diff options
Diffstat (limited to 'tvix/eval/src/value/thunk.rs')
| -rw-r--r-- | tvix/eval/src/value/thunk.rs | 155 |
1 files changed, 94 insertions, 61 deletions
diff --git a/tvix/eval/src/value/thunk.rs b/tvix/eval/src/value/thunk.rs index 2853a398de..3ff3bde659 100644 --- a/tvix/eval/src/value/thunk.rs +++ b/tvix/eval/src/value/thunk.rs @@ -79,6 +79,8 @@ enum ThunkRepr { content_span: Option<Span>, }, + // TODO(amjoseph): consider changing `Value` to `Rc<Value>` to avoid + // expensive clone()s in Thunk::force(). /// Fully evaluated thunk. Evaluated(Value), } @@ -203,69 +205,100 @@ impl Thunk { } } - // TODO(amjoseph): de-asyncify this - pub async fn force(self, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> { - // If the current thunk is already fully evaluated, return its evaluated - // value. The VM will continue running the code that landed us here. - if self.is_forced() { - return Ok(self.unwrap_or_clone()); - } - - // Begin evaluation of this thunk by marking it as a blackhole, meaning - // that any other forcing frame encountering this thunk before its - // evaluation is completed detected an evaluation cycle. - let inner = self.0.replace(self.prepare_blackhole(span)); - - match inner { - // If there was already a blackhole in the thunk, this is an - // evaluation cycle. - ThunkRepr::Blackhole { - forced_at, - suspended_at, - content_span, - } => Err(ErrorKind::InfiniteRecursion { - first_force: forced_at.span(), - suspended_at: suspended_at.map(|s| s.span()), - content_span, - }), - - // If there is a native function stored in the thunk, evaluate it - // and replace this thunk's representation with the result. - ThunkRepr::Native(native) => { - let value = native.0()?; - - // Force the returned value again, in case the native call - // returned a thunk. - let value = generators::request_force(&co, value).await; - - self.0.replace(ThunkRepr::Evaluated(value.clone())); - Ok(value) - } - - // When encountering a suspended thunk, request that the VM enters - // it and produces the result. - ThunkRepr::Suspended { - lambda, - upvalues, - light_span, - } => { - let value = - generators::request_enter_lambda(&co, lambda, upvalues, light_span).await; - - // This may have returned another thunk, so we need to request - // that the VM forces this value, too. - let value = generators::request_force(&co, value).await; - - self.0.replace(ThunkRepr::Evaluated(value.clone())); - Ok(value) + pub async fn force(mut myself: Thunk, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> { + // This vector of "thunks which point to the thunk-being-forced", to + // be updated along with it, is necessary in order to write this + // function in iterative (and later, mutual-tail-call) form. + let mut also_update: Vec<Rc<RefCell<ThunkRepr>>> = vec![]; + + loop { + // If the current thunk is already fully evaluated, return its evaluated + // value. The VM will continue running the code that landed us here. + if myself.is_forced() { + let val = myself.unwrap_or_clone(); + for other_thunk in also_update.into_iter() { + other_thunk.replace(ThunkRepr::Evaluated(val.clone())); + } + return Ok(val); } - // If an inner value is found, force it and then update. This is - // most likely an inner thunk, as `Thunk:is_forced` returned false. - ThunkRepr::Evaluated(val) => { - let value = generators::request_force(&co, val).await; - self.0.replace(ThunkRepr::Evaluated(value.clone())); - Ok(value) + // Begin evaluation of this thunk by marking it as a blackhole, meaning + // that any other forcing frame encountering this thunk before its + // evaluation is completed detected an evaluation cycle. + let inner = myself.0.replace(myself.prepare_blackhole(span.clone())); + + match inner { + // If there was already a blackhole in the thunk, this is an + // evaluation cycle. + ThunkRepr::Blackhole { + forced_at, + suspended_at, + content_span, + } => { + return Err(ErrorKind::InfiniteRecursion { + first_force: forced_at.span(), + suspended_at: suspended_at.map(|s| s.span()), + content_span, + }) + } + + // If there is a native function stored in the thunk, evaluate it + // and replace this thunk's representation with the result. + ThunkRepr::Native(native) => { + let value = native.0()?; + myself.0.replace(ThunkRepr::Evaluated(value)); + continue; + } + + // When encountering a suspended thunk, request that the VM enters + // it and produces the result. + ThunkRepr::Suspended { + lambda, + upvalues, + light_span, + } => { + // TODO(amjoseph): use #[tailcall::mutual] here. This can + // be turned into a tailcall to vm::execute_bytecode() by + // passing `also_update` to it. + let value = + generators::request_enter_lambda(&co, lambda, upvalues, light_span).await; + myself.0.replace(ThunkRepr::Evaluated(value)); + continue; + } + + // nested thunks -- try to flatten before forcing + ThunkRepr::Evaluated(Value::Thunk(inner_thunk)) => { + match Rc::try_unwrap(inner_thunk.0) { + Ok(refcell) => { + // we are the only reference to the inner thunk, + // so steal it + myself.0.replace(refcell.into_inner()); + continue; + } + Err(rc) => { + let inner_thunk = Thunk(rc); + if inner_thunk.is_forced() { + // tail call to force the inner thunk; note that + // this means the outer thunk remains unforced + // even after calling force() on it; however the + // next time it is forced we will be one + // thunk-forcing closer to it being + // fully-evaluated. + myself + .0 + .replace(ThunkRepr::Evaluated(inner_thunk.value().clone())); + continue; + } + also_update.push(myself.0.clone()); + myself = inner_thunk; + continue; + } + } + } + + ThunkRepr::Evaluated(val) => { + return Ok(val); + } } } } |