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author | Adam Joseph <adam@westernsemico.com> | 2023-11-14T18·23-0800 |
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committer | Adam Joseph <adam@westernsemico.com> | 2023-12-06T06·53+0000 |
commit | 49b34183e3f2591c6e18d44689acb6fd70eab1bf (patch) | |
tree | da5e566d1f8d948d2b5c6df5d8392b8dd1cb677f /.nixery/README.md | |
parent | 8135a8d38cefdc4632b3c85fdbb663d067f91248 (diff) |
feat(tvix/eval): rewrite Thunk::force() in nonrecursive form r/7120
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
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