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.

• Tail-call optimisation [hard]

  We can statically detect the conditions for tail-call optimisation. The compiler should do this, and it should then emit a new operation for doing the tail-calls.

• Optimise inner builtin access [medium]

  When accessing identifiers like builtins.foo, the compiler should not go through the trouble of setting up the attribute set on the stack and accessing foo from it if it knows that the scope for builtins is unpoisoned.

  The same thing goes for resolving with builtins;, which should definitely resolve statically.

• 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.