quan Anh
>