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    /// The core function type that is being lifted from / lowered to.
178    pub core_type: ModuleInternedTypeIndex,
179    /// The data model used by this adapter: linear memory or GC objects.
180    pub data_model: DataModel,
181}
182
183impl<'data> Translator<'_, 'data> {
184    /// This is the entrypoint of functionality within this module which
185    /// performs all the work of identifying adapter usages and organizing
186    /// everything into adapter modules.
187    ///
188    /// This will mutate the provided `component` in-place and fill out the dfg
189    /// metadata for adapter modules.
190    pub(super) fn partition_adapter_modules(&mut self, component: &mut dfg::ComponentDfg) {
191        // Visit each adapter, in order of its original definition, during the
192        // partitioning. This allows for the guarantee that dependencies are
193        // visited in a topological fashion ideally.
194        let mut state = PartitionAdapterModules::default();
195        for (id, adapter) in component.adapters.iter() {
196            state.adapter(component, id, adapter);
197        }
198        state.finish_adapter_module();
199
200        // Now that all adapters have been partitioned into modules this loop
201        // generates a core wasm module for each adapter module, translates
202        // the module using standard core wasm translation, and then fills out
203        // the dfg metadata for each adapter.
204        for (module_id, adapter_module) in state.adapter_modules.iter() {
205            let mut module =
206                fact::Module::new(self.types.types(), self.tunables.debug_adapter_modules);
207            let mut names = Vec::with_capacity(adapter_module.adapters.len());
208            for adapter in adapter_module.adapters.iter() {
209                let name = format!("adapter{}", adapter.as_u32());
210                module.adapt(&name, &component.adapters[*adapter]);
211                names.push(name);
212            }
213            let wasm = module.encode();
214            let imports = module.imports().to_vec();
215
216            // Extend the lifetime of the owned `wasm: Vec<u8>` on the stack to
217            // a higher scope defined by our original caller. That allows to
218            // transform `wasm` into `&'data [u8]` which is much easier to work
219            // with here.
220            let wasm = &*self.scope_vec.push(wasm);
221            if log::log_enabled!(log::Level::Trace) {
222                match wasmprinter::print_bytes(wasm) {
223                    Ok(s) => log::trace!("generated adapter module:\n{}", s),
224                    Err(e) => log::trace!("failed to print adapter module: {}", e),
225                }
226            }
227
228            // With the wasm binary this is then pushed through general
229            // translation, validation, etc. Note that multi-memory is
230            // specifically enabled here since the adapter module is highly
231            // likely to use that if anything is actually indirected through
232            // memory.
233            self.validator.reset();
234            let translation = ModuleEnvironment::new(
235                self.tunables,
236                &mut self.validator,
237                self.types.module_types_builder(),
238            )
239            .translate(Parser::new(0), wasm)
240            .expect("invalid adapter module generated");
241
242            // Record, for each adapter in this adapter module, the module that
243            // the adapter was placed within as well as the function index of
244            // the adapter in the wasm module generated. Note that adapters are
245            // paritioned in-order so we're guaranteed to push the adapters
246            // in-order here as well. (with an assert to double-check)
247            for (adapter, name) in adapter_module.adapters.iter().zip(&names) {
248                let index = translation.module.exports[name];
249                let i = component.adapter_partitionings.push((module_id, index));
250                assert_eq!(i, *adapter);
251            }
252
253            // Finally the metadata necessary to instantiate this adapter
254            // module is also recorded in the dfg. This metadata will be used
255            // to generate `GlobalInitializer` entries during the linearization
256            // final phase.
