[Truffle] Eliminating Calls to Side-Effect Free Methods?

[Stefan Marr]

Hi:

With the latest changes, TruffleSOM seems to get closer to ideal performance,
however, there seem to be some general issues that do not yet work as I would hope/expect.

I got a couple of micro benchmarks, which I would expect to be reduced to zero in an ideal situation.

For instance the Dispatch benchmark. Essentially, it is two nested loops and a mono-morphic message send to a method that returns it’s argument.

It looks more or less like this:

    benchmark = ( 1 to: 20000 do: [ :i | self method: i ] )
    method: argument = ( ^argument )

    outerBenchmarkLoop = (
        1 to: innerIterations do: [ self benchmark ]
    )

Now, the first tricky part is that a self send inside a loop implies the access to the outer lexical scope, and thereby the use of a materialized frame. I experimented a little with it, and it turns out that in the outer loop, the use of the materialized frame seems to be properly eliminated, in the inner loop however, it is not. As shown above, both loops are in separate methods, but are inlined as far as I can see in IGV.

That's one of the issues. The other one is the cost of introducing a sequence node.
When I increase the number of statements in the inner loop from 1 to 2, my compiler introduces an additional SequenceNode to hold them. I would expect that to be compiled away and not to imply any overhead, but its payload. However, going from 1 to 2 increases the runtime by more than the factor 2. Going to 3 or 4 statements shows then a more linear increase.

But again, the main point is that these methods don't do anything but producing heat. So, I would really like to see them eliminated. 

In order to identify the culprit in IGV, I don't have enough experience yet. There are to many things I cannot really correlate with the input program.

Are there perhaps known patterns I could look for to point out further optimization/specialization potential?

Thanks a lot
Stefan

PS: to see the issue, you can do the following:

git clone --recursive git at github.com:SOM-st/TruffleSOM.git
ant jar
mx --vm server vm -G:Dump=  -Xbootclasspath/a:build/classes:libs/truffle.jar som.vm.Universe  -cp Smalltalk:Examples/Benchmarks/DeltaBlue/ Examples/Benchmarks/BenchmarkHarness.som Dispatch 1000 0 2000

And there, in the output, the method is actually called #innerBenchmarkLoop as part of the benchmark harness.
Should be the second to last thing that’s compiled after the benchmark completed.

-- 
Stefan Marr
INRIA Lille - Nord Europe
http://stefan-marr.de/research/