Using Array.newInstance still generates checkcast instructions

[Vitaly Davidovich]

The whole use case is inlined here, which is the case that bothers me.  If
the underlying array access is inlined, couldn't its runtime type be
checked? I don't understand the heap pollution issue in this case.  The
field is not Object[] at JIT time.

sent from my phone
On Oct 27, 2015 1:40 PM, "John Rose" <john.r.rose at oracle.com> wrote:

> It's an old problem we have talked about before:  There is only limited
> type inference through field loads.  The JIT notes that the type of the
> heap variable is the erased Object[].  It does not attempt to attach
> special knowledge (T=String) to the type of 'strings' (ArrayList), because
> type parameter bindings are not completely reliable at runtime, due to the
> possibility of heap pollution (e.g., via reflective access).
>
> In some cases if the whole use case is inlined (as perhaps may happen
> here) then 'strings' can be scalarized and type propagation will be more
> accurate, since 'strings._items' would be lifted into a register.  (Type
> prop. through locals is not disturbed by the possibility of reflective
> access, heap pollution, etc.)  If you are seeing a cast then either (a)
> scalarization is incomplete (it can fail for even tiny reasons which is why
> we need un-aliasable value types), or (b) there is a bug somewhere.  The
> likely bet is (a).
>
> — John
>
> On Oct 27, 2015, at 10:23 AM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> >
> > Hi,
> >
> > I (intuitively) thought that using Array.newInstance() and specifying a
> final class (e.g. String) would remove checkcasts in the caller.  However,
> it appears that the check is still generated (at least on 8u51).  Why would
> this be? It seems the JIT should know the true runtime type of the array,
> and if accesses to it are inlined, the checkcast could be removed.  Am I
> missing something?
> >
> > e.g.
> >
> > public final class ArrayList<T> {
> >     private final T[] _items;
> >
> >     public ArrayList(Class<T> klass, int size) {
> >          _items = (T[])Array.newInstance(klass, size);
> >     }
> >
> >     // rest omitted for brevity
> >
> >     public T get(int index) { return _items[index]; }
> >
> > }
> >
> > ArrayList<String> strings = new ArrayList<>(String.class, 10);
> > String s = strings.get(0); // why is this checkcast not eliminated?
> >
> >
> > Thanks
>
>
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