a79bbad03b
Signed-off-by: Adam Joseph <adam@westernsemico.com> Change-Id: I460230863de853ca5248733bc977d4780b216f36 Reviewed-on: https://cl.tvl.fyi/c/depot/+/7096 Tested-by: BuildkiteCI Reviewed-by: sterni <sternenseemann@systemli.org>
112 lines
4.8 KiB
Markdown
112 lines
4.8 KiB
Markdown
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.
|
|
|
|
* 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 optimisation can also be done
|
|
for the other set operations like `builtins ? foo` and
|
|
`builtins.foo or alternative-implementation`.
|
|
|
|
The same thing goes for resolving `with builtins;`, which should
|
|
definitely resolve statically.
|
|
|
|
* Inline fully applied builtins with equivalent operators [medium]
|
|
|
|
Some `builtins` have equivalent operators, e.g. `builtins.add`
|
|
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 unpoisoned,
|
|
it could compile the equivalent operator bytecode instead: For
|
|
example, `builtins.add 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.
|