wasmtime_environ/component/translate/
adapt.rs

1//! Identification and creation of fused adapter modules in Wasmtime.
2//!
3//! A major piece of the component model is the ability for core wasm modules to
4//! talk to each other through the use of lifted and lowered functions. For
5//! example one core wasm module can export a function which is lifted. Another
6//! component could import that lifted function, lower it, and pass it as the
7//! import to another core wasm module. This is what Wasmtime calls "adapter
8//! fusion" where two core wasm functions are coming together through the
9//! component model.
10//!
11//! There are a few ingredients during adapter fusion:
12//!
13//! * A core wasm function which is "lifted".
14//! * A "lift type" which is the type that the component model function had in
15//!   the original component
16//! * A "lower type" which is the type that the component model function has
17//!   in the destination component (the one the uses `canon lower`)
18//! * Configuration options for both the lift and the lower operations such as
19//!   memories, reallocs, etc.
20//!
21//! With these ingredients combined Wasmtime must produce a function which
22//! connects the two components through the options specified. The fused adapter
23//! performs tasks such as validation of passed values, copying data between
24//! linear memories, etc.
25//!
26//! Wasmtime's current implementation of fused adapters is designed to reduce
27//! complexity elsewhere as much as possible while also being suitable for being
28//! used as a polyfill for the component model in JS environments as well. To
29//! that end Wasmtime implements a fused adapter with another wasm module that
30//! it itself generates on the fly. The usage of WebAssembly for fused adapters
31//! has a number of advantages:
32//!
33//! * There is no need to create a raw Cranelift-based compiler. This is where
34//!   majority of "unsafety" lives in Wasmtime so reducing the need to lean on
35//!   this or audit another compiler is predicted to weed out a whole class of
36//!   bugs in the fused adapter compiler.
37//!
38//! * As mentioned above generation of WebAssembly modules means that this is
39//!   suitable for use in JS environments. For example a hypothetical tool which
40//!   polyfills a component onto the web today would need to do something for
41//!   adapter modules, and ideally the adapters themselves are speedy. While
42//!   this could all be written in JS the adapting process is quite nontrivial
43//!   so sharing code with Wasmtime would be ideal.
44//!
45//! * Using WebAssembly insulates the implementation to bugs to a certain
46//!   degree. While logic bugs are still possible it should be much more
47//!   difficult to have segfaults or things like that. With adapters exclusively
48//!   executing inside a WebAssembly sandbox like everything else the failure
49//!   modes to the host at least should be minimized.
50//!
51//! * Integration into the runtime is relatively simple, the adapter modules are
52//!   just another kind of wasm module to instantiate and wire up at runtime.
53//!   The goal is that the `GlobalInitializer` list that is processed at runtime
54//!   will have all of its `Adapter`-using variants erased by the time it makes
55//!   its way all the way up to Wasmtime. This means that the support in
56//!   Wasmtime prior to adapter modules is actually the same as the support
57//!   after adapter modules are added, keeping the runtime fiddly bits quite
58//!   minimal.
59//!
60//! This isn't to say that this approach isn't without its disadvantages of
61//! course. For now though this seems to be a reasonable set of tradeoffs for
62//! the development stage of the component model proposal.
63//!
64//! ## Creating adapter modules
65//!
66//! With WebAssembly itself being used to implement fused adapters, Wasmtime
67//! still has the question of how to organize the adapter functions into actual
68//! wasm modules.
69//!
70//! The first thing you might reach for is to put all the adapters into the same
71//! wasm module. This cannot be done, however, because some adapters may depend
72//! on other adapters (transitively) to be created. This means that if
73//! everything were in the same module there would be no way to instantiate the
74//! module. An example of this dependency is an adapter (A) used to create a
75//! core wasm instance (M) whose exported memory is then referenced by another
76//! adapter (B). In this situation the adapter B cannot be in the same module
77//! as adapter A because B needs the memory of M but M is created with A which
78//! would otherwise create a circular dependency.
79//!
80//! The second possibility of organizing adapter modules would be to place each
81//! fused adapter into its own module. Each `canon lower` would effectively
82//! become a core wasm module instantiation at that point. While this works it's
83//! currently believed to be a bit too fine-grained. For example it would mean
84//! that importing a dozen lowered functions into a module could possibly result
85//! in up to a dozen different adapter modules. While this possibility could
86//! work it has been ruled out as "probably too expensive at runtime".
