Tuesday, October 8, 2013

Crack 0.8 Released

We're pleased to announce the release of Crack 0.8.  The new release includes
a number of bug-fixes and code cleanups as well as a number of new extension
modules, and brings us up to date with LLVM 3.3.

It's been almost a year since our last release, which is far longer than usual
and far longer than what we'd like.  The reason for the delay has been the
ongoing work on module caching.  Module caching provides a substantial
speed-up of program loading by allowing the executor to reuse the
artifacts of previous compiles instead of recompiling everything from source.
Unfortunately, it has proven to be very difficult to get this feature working
perfectly, and it still doesn't work quite right for complicated generics. 
Users are encouraged to experiment with it by adding the -C option.

The complete release notes for 0.8 follows:

    -   Lots of bug fixes and small features.
    -   Lots of refactors and code cleanups.
    -   Implemented most of the code for caching (which is still way too buggy
        for anything other than experimentation).
    -   Added flag driven tracing facility.
    -   Lots of new modules and extension modules:
        -   OpenSSL
        -   libpng
        -   alsa, midi, jack, and fluidsynth
        -   curl
        -   SHA1 digests
        -   base64 encoding
        -   Mongo DB and LMDB
        -   netCDF
        -   Mersenne Twister PRNG.
    -   enhanced the test framework to support composite tests.
    -   added the Wurld OpenGL example program.
    -   Ported to LLVM 3.3
    -   Changed annotation-in-generic semantics.

Tuesday, September 10, 2013

Annotation Semantics Changing for Generics

Annotations are a useful feature that essentially allow you to program the Crack compiler at compile time.  They provide the functionality of macros in C plus a whole lot more - you can actually use them to implement your own domain specific languages, if you like.

Annotations are stored in a "compile time namespace." This is separate from the normal namespace that functions, variables and classes live in, but it follows the same resolution rules: annotations are scoped to the lexical scope in which they are defined.

Until now, annotations used in generics were no different from annotations in any other context.  When the generic was instantiated with new parameters, it was essentially replayed into the same compile time context (including the same compile-time namespace).

Unfortunately, this approach doesn't work with caching.  When a generic is cached, the original compile context is gone.  We have to restore parts of that context (notably, the normal namespace) but the contents of the compile time namespace cannot be persisted or restored: it can contain arbitrary objects created by the annotation system, and, in fact, it routinely contains pointers to primitive functions.

The only way to restore the compile namespace of a generic is to replay the original source file it was defined in.  This would be contrary to the purpose of caching, and it would also require us to create a dummy environment so as not to actually regenerate existing code and representational datastructures.

Rather than try to go down this path and further delay the 1.0 release, I've decided to impose some limitations on the way that annotations can be used with generics.  This was a painful decision: I don't like having non-uniform semantics in the language.  Annotations should work the same for generics as for any other code in a module, but something had to give and after discussing it with the team I feel this is the best compromise.

As of this morning's check-in, generics now preserve only imported annotations.  So for example, the following code will no longer compile:

@import crack.ann define;
@define my_func() { void f() {} }
class A[T] {  @my_func }

It could be rewritten like this:

@import crack.ann define;
class A[T] {
  @define my_func() { void f() {} }

Note that the symbols defined with @import can be reused -- it's possible for us to replay imports safely.  But macros defined with @define must be defined (or redefined) within the scope of the generic itself.


Monday, May 13, 2013

LLVM 3.2 and Caching

We've been relatively quiet for a long time (since last October) so I just wanted to post an update of some of the big changes that have been checked in last week.

First of all, the default branch now uses LLVM 3.2.  Arno Rehn did the port on a branch some time ago, but I had caching work happening on another branch and didn't have the bandwidth to do the merge at the time.  The introduction of LLVM 3.2 doesn't change very much, but it does keep us in sync with LLVM and the major distros.  It will also help us when we port to 3.3, which currently has a release candidate in testing.

The most exciting thing happening is that after a period of over 6 months of development, caching finally (mostly) works.  That is to say, we can run all but one of the non-bootstrapped tests with the -C option (which enables module caching) before and after persistence.

For the uninitiated, module caching has been an important planned feature for Crack, so much so that we've made it an essential feature for the 1.0 release.  With caching enabled, when we compile a module we store the products of the compile (meta-data and an LLVM bit-code file) to a persistent cache, so the next time you run something that needs that module it gets loaded from the cache, saving the executor from having to compile it and substantially reducing startup time.  This is similar to what Python does with its .pyc files, though it's harder to implement in more static language like Crack.

Over the past month, I've finally managed to solve the hardest problem of caching, which is that of cyclic ephemeral modules.  When generics are instantiated, an "ephemeral module" is created for the generic.  An ephemeral module is a module that does not directly correspond to its own source file.  So if module "x" defines generic A, when we instantiate A[int], we create the ephemeral module x.A[int].  When caching, this allows the same instantiation of a generic to be referenced by all of the modules that use it.  However, this feature also opens the possibility of dependency cycles: if A[int] uses symbols in x, and x uses A[int], then x depends on x.A[int] and x.A[int] also depends on x.  This breaks a basic assumption about how modules are loaded and has implications at many levels of the system.  Solving these problems was the last major hurdle to having a working caching system, and it looks like we're finally there.

There are a few things that are still broken in caching.  I'm currently working on fixing the "import from a non-extension shared library" feature.  I also might end up essentially rewriting the LLVM linker to correctly deal with referencing isomorphic types.  But most of the big plumbing changes are in, and I've merged the changes to the default branch.

So please try out the new caching functionality, it does noticeably speed up start-up time.  Feel free to post bugs for whatever problems you find.  Happy cracking!