77 lines
2.6 KiB
Markdown
77 lines
2.6 KiB
Markdown
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tvix-eval VM loop
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=================
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Date: 2023-02-14
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Author: tazjin
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## Background
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The VM loop implemented in `src/vm.rs` currently has a couple of functions:
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1. Advance the instruction pointer and execute instructions, in a loop
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(unsurprisingly).
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2. Tracking Nix call frames as functions/thunks are entered/exited.
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3. Catch trampoline requests returned from instruction executions, and
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resuming of executions afterwards.
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4. Invoking the inner trampoline loop, handling actions and
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continuations from the trampoline.
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The current implementation of the trampoline logic was added on to the
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existing VM, which recursed for thunk forcing. As a result, it is
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currently a little difficult to understand what exactly is going on in
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the VM loop and how the trampoline logic works.
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This has also led to several bugs, for example: b/251, b/246, b/245,
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and b/238.
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These bugs are very tricky to deal with, as we have to try and make
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the VM do things that are somewhat difficult to fit into the current
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model. We could of course keep extending the trampoline logic to
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accomodate all sorts of concepts (such as finalisers), but that seems
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difficult.
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There are also additional problems, such as forcing inside of builtin
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implementations, which leads to a situation where we would like to
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suspend the builtin and return control flow to the VM until the value
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is forced.
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## Proposal
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We rewrite parts of the VM loop to elevate trampolining and
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potentially other modes of execution to a top-level concept of the VM.
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We achieve this by replacing the current concept of call frames with a
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"VM context" (naming tbd), which can represent multiple different
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states of the VM:
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1. Tvix code execution, equivalent to what is currently a call frame,
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executing bytecode until the instruction pointer reaches the end of
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a chunk, then returning one level up.
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2. Trampolining the forcing of a thunk, equivalent to the current
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trampolining logic.
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3. Waiting for the result of a trampoline, to ensure that in case of
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nested thunks all representations are correctly transformed.
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4. Trampolining the execution of a builtin. This is not in scope for
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the initial implementation, but it should be conceptually possible.
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It is *not* in scope for this proposal to enable parallel suspension
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of thunks. This is a separate topic which is discussed in [waiting for
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the store][wfs].
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## Alternatives considered
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1. Tacking on more functionality onto the existing VM loop
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implementation to accomodate problems as they show up. This is not
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preferred as the code is already getting messy.
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2. ... ?
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[wfs]: https://docs.google.com/document/d/1Zuw9UdMy95hcqsd-KudTw5yeeUkEXrqOi0rooGB7GWA/edit
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