87//!
88//! Thus the purpose and existence of this module is now evident -- this module
89//! exists to identify what exactly goes into which adapter module. This will
90//! evaluate the `GlobalInitializer` lists coming out of the `inline` pass and
91//! insert `InstantiateModule` entries for where adapter modules should be
92//! created.
93//!
94//! ## Partitioning adapter modules
95//!
96//! Currently this module does not attempt to be really all that fancy about
97//! grouping adapters into adapter modules. The main idea is that most items
98//! within an adapter module are likely to be close together since they're
99//! theoretically going to be used for an instantiation of a core wasm module
100//! just after the fused adapter was declared. With that in mind the current
101//! algorithm is a one-pass approach to partitioning everything into adapter
102//! modules.
103//!
104//! Adapters were identified in-order as part of the inlining phase of
105//! translation where we're guaranteed that once an adapter is identified
106//! it can't depend on anything identified later. The pass implemented here is
107//! to visit all transitive dependencies of an adapter. If one of the
108//! dependencies of an adapter is an adapter in the current adapter module
109//! being built then the current module is finished and a new adapter module is
110//! started. This should quickly partition adapters into contiugous chunks of
111//! their index space which can be in adapter modules together.
112//!
113//! There's probably more general algorithms for this but for now this should be
114//! fast enough as it's "just" a linear pass. As we get more components over
115//! time this may want to be revisited if too many adapter modules are being
116//! created.
117
118use crate::EntityType;
119use crate::component::translate::*;
120use crate::fact;
121use std::collections::HashSet;
122
123/// Metadata information about a fused adapter.
124#[derive(Debug, Clone, Hash, Eq, PartialEq)]
125pub struct Adapter {
126    /// The type used when the original core wasm function was lifted.
127    ///
128    /// Note that this could be different than `lower_ty` (but still matches
129    /// according to subtyping rules).
130    pub lift_ty: TypeFuncIndex,
131    /// Canonical ABI options used when the function was lifted.
132    pub lift_options: AdapterOptions,
133    /// The type used when the function was lowered back into a core wasm
134    /// function.
135    ///
136    /// Note that this could be different than `lift_ty` (but still matches
137    /// according to subtyping rules).
138    pub lower_ty: TypeFuncIndex,
139    /// Canonical ABI options used when the function was lowered.
140    pub lower_options: AdapterOptions,
141    /// The original core wasm function which was lifted.
142    pub func: dfg::CoreDef,
143}
144
145/// The data model for objects that are not unboxed in locals.
146#[derive(Debug, Clone, Hash, Eq, PartialEq)]
147pub enum DataModel {
148    /// Data is stored in GC objects.
149    Gc {},
150
151    /// Data is stored in a linear memory.
152    LinearMemory {
153        /// An optional memory definition supplied.
154        memory: Option<dfg::CoreExport<MemoryIndex>>,
155        /// If `memory` is specified, whether it's a 64-bit memory.
156        memory64: bool,
157        /// An optional definition of `realloc` to used.
158        realloc: Option<dfg::CoreDef>,
159    },
160}
161
162/// Configuration options which can be specified as part of the canonical ABI
163/// in the component model.
164#[derive(Debug, Clone, Hash, Eq, PartialEq)]
165pub struct AdapterOptions {
166    /// The Wasmtime-assigned component instance index where the options were
167    /// originally specified.
168    pub instance: RuntimeComponentInstanceIndex,
169    /// How strings are encoded.
170    pub string_encoding: StringEncoding,
171    /// The async callback function used by these options, if specified.
172    pub callback: Option<dfg::CoreDef>,
173    /// An optional definition of a `post-return` to use.
174    pub post_return: Option<dfg::CoreDef>,
175    /// Whether to use the async ABI for lifting or lowering.
176    pub async_: bool,
177    /// Whether or not this intrinsic can consume a task cancellation
178    /// notification.
179    pub cancellable: bool,
180    /// The core function type that is being lifted from / lowered to.
181    pub core_type: ModuleInternedTypeIndex,
182    /// The data model used by this adapter: linear memory or GC objects.
183    pub data_model: DataModel,
184}
185
186impl<'data> Translator<'_, 'data> {
187    /// This is the entrypoint of functionality within this module which
188    /// performs all the work of identifying adapter usages and organizing
189    /// everything into adapter modules.
