hg: lambda/lambda/jdk: - ensure null values are retained

[Paul Sandoz]

On Dec 4, 2012, at 2:26 PM, Remi Forax <forax at univ-mlv.fr> wrote:

> On 12/04/2012 11:44 AM, Paul Sandoz wrote:
>> Hi Remi,
>> 
>> Many thanks, that is just the kind of VM-perspective advice i was looking for. I was assuming that the VM would be able to somehow inline the hot path to the else block.
> 
> yes, but you don't need a supplementary field for that.
> 

Right, but subjectively i think it is marginally clearer code.


>> 
>> So you are suggesting something like this:
>> 
>>                     T _t = (T) maskNull(t);
>>                     if (lastSeen == null || !lastSeen.equals(_t)) {
>>                         lastSeen = _t;
>>                         downstream.accept(t);
>>                     }
> 
> I was thinking to something like this:
> 
>             return new Sink.ChainedReference<T>(sink) {
>                 private Object lastSeen;
> 
>                 @Override
>                 public void begin(long size) {
>                     lastSeen = null;            // not needed if end() is called in a finally ?

It's not needed.


>                     downstream.begin(-1);
>                 }
> 
>                 @Override
>                 public void end() {
>                     lastSeen = null;
>                     downstream.end();
>                 }
> 
>                 @Override
>                 public void accept(T t) {
>                     if (t == null) {
>                         if (lastSeen != NULL_OBJECT) {
>                             lastSeen = NULL_OBJECT
>                             downstream.accept(null);
>                         }
>                     } else if (lastSeen == null || !t.equals(lastSeen)) {
>                         lastSeen = t;
>                         downstream.accept((T)t);
>                     }
>                 }
>             };
> 

If one assumes nulls are rare is there any performance advantage (beyond the extra field not being used) to using the null object design pattern in this context?

I am trying and failing to correlate what you say below with the code base:

"Your solution require to load/store value from the RAM because the VM will not able to optimize this code (inlining is often  defeated due to the fact that the call between ops is a virtual call)."

Paul.

>> 
>> rather than:
>> 
>>                     if (t == null) {
>>                         if (!seenNull) {
>>                             seenNull = true;
>>                             downstream.accept(lastSeen = null);
>>                         }
>>                     } else if (lastSeen == null || !t.equals(lastSeen)) {
>>                         downstream.accept(lastSeen = t);
>>                     }
>> 
>> Paul.
> 
> Rémi
> 
>