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/value/mod.rs | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'tvix/eval/src/value/mod.rs') diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs index c9c7f3ca4745..f57cfda5eecf 100644 --- a/tvix/eval/src/value/mod.rs +++ b/tvix/eval/src/value/mod.rs @@ -184,6 +184,7 @@ pub enum PointerEquality { } impl Value { + // TODO(amjoseph): de-asyncify this (when called directly by the VM) /// Deeply forces a value, traversing e.g. lists and attribute sets and forcing /// their contents, too. /// @@ -242,6 +243,7 @@ impl Value { Ok(value) } + // TODO(amjoseph): de-asyncify this (when called directly by the VM) /// Coerce a `Value` to a string. See `CoercionKind` for a rundown of what /// input types are accepted under what circumstances. pub async fn coerce_to_string(self, co: GenCo, kind: CoercionKind) -> Result { @@ -331,6 +333,7 @@ impl Value { } } + // TODO(amjoseph): de-asyncify this (when called directly by the VM) /// Compare two Nix values for equality, forcing nested parts of the structure /// as needed. /// @@ -539,6 +542,7 @@ impl Value { gen_is!(is_number, Value::Integer(_) | Value::Float(_)); gen_is!(is_bool, Value::Bool(_)); + // TODO(amjoseph): de-asyncify this (when called directly by the VM) /// Compare `self` against other using (fallible) Nix ordering semantics. /// /// Note that as this returns an `Option` it can not directly be @@ -606,6 +610,7 @@ impl Value { } } + // TODO(amjoseph): de-asyncify this (when called directly by the VM) pub async fn force(self, co: GenCo) -> Result { if let Value::Thunk(thunk) = self { return thunk.force(co).await; -- cgit 1.4.1