docs(tvix/eval): StackIdx, LocalIdx UpvalueIdx r/5201

This adds a comment noting that StackIdx is an offset relative to
the base of the current CallFrame, whereas UpvalueIdx is an absolute
index into the upvalues array.

It also removes the confusing mention of StackIdx in the descriptive
comment for LocalIdx.  They index into totally different structures;
one exists at runtime and the other exists at compile time.

Change-Id: Ib932b1b0679734c15001e8c5c95a08293fa016b4
Signed-off-by: Adam Joseph <adam@westernsemico.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7017
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 6 insertions, 2 deletions

diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index 382a857ef93d..b7367cd815ea 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -27,12 +27,16 @@ impl Sub<usize> for CodeIdx {
     }
 }
 
-/// Index of a value in the runtime stack.
+/// Index of a value in the runtime stack.  This is an offset
+/// *relative to* the VM value stack_base of the CallFrame
+/// containing the opcode which contains this StackIdx.
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
 pub struct StackIdx(pub usize);
 
-/// Index of an upvalue within a closure's upvalue list.
+/// Index of an upvalue within a closure's bound-variable upvalue
+/// list.  This is an absolute index into the Upvalues of the
+/// CallFrame containing the opcode which contains this UpvalueIdx.
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct UpvalueIdx(pub usize);