JEPs proposed to target JDK 9 (2015/7/16)

[John Rose]

On Jul 16, 2015, at 5:32 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> 
> Varhandles JEP talks about replacing Unsafe array accesses and trying to be
> smarter about range check elimination to match performance of check-less
> accesses using Unsafe today.  One use case that will suffer is parallel
> arrays of equal length.  The programmer knows they're all the same length
> (e.g. arrays are final fields allocated in ctor), and a handwritten check
> for one suffices for the others; the accesses can then be safely done using
> Unsafe with no range checks.  With varhandles, the JIT is not going to be
> able to see this relationship between them and will issue range checks on
> all accesses.
> 
> Has such a use case been discussed/thought about?

There are *always* going to be places where the programmer will
be able to prove that x==y (or x<=y) for some distinct expressions
x,y, yet the JIT will not be able to do this.

Here, x is a.length and y is b.length, and I suppose you want one
range check to serve for both limits.

With Unsafe you can say in effect "trust me they are the same".
With VH's you won't be able to.  So there will be places where we
will have to add new optimizations or repurpose old ones.

In this case, existing range check optimizations may be applicable.
The current optimizations are able to collect several array lengths
and determine which is the most restrictive, with respect to a given
loop body.  That might cover your case also, and many similar ones.

— John