Wasmtime GitHub notifications bot (Jul 21 2022 at 19:35): Wasmtime GitHub notifications bot (Jul 21 2022 at 19:35):alexcrichton opened PR #4501 from trampoline-compiler to main:
This commit lands a significant new piece of functionality to Wasmtime's
implementation of the component model in the form of the implementation
of fused adapter trampolines. Internally within a component core wasm
modules can communicate with each other by having their exports
canon lift'd to getcanon lower'd into a different component. This
signifies that two components are communicating through a statically
known interface via the canonical ABI at this time. Previously Wasmtime
was able to identify that this communication was happening but it simply
panicked withunimplemented!upon seeing it. This commit is the
beginning of filling out this panic location with an actual
implementation.The implementation route chosen here for fused adapters is to use a
WebAssembly module itself for the implementation. This means that, at
compile time of a component, Wasmtime is generating core WebAssembly
modules which then get recursively compiled within Wasmtime as well. The
choice to use WebAssembly itself as the implementation of fused adapters
stems from a few motivations:
This does not represent a significant increase in the "trusted
compiler base" of Wasmtime. Getting the Wasm -> CLIF translation
correct once is hard enough much less for an entirely different IR to
CLIF. By generating WebAssembly no new interactions with Cranelift are
added which drastically reduces the possibilities for mistakes.Using WebAssembly means that component adapters are insulated from
miscompilations and mistakes. If something goes wrong it's defined
well within the WebAssembly specification how it goes wrong and what
happens as a result. This means that the "blast zone" for a wrong
adapter is the component instance but not the entire host itself.
Accesses to linear memory are guaranteed to be in-bounds and otherwise
handled via well-defined traps.A fully-finished fused adapter compiler is expected to be a
significant and quite complex component of Wasmtime. Functionality
along these lines is expected to be needed for Web-based polyfills of
the component model and by using core WebAssembly it provides the
opportunity to share code between Wasmtime and these polyfills for the
component model.Finally the runtime implementation of managing WebAssembly modules is
already implemented and quite easy to integrate with, so representing
fused adapters with WebAssembly results in very little extra support
necessary for the runtime implementation of instantiating and managing
a component.The compiler added in this commit is dubbed Wasmtime's Fused Adapter
Compiler of Trampolines (FACT) because who doesn't like deriving a name
from an acronym. Currently the trampoline compiler is limited in its
support for interface types and only supports a few primitives. I plan
on filing future PRs to flesh out the support here for all the variants
ofInterfaceType. For now this PR is primarily focused on all of the
other infrastructure for the addition of a trampoline compiler.With the choice to use core WebAssembly to implement fused adapters it
means that adapters need to be inserted into a module. Unfortunately
adapters cannot all go into a single WebAssembly module because adapters
themselves have dependencies which may be provided transitively through
instances that were instantiated with other adapters. This means that a
significant chunk of this PR (adapt.rs) is dedicated to determining
precisely which adapters go into precisely which adapter modules. This
partitioning process attempts to make large modules wherever it can to
cut down on core wasm instantiations but is likely not optimal as
it's just a simple heuristic today.With all of this added together it's now possible to start writing
*.wasttests that internally have adapted modules communicating with
one another. Afused.wasttest suite was added as part of this PR
which is the beginning of tests for the support of the fused adapter
compiler added in this PR. Currently this is primarily testing some
various topologies of adapters along with direct/indirect modes. This
will grow many more tests over time as more types are supported.Overall I'm not 100% satisfied with the testing story of this PR. When a
test fails it's very difficult to debug since everything is written in
the text format of WebAssembly meaning there's no "conveniences" to
print out the state of the world when things go wrong and easily debug.
I think this will become even more apparent as more tests are written
for more types in subsequent PRs. At this time though I know of no
better alternative other than leaning pretty heavily on fuzz-testing to
ensure this is all exercised.<!--
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Wasmtime GitHub notifications bot (Jul 21 2022 at 19:35):alexcrichton requested fitzgen for a review on PR #4501.
Wasmtime GitHub notifications bot (Jul 21 2022 at 19:44):alexcrichton updated PR #4501 from trampoline-compiler to main.
Wasmtime GitHub notifications bot (Jul 21 2022 at 20:03):alexcrichton updated PR #4501 from trampoline-compiler to main.
Wasmtime GitHub notifications bot (Jul 22 2022 at 17:06):alexcrichton updated PR #4501 from trampoline-compiler to main.
Wasmtime GitHub notifications bot (Jul 25 2022 at 18:37):alexcrichton updated PR #4501 from trampoline-compiler to main.
Wasmtime GitHub notifications bot (Jul 25 2022 at 20:04):fitzgen submitted PR review.
Wasmtime GitHub notifications bot (Jul 25 2022 at 20:04):fitzgen submitted PR review.
Wasmtime GitHub notifications bot (Jul 25 2022 at 20:04):fitzgen created PR review comment:
*imports
Wasmtime GitHub notifications bot (Jul 25 2022 at 21:44):alexcrichton updated PR #4501 from trampoline-compiler to main.
Wasmtime GitHub notifications bot (Jul 25 2022 at 21:48):alexcrichton updated PR #4501 from trampoline-compiler to main.
Wasmtime GitHub notifications bot (Jul 25 2022 at 21:48):alexcrichton has enabled auto merge for PR #4501.
Wasmtime GitHub notifications bot (Jul 25 2022 at 23:13):alexcrichton merged PR #4501.
Last updated: Nov 22 2024 at 16:03 UTC