In late November, Andy Wingo pushed a new register VM to Guile's (the GNU implementation of the Scheme language) master branch. It brought a number of performance improvements, but led to a bit of a conceptual mismatch between the compiler's direct-style intermediate language and the virtual machine. Earlier this week Andy Wingo announced a new continuation-passing style intermediate language for Guile. From the article: "To recap, we switched from a stack machine to a register machine because, among other reasons, register machines can consume and produce named intermediate results in fewer instructions than stack machines, and that makes things faster. To take full advantage of this new capability, it is appropriate to switch at the same time from the direct-style intermediate language (IL) that we had to an IL that names all intermediate values. ... In Guile I chose a continuation-passing style language. ... Guile's CPS language is composed of terms, expressions, and continuations. It was heavily inspired by Andrew Kennedy's 'Compiling with Continuations, Continued' paper. ... The optimizations I have currently implemented for CPS are fairly basic. Contification was tricky. One thing I did recently was to make all non-tail $call nodes require $kreceive continuations; if, as in the common case, extra values were unused, that was reflected in an unused rest argument. This required a number of optimizations to clean up and remove the extra rest arguments for other kinds of source expressions: dead-code elimination, the typical beta/eta reduction, and some code generation changes." The article describes the CPS language provided by Guile and explains the reasons behind choosing CPS over SSA or A-Normal Form. The Guile manual contains draft documentation. The new VM and Intermediate Language will be released with Guile 2.2, which should be out later this year.
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