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Programming GNU is Not Unix

GNU Guile Scheme Gets a Register VM and CPS-Based IL 42

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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GNU Guile Scheme Gets a Register VM and CPS-Based IL

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  • Never done anything substantial in Scheme, but was probably my favorite language to learn in college, in the context of a function programming course. I did not know there as a GNU interpreter. Will have to check it out. Speaking of functional languages, someone told me that the XKCD website is programmed entirely in Haskell. True?
  • I don't think I have ever encountered a title at Slashdot more meaningless to me than this one. There might be a lesson in that, and I don't think the lesson is for me.

    • by ari_j ( 90255 ) on Wednesday January 15, 2014 @08:40PM (#45971889)

      To those interested in the implementation of programming languages, it is immediately apparent that this is a fundamental change in the compiler behind the GNU Guile system which implements the Scheme programming language, inasmuch as it now has a virtual machine based on the register model instead of the implied stack model, along with an intermediate language in its compilation path that is based on continuation-passing style.

      I think that the lesson here is for everyone: There are many segments of nerd culture, and it is very unlikely that any randomly-selected Slashdot reader understands and appreciates all of those segments. For example, the earlier headline today "Why Transivity Violations Can Be Rational" has no meaning to many readers, even after the title was corrected to spell transitivity correctly. After reading a little about that topic, I see that it is an area of of obvious interest to many nerds.

      That being said, there are plenty of topics Slashdot poorly reports on which are not of interest to any segment of nerd culture, at least not beyond the overlap between nerd culture and the mainstream news where we already read the same information three days earlier except through the words of a literate, competent reporter with real editing before it hit the press.

    • It's about defining an intermediate syntax for the compiler to use, so that optimisations can be performed efficiently. As TFA suggests, if you need an overview on the differences & similarities of SSA and CPS, you could read his earlier article [wingolog.org].

      I've experimented a little with LLVM's SSA form for compiling and optimising high level procedural languages. And I think he makes a few quite valid criticisms of the way SSA form is defined and used.

  • by guacamole ( 24270 ) on Thursday January 16, 2014 @01:54AM (#45973613)

    The Guile team is doing amazing work. Starting with version 2, Scheme is no longer something that always runs slow. My simple test code run several times faster in Guile 2 compared to similar code in python 3. The problem is that no one seems to be interested. It's quite sad. Guile is simply a fantastic, fairly complete programming environment with a very fast VM. In the early days of Gnome, GNU elevated Scheme to the number 1 status among the Gnome languages, but it was dropped since then. I don't know whether this is from the perception of LISP as the old-school language or the rejection of the code aesthetics due to meticulous use of parenthesis.

If all the world's economists were laid end to end, we wouldn't reach a conclusion. -- William Baumol