Bounds checks with unsafe array access

[Paul Sandoz]

On Sep 11, 2014, at 10:51 PM, John Rose <john.r.rose at oracle.com> wrote:
> In some cases of highly tuned code the author can work with the JIT engineer to insert the right code shapes from the start.
> The "test length against zero" code shape is one of these, and can often be merged with a check that is required any way (e.g., is the data structure logically empty?).
> For example:
>   http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/f08705540498/src/java.base/share/classes/java/util/HashMap.java#l569
> 

A great example, i was just recently looking at that :-)

My understanding is that check ("(n = tab.length) > 0") is in place solely to generate more efficient bounds checks, since it should always return true (table is either null or non-null with a length > 0). If/when JDK-8003585 gets fixed we might be able to update the code and remove that check (or turn it into an assert if it did not interfere with inlining...).

I was naively pondering whether that table field:

  http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/f08705540498/src/java.base/share/classes/java/util/HashMap.java#l395

could be annotated with something like @ArrayHasElementsIfNonNull, so that a write to that field would be guarded by a check which would throw an assertion error if the field would otherwise be set to an empty array.


Final field optimizations would be a bigger bang for the buck though. 

Combine that with specialization and perhaps there is an alternative implementation approach to the current VarHandles prototype?

class FieldHandle<R, any V> {
   /* really */ final Class<?> rType;
   /* really */ final Class<?> vType;
   /* really */ final long foffset;

   <where reference V>
   V getVolatile(R r) {
       r.getClass();
       rType.cast(r); // Should go away (well almost!) if FieldHandle instance is constant
       return vType.cast(U.getObjectVolatile(r, foffset));
   }

   <where V = int>
   int getVolatile(R r) {
       r.getClass();
       rType.cast(r);
       return U.getIntVolatile(r, foffset);
   }
}

Paul.
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