Stream: git-wasmtime

Topic: wasmtime / PR #4623 Implement strings in adapter modules

Wasmtime GitHub notifications bot (Aug 05 2022 at 15:17):

alexcrichton opened PR #4623 from adapt-string to main:

This commit is a hefty addition to Wasmtime's support for the component
model. This implements the final remaining type (in the current type
hierarchy) unimplemented in adapter module trampolines: strings. Strings
are the most complicated type to implement in adapter trampolines
because they are highly structured chunks of data in memory (according
to specific encodings). Additionally each lift/lower operation can
choose its own encoding for strings meaning that Wasmtime, the host, may
have to convert between any pairwise ordering of string encodings.

The CanonicalABI.md in the component-model repo in general specifies
all the fiddly bits of string encoding so there's not a ton of wiggle
room for Wasmtime to get creative. This PR largely "just" implements
that. The high-level architecture of this implementation is:

• Fused adapters are first identified to determine src/dst string
  encodings. This statically fixes what transcoding operation is being
  performed.

• The generated adapter will be responsible for managing calls to
  realloc and performing bounds checks. The adapter itself does not
  perform memory copies or validation of string contents, however.
  Instead each transcoding operation is modeled as an imported function
  into the adapter module. This means that the adapter module
  dynamically, during compile time, determines what string transcoders
  are needed. Note that an imported transcoder is not only parameterized
  over the transcoding operation but additionally which memory is the
  source and which is the destination.

• The imported core wasm functions are modeled as a new
  CoreDef::Transcoder structure. These transcoders end up being small
  Cranelift-compiled trampolines. The Cranelift-compiled trampoline will
  load the actual base pointer of memory and add it to the relative
  pointers passed as function arguments. This trampoline then calls a
  transcoder "libcall" which enters Rust-defined functions for actual
  transcoding operations.

• Each possible transcoding operation is implemented in Rust with a
  unique name and a unique signature depending on the needs of the
  transcoder. I've tried to document inline what each transcoder does.

This means that the Module::translate_string in adapter modules is by
far the largest translation method. The main reason for this is due to
the management around calling the imported transcoder functions in the
face of validating string pointer/lengths and performing the dance of
realloc-vs-transcode at the right time. I've tried to ensure that each
individual case in transcoding is documented well enough to understand
what's going on as well.

Additionally in this PR is a full implementation in the host for the
latin1+utf16 encoding which means that both lifting and lowering host
strings now works with this encoding.

Currently the implementation of each transcoder function is likely far
from optimal. Where possible I've leaned on the standard library itself
and for latin1-related things I'm leaning on the encoding_rs crate. I
initially tried to implement everything with encoding_rs but was
unable to uniformly do so easily. For now I settled on trying to get a
known-correct (even in the face of endianness) implementation for all of
these transcoders. If an when performance becomes an issue it should be
possible to implement more optimized versions of each of these
transcoding operations.

Testing this commit has been somewhat difficult and my general plan,
like with the (list T) type, is to rely heavily on fuzzing to cover
the various cases here. In this PR though I've added a simple test that
pushes some statically known strings through all the pairs of encodings
between source and destination. I've attempted to pick "interesting"
strings that one way or another stress the various paths in each
transcoding operation to ideally get full branch coverage there.
Additionally a suite of "negative" tests have also been added to ensure
that validity of encoding is actually checked.

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Closes #4309

Wasmtime GitHub notifications bot (Aug 05 2022 at 15:17):

alexcrichton requested fitzgen for a review on PR #4623.

Wasmtime GitHub notifications bot (Aug 05 2022 at 15:33):

alexcrichton updated PR #4623 from adapt-string to main.

Wasmtime GitHub notifications bot (Aug 05 2022 at 17:26):

alexcrichton updated PR #4623 from adapt-string to main.

Wasmtime GitHub notifications bot (Aug 05 2022 at 18:09):

fitzgen created PR review comment:

This isn't us being optimistic tho, right? This optimism is prescribed by the spec, correct?

Wasmtime GitHub notifications bot (Aug 05 2022 at 18:09):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Aug 05 2022 at 18:09):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Aug 05 2022 at 18:09):

fitzgen created PR review comment:

I feel like this method is already super large, so adding a bunch of large closures into it is not helping. How much stuff is this really closing over? Can it and the other closures be a separate method?

Wasmtime GitHub notifications bot (Aug 05 2022 at 18:09):

fitzgen created PR review comment:

I thought that we settled on bounds checks regardless if the string is empty or not?

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:29):

alexcrichton updated PR #4623 from adapt-string to main.

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:29):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:29):

alexcrichton created PR review comment:

Sure, I've split out everything now.

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:29):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:29):

alexcrichton created PR review comment:

Indeed!

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:30):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:30):

alexcrichton created PR review comment:

This was written before the spec changed, and I was originally going to land this first and then fixup all our handling of zero-length arrays and things but I went ahead and updated this.

Wasmtime GitHub notifications bot (Aug 08 2022 at 15:30):

alexcrichton has enabled auto merge for PR #4623.

Wasmtime GitHub notifications bot (Aug 08 2022 at 16:01):

alexcrichton merged PR #4623.

Last updated: Dec 23 2024 at 12:05 UTC