tvl-depot/tvix/eval
Adam Joseph 49b34183e3 feat(tvix/eval): rewrite Thunk::force() in nonrecursive form
This commit rewrites Thunk::force() so that it is not (directly)
self-recursive.  It maintains a Vec of all the
previously-encountered thunks which point to the one it is currently
forcing, rather than recursively calling itself.

Benefits:

- Short term:

  This commit saves the cost of a round-trip through the generator
  machinery for the generators::request_force() which is removed by
  this commit.

- Medium term:

  Once a similar transformation has been applied to nix_cmp(),
  nix_add(), nix_eq(), and coerce_to_string(), those four functions,
  along with Thunk::force(), will make non-tail calls only to each
  other.  They can then be merged into a single tail-recursive
  function which does not use the generator machinery at all:

    enum Task { Cmp, Add, Eq, CoerceToString, Force};

    fn Value::walk(task:Task, v1:Value, v2:Value) {
      // ...

- Long term:

  The long-term goal here is to use generators **only for builtins**
  and [Marionette]-style remote control of the VM.  In other words:
  use `async` for things that actually involve concurrency.  Calls
  from the VM to builtins can then be blocking calls, because even
  cppnix will overflow the stack if you make a MAX_STACK_DEPTH-deep
  recursive call which passes through a builtin at every stack frame
  (e.g. `{ func = builtins.sort (a: b: ... func ...) ...}`).

  This way the inner "tight loop" of the interpreter doesn't pay the
  costs of `async` and generators.  These costs manifest in terms
  of: performance, complex nonlocal control flow, and language
  impediments (async Rust is a restricted subset of real Rust, and
  is missing things like traits).

[Marionette]: https://firefox-source-docs.mozilla.org/testing/marionette/Intro.html

Change-Id: I6179b8abb2ea0492180fcb347f37595a14665777
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10039
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
2023-12-06 06:53:01 +00:00
..
benches fix(tvix/eval/benches): use black_box properly 2023-11-02 20:39:52 +00:00
builtin-macros chore(tvix): fix trivial clippy lints 2023-11-05 20:28:37 +00:00
docs docs(tvix/eval): optimization potential for inherit (from) exprs 2023-11-24 23:36:15 +00:00
proptest-regressions/value refactor(tvix/eval): flatten call stack of VM using generators 2023-03-13 20:30:59 +00:00
src feat(tvix/eval): rewrite Thunk::force() in nonrecursive form 2023-12-06 06:53:01 +00:00
tests refactor(tvix/eval/tests): migrate to tempfile 2023-10-08 22:47:33 +00:00
.skip-subtree feat(tvix/tests): check in Nix' language test suite 2022-08-24 21:25:41 +00:00
build.rs feat(tvix/eval): implement builtins.currentSystem 2022-10-24 12:20:01 +00:00
Cargo.toml chore(tvix): bump proptest dependency 2023-11-05 20:28:38 +00:00
default.nix refactor(tvix): build Rust projects using crate2nix 2022-12-15 17:26:45 +00:00
README.md docs(tvix/eval): update test suite documentation 2023-06-15 19:28:16 +00:00

Tvix Evaluator

This project implements an interpreter for the Nix programming language. You can experiment with an online version of the evaluator: tvixbolt.

The interpreter aims to be compatible with nixpkgs, on the foundation of Nix 2.3.

Important note: The evaluator is not yet feature-complete, and while the core mechanisms (compiler, runtime, ...) have stabilised somewhat, a lot of components are still changing rapidly.

Please contact TVL with any questions you might have.

Building tvix-eval

Please check the README.md one level up for instructions on how to build this.

The evaluator itself can also be built with standard Rust tooling (i.e. cargo build).

If you would like to clone only the evaluator and build it directly with Rust tooling, you can do:

git clone https://code.tvl.fyi/depot.git:/tvix/eval.git tvix-eval

cd tvix-eval && cargo build

Tests

Tvix currently has three language test suites for tvix-eval:

  • nix_tests and tvix_tests are based on the same mechanism borrowed from the C++ Nix implementation. They consist of Nix files as well as expected output (if applicable). The test cases are split into four categories: eval-okay (evaluates successfully with the expected output), eval-fail (fails to evaluate, no expected output), parse-okay (expression parses successfully, no expected output) and parse-fail (expression fails to parse, no expected output). Tvix currently ignores the last two types of test cases, since it doesn't implement its own parser.

    Both test suites have a notyetpassing directory. All test cases in here test behavior that is not yet supported by Tvix. They are considered to be expected failures, so you can't forget to move them into the test suite proper when fixing the incompatibility.

    Additionally, separate targets in the depot pipeline, under //tvix/verify-lang-tests, check both test suites (including notyetpassing directories) against C++ Nix 2.3 and the default C++ Nix version in nixpkgs. This way we can prevent accidentally introducing test cases for behavior that C++ Nix doesn't exhibit.

    • nix_tests has the test cases from C++ Nix's language test suite and is sporadically updated by manually syncing the directories. The notyetpassing directory shows how far it is until we pass it completely.

    • tvix_tests contains test cases written by the Tvix contributors. Some more or less duplicate test cases contained in nix_tests, but many cover relevant behavior that isn't by nix_tests. Consequently, it'd be nice to eventually merge the two test suites into a jointly maintained, common Nix language test suite.

      It also has a notyetpassing directory for missing behavior that is discovered while working on Tvix and isn't covered by the nix_tests suite.

  • nix_oracle can evaluate Nix expressions in Tvix and compare the result against C++ Nix (2.3) directly. Eventually it should gain the ability to property test generated Nix expressions. An additional feature is that it can evaluate expressions without --strict, so thunking behavior can be verified more easily.

rnix-parser

Tvix is written in memory of jD91mZM2, the author of rnix-parser who sadly passed away.

Tvix makes heavy use of rnix-parser in its bytecode compiler. The parser is now maintained by Nix community members.