gistfile1.md

• Clone JRuby 9k master
• Download http://lafo.ssw.uni-linz.ac.at/graalvm/openjdk-8-graalvm-b132-macosx-x86_64-0.7.tar.gz and untar into JRuby directory
• or http://lafo.ssw.uni-linz.ac.at/graalvm/openjdk-8-graalvm-b132-linux-x86_64-0.7.tar.gz
• $ function jt { ruby tool/jt.rb $@; }
• $ jt build
• $ jt run --graal test/truffle/can-we-fold-yet.rb

$ jt run --graal test/truffle/can-we-fold-yet.rb test/truffle/can-we-fold-yet.rb
Can Truffle constant fold yet?
> 14
Yes! Truffle can constant fold this to 14
> 14 + 2
Yes! Truffle can constant fold this to 16
> eval([1, 2, 3].inspect).sort[1] * 2
Yes! Truffle can constant fold this to 4
> eval(rand < 0.5 ? '14 - 2' : '10 + 2')
Yes! Truffle can constant fold this to 12
> eval('rand')
No :( Truffle can't constant fold that

Some more examples:

Proc#binding returns an object representing the environment captured in a block - even if the variables aren't actually lexically referenced. People have said it makes it 'impossible' to optimise local variables like the x in this example http://www.sitepoint.com/laurent-sansonetti-on-rubymotion-internals/. This example proves that wrong:

> x = 14; p = Proc.new { }; p.binding.local_variable_get(:x)
Yes! Truffle can constant fold this to 14

We can merge equal results from two control flow paths, even if we don't know which path is actually taken:

> eval(rand < 0.5 ? '14 - 2' : '10 + 2')
Yes! Truffle can constant fold this to 12

We can constant fold through Hash creation and lookup:

> ({x: 1, y: 2, z: 3})[:y]
Yes! Truffle can constant fold this to 2

We can constant fold through higher order functions like map:

> ({x: 1, y: 2, z: 3}).map { |k, v| v } [1]
Yes! Truffle can constant fold this to 2

We can constant fold through metaprogramming, even if the method that we call is not determinable statically:

> 14.send(:*, 2)
Yes! Truffle can constant fold this to 28
> 14.send('*', 2)
Yes! Truffle can constant fold this to 28
> 14.send((')'.ord + 1).chr, 2)
Yes! Truffle can constant fold this to 28

Metadata can also be constant folded:

> 14.object_id
Yes! Truffle can constant fold this to 29

Some examples of things that don't work for balance:

> rand * 2
No :( Truffle can't constant fold that
> 14.5.object_id
No :( Truffle can't constant fold that
> 'foo' + 'bar'
No :( Truffle can't constant fold that

'foo' + 'bar'
No :( Truffle can't constant fold that

Huh? I am not a Rubyist (anymore) but is there some bizarre gotcha in this example? How comes you can constant fold 14.send((')'.ord + 1).chr, 2) but not two strings being concatenated?

It's because strings are mutable. A dynamic value 14 is the same as a constant value 14, but 'foo' + 'bar' generates a new value each time, so it's not clear how to make that constant.