From 1e80b9ea8b9a28588a90a0835988bb3b3067e7d6 Mon Sep 17 00:00:00 2001 From: Adam Joseph Date: Mon, 13 Mar 2023 05:58:33 -0700 Subject: chore(tvix/eval): mark async functions which are called by the VM Given Rust's current lack of support for tail calls, we cannot avoid using `async` for builtins. This is the only way to avoid overflowing the cpu stack when we have arbitrarily deep builtin/interpreted/builtin/interpreted/... "sandwiches" There are only five `async fn` functions which are not builtins (some come in multiple "flavors"): - add_values - resolve_with - force, final_deep_force - nix_eq, nix_cmp_eq - coerce_to_string These can be written iteratively rather than recursively (and in fact nix_eq used to be written that way!). I volunteer to rewrite them. If written iteratively they would no longer need to be `async`. There are two motivations for limiting our reliance on `async` to only the situation (builtins) where we have no other choice: 1. Performance. We don't really have any good measurement of the performance hit that the Boxs impose on us. Right now all of our large (nixpkgs-eval) tests are swamped by the cost of other things (e.g. fork()ing `nix-store`) so we can't really measure it. Builtins tend to be expensive operations anyways (regexp-matching, sorting, etc) that are likely to already cost more than the `async` overhead. 2. Preserving the ability to switch to `musttail` calls. Clang/LLVM recently got `musttail` (mandatory-elimination tail calls). Rust has refused to add this mainly because WASM doesn't support, but WASM `tail_call` has been implemented and was recently moved to phase 4 (standardization). It is very likely that Rust will get tail calls sometime in the next year; if it does, we won't need async anymore. In the meantime, I'd like to avoid adding any further reliance on `async` in places where it wouldn't be straightforward to replace it with a tail call. https://reviews.llvm.org/D99517 https://github.com/WebAssembly/proposals/pull/157 https: //github.com/rust-lang/rfcs/issues/2691#issuecomment-1462152908 Change-Id: Id15945d5a92bf52c16d93456e3437f91d93bdc57 Reviewed-on: https://cl.tvl.fyi/c/depot/+/8290 Reviewed-by: tazjin Tested-by: BuildkiteCI Autosubmit: Adam Joseph --- tvix/eval/src/vm/mod.rs | 3 +++ 1 file changed, 3 insertions(+) (limited to 'tvix/eval/src/vm') diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs index febd4ebd9777..610246abe0c8 100644 --- a/tvix/eval/src/vm/mod.rs +++ b/tvix/eval/src/vm/mod.rs @@ -998,6 +998,7 @@ impl<'o> VM<'o> { } } +// TODO(amjoseph): de-asyncify this /// Resolve a dynamically bound identifier (through `with`) by looking /// for matching values in the with-stacks carried at runtime. async fn resolve_with( @@ -1050,6 +1051,7 @@ async fn resolve_with( Err(ErrorKind::UnknownDynamicVariable(ident)) } +// TODO(amjoseph): de-asyncify this async fn add_values(co: GenCo, a: Value, b: Value) -> Result { let result = match (a, b) { (Value::Path(p), v) => { @@ -1079,6 +1081,7 @@ pub struct RuntimeResult { pub warnings: Vec, } +// TODO(amjoseph): de-asyncify this /// Generator that retrieves the final value from the stack, and deep-forces it /// before returning. async fn final_deep_force(co: GenCo) -> Result { -- cgit 1.4.1