190    ///
191    /// This will mutate the provided `component` in-place and fill out the dfg
192    /// metadata for adapter modules.
193    pub(super) fn partition_adapter_modules(&mut self, component: &mut dfg::ComponentDfg) {
194        // Visit each adapter, in order of its original definition, during the
195        // partitioning. This allows for the guarantee that dependencies are
196        // visited in a topological fashion ideally.
197        let mut state = PartitionAdapterModules::default();
198        for (id, adapter) in component.adapters.iter() {
199            state.adapter(component, id, adapter);
200        }
201        state.finish_adapter_module();
202
203        // Now that all adapters have been partitioned into modules this loop
204        // generates a core wasm module for each adapter module, translates
205        // the module using standard core wasm translation, and then fills out
206        // the dfg metadata for each adapter.
207        for (module_id, adapter_module) in state.adapter_modules.iter() {
208            let mut module =
209                fact::Module::new(self.types.types(), self.tunables.debug_adapter_modules);
210            let mut names = Vec::with_capacity(adapter_module.adapters.len());
211            for adapter in adapter_module.adapters.iter() {
212                let name = format!("adapter{}", adapter.as_u32());
213                module.adapt(&name, &component.adapters[*adapter]);
214                names.push(name);
215            }
216            let wasm = module.encode();
217            let imports = module.imports().to_vec();
218
219            // Extend the lifetime of the owned `wasm: Vec<u8>` on the stack to
220            // a higher scope defined by our original caller. That allows to
221            // transform `wasm` into `&'data [u8]` which is much easier to work
222            // with here.
223            let wasm = &*self.scope_vec.push(wasm);
224            if log::log_enabled!(log::Level::Trace) {
225                match wasmprinter::print_bytes(wasm) {
226                    Ok(s) => log::trace!("generated adapter module:\n{s}"),
227                    Err(e) => log::trace!("failed to print adapter module: {e}"),
228                }
229            }
230
231            // With the wasm binary this is then pushed through general
232            // translation, validation, etc. Note that multi-memory is
233            // specifically enabled here since the adapter module is highly
234            // likely to use that if anything is actually indirected through
235            // memory.
236            self.validator.reset();
237            let static_module_index = self.static_modules.next_key();
238            let translation = ModuleEnvironment::new(
239                self.tunables,
240                &mut self.validator,
241                self.types.module_types_builder(),
242                static_module_index,
243            )
244            .translate(Parser::new(0), wasm)
245            .expect("invalid adapter module generated");
246
247            // Record, for each adapter in this adapter module, the module that
248            // the adapter was placed within as well as the function index of
249            // the adapter in the wasm module generated. Note that adapters are
250            // partitioned in-order so we're guaranteed to push the adapters
251            // in-order here as well. (with an assert to double-check)
252            for (adapter, name) in adapter_module.adapters.iter().zip(&names) {
253                let index = translation.module.exports[name];
254                let i = component.adapter_partitionings.push((module_id, index));
255                assert_eq!(i, *adapter);
256            }
257
258            // Finally the metadata necessary to instantiate this adapter
259            // module is also recorded in the dfg. This metadata will be used
260            // to generate `GlobalInitializer` entries during the linearization
261            // final phase.
