Truffle Splitting Heuristics cause compilation to never stabilize

[Thomas Wuerthinger]

Thanks Stefan. This is a great catch! We need to fix the splitting heuristics to cover indirect recursion. - thomas

> On 26 Jul 2017, at 17:57, Stefan Marr <java at stefan-marr.de> wrote:
> 
> Hi:
> 
> I have been running into a stabilizing issue with a small benchmark from the Newspeak benchmark suite.
> 
> The issue exists however also in other languages.
> Let’s take Ruby as example, which might be more legible:
> 
> def fib(x)
>  -> n {
>    if n < 2
>      1
>    else
>      fib(n-1) + fib(n-2)
>    end
>  }.call(x)
> end
> 
> loop { fib(25) }
> 
> The main problem here is that we have an indirectly recursive function, which itself has only a single call.
> 
> This means, DefaultTruffleSplittingStrategy will see fib and say hey isMaxSingleCall==true, let’s split it.
> In the block itself, fib is then split again and again, because it is not directly recursive, but one indirection removed. So, the recursion heuristic doesn’t trigger.
> 
> In my Newspeak (SOMns), the problem is further exaggerated because when a method gets split, I also split its embedded blocks to allow type specialization of local variables. This isn’t the case in TruffleRuby however.
> 
> Is there anything that could be done to make DefaultTruffleSplittingStrategy a little smarter about this?
> 
> Thanks
> Stefan
> 
> -- 
> Stefan Marr
> Johannes Kepler Universität Linz
> http://stefan-marr.de/research/
> 
> 
>