chore(tvix/eval): move eval docs to tvix/docs

Change-Id: I75b33c43456389de6e521b4f0ad46d68bc9e98f6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11809
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/docs/src/eval/known-optimisation-potential.md

@@ -0,0 +1,162 @@
+Known Optimisation Potential
+============================
+
+There are several areas of the Tvix evaluator code base where
+potentially large performance gains can be achieved through
+optimisations that we are already aware of.
+
+The shape of most optimisations is that of moving more work into the
+compiler to simplify the runtime execution of Nix code. This leads, in
+some cases, to drastically higher complexity in both the compiler
+itself and in invariants that need to be guaranteed between the
+runtime and the compiler.
+
+For this reason, and because we lack the infrastructure to adequately
+track their impact (WIP), we have not yet implemented these
+optimisations, but note the most important ones here.
+
+* Use "open upvalues" [hard]
+
+  Right now, Tvix will immediately close over all upvalues that are
+  created and clone them into the `Closure::upvalues` array.
+
+  Instead of doing this, we can statically determine most locals that
+  are closed over *and escape their scope* (similar to how the
+  `compiler::scope::Scope` struct currently tracks whether locals are
+  used at all).
+
+  If we implement the machinery to track this, we can implement some
+  upvalues at runtime by simply sticking stack indices in the upvalue
+  array and only copy the values where we know that they escape.
+
+* Avoid `with` value duplication [easy]
+
+  If a `with` makes use of a local identifier in a scope that can not
+  close before the with (e.g. not across `LambdaCtx` boundaries), we
+  can avoid the allocation of the phantom value and duplication of the
+  `NixAttrs` value on the stack. In this case we simply push the stack
+  index of the known local.
+
+* Multiple attribute selection [medium]
+
+  An instruction could be introduced that avoids repeatedly pushing an
+  attribute set to/from the stack if multiple keys are being selected
+  from it. This occurs, for example, when inheriting from an attribute
+  set or when binding function formals.
+
+* Split closure/function representation [easy]
+
+  Functions have fewer fields that need to be populated at runtime and
+  can directly use the `value::function::Lambda` representation where
+  possible.
+
+* Apply `compiler::optimise_select` to other set operations [medium]
+
+  In addition to selects, statically known attribute resolution could
+  also be used for things like `?` or `with`. The latter might be a
+  little more complicated but is worth investigating.
+
+* Inline fully applied builtins with equivalent operators [medium]
+
+  Some `builtins` have equivalent operators, e.g. `builtins.sub`
+  corresponds to the `-` operator, `builtins.hasAttr` to the `?`
+  operator etc. These operators additionally compile to a primitive
+  VM opcode, so they should be just as cheap (if not cheaper) as
+  a builtin application.
+
+  In case the compiler encounters a fully applied builtin (i.e.
+  no currying is occurring) and the `builtins` global is unshadowed,
+  it could compile the equivalent operator bytecode instead: For
+  example, `builtins.sub 20 22` would be compiled as `20 - 22`.
+  This would ensure that equivalent `builtins` can also benefit
+  from special optimisations we may implement for certain operators
+  (in the absence of currying). E.g. we could optimise access
+  to the `builtins` attribute set which a call to
+  `builtins.getAttr "foo" builtins` should also profit from.
+
+* Avoid nested `VM::run` calls [hard]
+
+  Currently when encountering Nix-native callables (thunks, closures)
+  the VM's run loop will nest and return the value of the nested call
+  frame one level up. This makes the Rust call stack almost mirror the
+  Nix call stack, which is usually undesirable.
+
+  It is possible to detect situations where this is avoidable and
+  instead set up the VM in such a way that it continues and produces
+  the desired result in the same run loop, but this is kind of tricky
+  to get right - especially while other parts are still in flux.
+
+  For details consult the commit with Gerrit change ID
+  `I96828ab6a628136e0bac1bf03555faa4e6b74ece`, in which the initial
+  attempt at doing this was reverted.
+
+* Avoid thunks if only identifier closing is required [medium]
+
+  Some constructs, like `with`, mostly do not change runtime behaviour
+  if thunked. However, they are wrapped in thunks to ensure that
+  deferred identifiers are resolved correctly.
+
+  This can be avoided, as we statically analyse the scope and should
+  be able to tell whether any such logic was required.
+
+* Intern literals [easy]
+
+  Currently, the compiler emits a separate entry in the constant
+  table for each literal.  So the program `1 + 1 + 1` will have
+  three entries in its `Chunk::constants` instead of only one.
+
+* Do some list and attribute set operations in place [hard]
+
+  Algorithms that can not do a lot of work inside `builtins` like `map`,
+  `filter` or `foldl'` usually perform terribly if they use data structures like
+  lists and attribute sets.
+
+  `builtins` can do work in place on a copy of a `Value`, but naïvely expressed
+  recursive algorithms will usually use `//` and `++` to do a single change to a
+  `Value` at a time, requiring a full copy of the data structure each time.
+  It would be a big improvement if we could do some of these operations in place
+  without requiring a new copy.
+
+  There are probably two approaches: We could determine statically if a value is
+  reachable from elsewhere and emit a special in place instruction if not. An
+  easier alternative is probably to rely on reference counting at runtime: If no
+  other reference to a value exists, we can extend the list or update the
+  attribute set in place.
+
+  An **alternative** to this is using [persistent data
+  structures](https://en.wikipedia.org/wiki/Persistent_data_structure) or at the
+  very least [immutable data structures](https://docs.rs/im/latest/im/) that can
+  be copied more efficiently than the stock structures we are using at the
+  moment.
+
+* Skip finalising unfinalised thunks or non-thunks instead of crashing [easy]
+
+  Currently `OpFinalise` crashes the VM if it is called on values that don't
+  need to be finalised. This helps catching miscompilations where `OpFinalise`
+  operates on the wrong `StackIdx`. In the case of function argument patterns,
+  however, this means extra VM stack and instruction overhead for dynamically
+  determining if finalisation is necessary or not. This wouldn't be necessary
+  if `OpFinalise` would just noop on any values that don't need to be finalised
+  (anymore).
+
+* Phantom binding for from expression of inherits [easy]
+
+  The from expression of an inherit is reevaluated for each inherit. This can
+  be demonstrated using the following Nix expression which, counter-intuitively,
+  will print “plonk” twice.
+
+  ```nix
+  let
+    inherit (builtins.trace "plonk" { a = null; b = null; }) a b;
+  in
+  builtins.seq a (builtins.seq b null)
+  ```
+
+  In most Nix code, the from expression is just an identifier, so it is not
+  terribly inefficient, but in some cases a more expensive expression may
+  be used. We should create a phantom binding for the from expression that
+  is reused in the inherits, so only a single thunk is created for the from
+  expression.
+
+  Since we discovered this, C++ Nix has implemented a similar optimization:
+  <https://github.com/NixOS/nix/pull/9847>.