262            assert_eq!(imports.len(), translation.module.imports().len());
263            let args = imports
264                .iter()
265                .zip(translation.module.imports())
266                .map(|(arg, (_, _, ty))| fact_import_to_core_def(component, arg, ty))
267                .collect::<Vec<_>>();
268            let static_module_index2 = self.static_modules.push(translation);
269            assert_eq!(static_module_index, static_module_index2);
270            let id = component.adapter_modules.push((static_module_index, args));
271            assert_eq!(id, module_id);
272        }
273    }
274}
275
276fn fact_import_to_core_def(
277    dfg: &mut dfg::ComponentDfg,
278    import: &fact::Import,
279    ty: EntityType,
280) -> dfg::CoreDef {
281    fn unwrap_memory(def: &dfg::CoreDef) -> dfg::CoreExport<MemoryIndex> {
282        match def {
283            dfg::CoreDef::Export(e) => e.clone().map_index(|i| match i {
284                EntityIndex::Memory(i) => i,
285                _ => unreachable!(),
286            }),
287            _ => unreachable!(),
288        }
289    }
290
291    let mut simple_intrinsic = |trampoline: dfg::Trampoline| {
292        let signature = ty.unwrap_func();
293        let index = dfg
294            .trampolines
295            .push((signature.unwrap_module_type_index(), trampoline));
296        dfg::CoreDef::Trampoline(index)
297    };
298    match import {
299        fact::Import::CoreDef(def) => def.clone(),
300        fact::Import::Transcode {
301            op,
302            from,
303            from64,
304            to,
305            to64,
306        } => {
307            let from = dfg.memories.push(unwrap_memory(from));
308            let to = dfg.memories.push(unwrap_memory(to));
309            let signature = ty.unwrap_func();
310            let index = dfg.trampolines.push((
311                signature.unwrap_module_type_index(),
312                dfg::Trampoline::Transcoder {
313                    op: *op,
314                    from,
315                    from64: *from64,
316                    to,
317                    to64: *to64,
318                },
319            ));
320            dfg::CoreDef::Trampoline(index)
321        }
322        fact::Import::ResourceTransferOwn => simple_intrinsic(dfg::Trampoline::ResourceTransferOwn),
323        fact::Import::ResourceTransferBorrow => {
324            simple_intrinsic(dfg::Trampoline::ResourceTransferBorrow)
325        }
326        fact::Import::ResourceEnterCall => simple_intrinsic(dfg::Trampoline::ResourceEnterCall),
327        fact::Import::ResourceExitCall => simple_intrinsic(dfg::Trampoline::ResourceExitCall),
328        fact::Import::PrepareCall { memory } => simple_intrinsic(dfg::Trampoline::PrepareCall {
329            memory: memory.as_ref().map(|v| dfg.memories.push(unwrap_memory(v))),
330        }),
331        fact::Import::SyncStartCall { callback } => {
332            simple_intrinsic(dfg::Trampoline::SyncStartCall {
333                callback: callback.clone().map(|v| dfg.callbacks.push(v)),
334            })
335        }
336        fact::Import::AsyncStartCall {
337            callback,
338            post_return,
339        } => simple_intrinsic(dfg::Trampoline::AsyncStartCall {
340            callback: callback.clone().map(|v| dfg.callbacks.push(v)),
341            post_return: post_return.clone().map(|v| dfg.post_returns.push(v)),
342        }),
343        fact::Import::FutureTransfer => simple_intrinsic(dfg::Trampoline::FutureTransfer),
344        fact::Import::StreamTransfer => simple_intrinsic(dfg::Trampoline::StreamTransfer),
345        fact::Import::ErrorContextTransfer => {
346            simple_intrinsic(dfg::Trampoline::ErrorContextTransfer)
347        }
348    }
349}
350
351#[derive(Default)]
352struct PartitionAdapterModules {
353    /// The next adapter module that's being created. This may be empty.
354    next_module: AdapterModuleInProgress,
355
356    /// The set of items which are known to be defined which the adapter module
357    /// in progress is allowed to depend on.
358    defined_items: HashSet<Def>,
359
360    /// Finished adapter modules that won't be added to.
361    ///
362    /// In theory items could be added to preexisting modules here but to keep
363    /// this pass linear this is never modified after insertion.
364    adapter_modules: PrimaryMap<dfg::AdapterModuleId, AdapterModuleInProgress>,
365}
366
367#[derive(Default)]
368struct AdapterModuleInProgress {
369    /// The adapters which have been placed into this module.
370    adapters: Vec<dfg::AdapterId>,
371}
372
373/// Items that adapters can depend on.
374///
375/// Note that this is somewhat of a flat list and is intended to mostly model
376/// core wasm instances which are side-effectful unlike other host items like
377/// lowerings or always-trapping functions.
378#[derive(Copy, Clone, Hash, Eq, PartialEq)]
379enum Def {
380    Adapter(dfg::AdapterId),
381    Instance(dfg::InstanceId),
382}
383
384impl PartitionAdapterModules {
385    fn adapter(&mut self, dfg: &dfg::ComponentDfg, id: dfg::AdapterId, adapter: &Adapter) {
386        // Visit all dependencies of this adapter and if anything depends on
387        // the current adapter module in progress then a new adapter module is
388        // started.
389        self.adapter_options(dfg, &adapter.lift_options);
390        self.adapter_options(dfg, &adapter.lower_options);
391        self.core_def(dfg, &adapter.func);
392
393        // With all dependencies visited this adapter is added to the next
394        // module.
