refactor(tvix/eval): add a LightSpan type for lighter span tracking r/5457

This type carries the information required for calculating a
span (i.e. the chunk and offset), instead of the span itself. The span
is then only calculated in cases where it is required (when throwing
errors).

This reduces the eval time for
`builtins.length (builtins.attrNames (import <nixpkgs> {}))` by *one
third*!

The data structure in chunks that carries span information reduces
in-memory size by trading off the speed of retrieving span
information. This is because the span information is only actually
required when throwing errors (or emitting warnings).

However, somewhere along the way we grew a dependency on carrying span
information in thunks (for correctly reporting error chains). Hitting
the code paths for span retrieval was expensive, and carrying the
spans in a different way would still be less cache-efficient. This
change is the best tradeoff I could come up with.

Refs: b/229.
Change-Id: I27d4c4b5c5f9be90ac47f2db61941e123a78a77b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7558
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

1 files changed, 8 insertions, 1 deletions

diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index f0764c314f..37d30a6c01 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -10,6 +10,7 @@ use crate::{
     nix_search_path::NixSearchPath,
     observer::RuntimeObserver,
     opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
+    spans::LightSpan,
     unwrap_or_clone_rc,
     upvalues::Upvalues,
     value::{Builtin, Closure, CoercionKind, Lambda, NixAttrs, NixList, Thunk, Value},
@@ -211,6 +212,12 @@ impl<'o> VM<'o> {
         self.chunk().get_span(self.frame().ip - 1)
     }
 
+    /// Returns the information needed to calculate the current span,
+    /// but without performing that calculation.
+    fn current_light_span(&self) -> LightSpan {
+        LightSpan::new_delayed(self.frame().lambda.clone(), self.frame().ip - 1)
+    }
+
     /// Construct an error from the given ErrorKind and the source
     /// span of the current instruction.
     pub fn error(&self, kind: ErrorKind) -> Error {
@@ -884,7 +891,7 @@ impl<'o> VM<'o> {
                     );
                     Thunk::new_closure(blueprint)
                 } else {
-                    Thunk::new_suspended(blueprint, self.current_span())
+                    Thunk::new_suspended(blueprint, self.current_light_span())
                 };
                 let upvalues = thunk.upvalues_mut();
                 self.push(Value::Thunk(thunk.clone()));