395        //
396        // This will either get added the preexisting module if this adapter
397        // didn't depend on anything in that module itself or it will be added
398        // to a fresh module if this adapter depended on something that the
399        // current adapter module created.
400        log::debug!("adding {id:?} to adapter module");
401        self.next_module.adapters.push(id);
402    }
403
404    fn adapter_options(&mut self, dfg: &dfg::ComponentDfg, options: &AdapterOptions) {
405        if let Some(def) = &options.callback {
406            self.core_def(dfg, def);
407        }
408        if let Some(def) = &options.post_return {
409            self.core_def(dfg, def);
410        }
411        match &options.data_model {
412            DataModel::Gc {} => {
413                // Nothing to do here yet.
414            }
415            DataModel::LinearMemory {
416                memory,
417                memory64: _,
418                realloc,
419            } => {
420                if let Some(memory) = memory {
421                    self.core_export(dfg, memory);
422                }
423                if let Some(def) = realloc {
424                    self.core_def(dfg, def);
425                }
426            }
427        }
428    }
429
430    fn core_def(&mut self, dfg: &dfg::ComponentDfg, def: &dfg::CoreDef) {
431        match def {
432            dfg::CoreDef::Export(e) => self.core_export(dfg, e),
433            dfg::CoreDef::Adapter(id) => {
434                // If this adapter is already defined then we can safely depend
435                // on it with no consequences.
436                if self.defined_items.contains(&Def::Adapter(*id)) {
437                    log::debug!("using existing adapter {id:?} ");
438                    return;
439                }
440
441                log::debug!("splitting module needing {id:?} ");
442
443                // .. otherwise we found a case of an adapter depending on an
444                // adapter-module-in-progress meaning that the current adapter
445                // module must be completed and then a new one is started.
446                self.finish_adapter_module();
447                assert!(self.defined_items.contains(&Def::Adapter(*id)));
448            }
449
450            // These items can't transitively depend on an adapter
451            dfg::CoreDef::Trampoline(_) | dfg::CoreDef::InstanceFlags(_) => {}
452        }
453    }
454
455    fn core_export<T>(&mut self, dfg: &dfg::ComponentDfg, export: &dfg::CoreExport<T>) {
456        // When an adapter depends on an exported item it actually depends on
457        // the instance of that exported item. The caveat here is that the
458        // adapter not only depends on that particular instance, but also all
459        // prior instances to that instance as well because instance
460        // instantiation order is fixed and cannot change.
461        //
462        // To model this the instance index space is looped over here and while
463        // an instance hasn't been visited it's visited. Note that if an
464        // instance has already been visited then all prior instances have
465        // already been visited so there's no need to continue.
466        let mut instance = export.instance;
467        while self.defined_items.insert(Def::Instance(instance)) {
468            self.instance(dfg, instance);
469            if instance.as_u32() == 0 {
470                break;
471            }
472            instance = dfg::InstanceId::from_u32(instance.as_u32() - 1);
473        }
474    }
475
476    fn instance(&mut self, dfg: &dfg::ComponentDfg, instance: dfg::InstanceId) {
477        log::debug!("visiting instance {instance:?}");
478
479        // ... otherwise if this is the first timet he instance has been seen
480        // then the instances own arguments are recursively visited to find
481        // transitive dependencies on adapters.
482        match &dfg.instances[instance] {
483            dfg::Instance::Static(_, args) => {
484                for arg in args.iter() {
485                    self.core_def(dfg, arg);
486                }
487            }
488            dfg::Instance::Import(_, args) => {
489                for (_, values) in args {
490                    for (_, def) in values {
491                        self.core_def(dfg, def);
492                    }
493                }
494            }
495        }
496    }
497
498    fn finish_adapter_module(&mut self) {
499        if self.next_module.adapters.is_empty() {
500            return;
501        }
502
503        // Reset the state of the current module-in-progress and then flag all
504        // pending adapters as now defined since the current module is being
505        // committed.
506        let module = mem::take(&mut self.next_module);
507        for adapter in module.adapters.iter() {
508            let inserted = self.defined_items.insert(Def::Adapter(*adapter));
509            assert!(inserted);
510        }
511        let idx = self.adapter_modules.push(module);
512        log::debug!("finishing adapter module {idx:?}");
513    }
514}