257            assert_eq!(imports.len(), translation.module.imports().len());
258            let args = imports
259                .iter()
260                .zip(translation.module.imports())
261                .map(|(arg, (_, _, ty))| fact_import_to_core_def(component, arg, ty))
262                .collect::<Vec<_>>();
263            let static_index = self.static_modules.push(translation);
264            let id = component.adapter_modules.push((static_index, args));
265            assert_eq!(id, module_id);
266        }
267    }
268}
269
270fn fact_import_to_core_def(
271    dfg: &mut dfg::ComponentDfg,
272    import: &fact::Import,
273    ty: EntityType,
274) -> dfg::CoreDef {
275    fn unwrap_memory(def: &dfg::CoreDef) -> dfg::CoreExport<MemoryIndex> {
276        match def {
277            dfg::CoreDef::Export(e) => e.clone().map_index(|i| match i {
278                EntityIndex::Memory(i) => i,
279                _ => unreachable!(),
280            }),
281            _ => unreachable!(),
282        }
283    }
284
285    let mut simple_intrinsic = |trampoline: dfg::Trampoline| {
286        let signature = ty.unwrap_func();
287        let index = dfg
288            .trampolines
289            .push((signature.unwrap_module_type_index(), trampoline));
290        dfg::CoreDef::Trampoline(index)
291    };
292    match import {
293        fact::Import::CoreDef(def) => def.clone(),
294        fact::Import::Transcode {
295            op,
296            from,
297            from64,
298            to,
299            to64,
300        } => {
301            let from = dfg.memories.push(unwrap_memory(from));
302            let to = dfg.memories.push(unwrap_memory(to));
303            let signature = ty.unwrap_func();
304            let index = dfg.trampolines.push((
305                signature.unwrap_module_type_index(),
306                dfg::Trampoline::Transcoder {
307                    op: *op,
308                    from,
309                    from64: *from64,
310                    to,
311                    to64: *to64,
312                },
313            ));
314            dfg::CoreDef::Trampoline(index)
315        }
316        fact::Import::ResourceTransferOwn => simple_intrinsic(dfg::Trampoline::ResourceTransferOwn),
317        fact::Import::ResourceTransferBorrow => {
318            simple_intrinsic(dfg::Trampoline::ResourceTransferBorrow)
319        }
320        fact::Import::ResourceEnterCall => simple_intrinsic(dfg::Trampoline::ResourceEnterCall),
321        fact::Import::ResourceExitCall => simple_intrinsic(dfg::Trampoline::ResourceExitCall),
322        fact::Import::PrepareCall { memory } => simple_intrinsic(dfg::Trampoline::PrepareCall {
323            memory: memory.as_ref().map(|v| dfg.memories.push(unwrap_memory(v))),
324        }),
325        fact::Import::SyncStartCall { callback } => {
326            simple_intrinsic(dfg::Trampoline::SyncStartCall {
327                callback: callback.clone().map(|v| dfg.callbacks.push(v)),
328            })
329        }
330        fact::Import::AsyncStartCall {
331            callback,
332            post_return,
333        } => simple_intrinsic(dfg::Trampoline::AsyncStartCall {
334            callback: callback.clone().map(|v| dfg.callbacks.push(v)),
335            post_return: post_return.clone().map(|v| dfg.post_returns.push(v)),
336        }),
337        fact::Import::FutureTransfer => simple_intrinsic(dfg::Trampoline::FutureTransfer),
338        fact::Import::StreamTransfer => simple_intrinsic(dfg::Trampoline::StreamTransfer),
339        fact::Import::ErrorContextTransfer => {
340            simple_intrinsic(dfg::Trampoline::ErrorContextTransfer)
341        }
342    }
343}
344
345#[derive(Default)]
346struct PartitionAdapterModules {
347    /// The next adapter module that's being created. This may be empty.
348    next_module: AdapterModuleInProgress,
349
350    /// The set of items which are known to be defined which the adapter module
351    /// in progress is allowed to depend on.
352    defined_items: HashSet<Def>,
353
354    /// Finished adapter modules that won't be added to.
355    ///
356    /// In theory items could be added to preexisting modules here but to keep
357    /// this pass linear this is never modified after insertion.
358    adapter_modules: PrimaryMap<dfg::AdapterModuleId, AdapterModuleInProgress>,
359}
360
361#[derive(Default)]
362struct AdapterModuleInProgress {
363    /// The adapters which have been placed into this module.
364    adapters: Vec<dfg::AdapterId>,
365}
366
367/// Items that adapters can depend on.
368///
369/// Note that this is somewhat of a flat list and is intended to mostly model
370/// core wasm instances which are side-effectful unlike other host items like
371/// lowerings or always-trapping functions.
372#[derive(Copy, Clone, Hash, Eq, PartialEq)]
373enum Def {
374    Adapter(dfg::AdapterId),
375    Instance(dfg::InstanceId),
376}
377
378impl PartitionAdapterModules {
379    fn adapter(&mut self, dfg: &dfg::ComponentDfg, id: dfg::AdapterId, adapter: &Adapter) {
380        // Visit all dependencies of this adapter and if anything depends on
381        // the current adapter module in progress then a new adapter module is
382        // started.
383        self.adapter_options(dfg, &adapter.lift_options);
384        self.adapter_options(dfg, &adapter.lower_options);
385        self.core_def(dfg, &adapter.func);
386
387        // With all dependencies visited this adapter is added to the next
388        // module.
389        //
390        // This will either get added the preexisting module if this adapter
391        // didn't depend on anything in that module itself or it will be added
392        // to a fresh module if this adapter depended on something that the
393        // current adapter module created.
394        log::debug!("adding {id:?} to adapter module");
395        self.next_module.adapters.push(id);
396    }
397
398    fn adapter_options(&mut self, dfg: &dfg::ComponentDfg, options: &AdapterOptions) {
399        if let Some(def) = &options.callback {
400            self.core_def(dfg, def);
401        }
402        if let Some(def) = &options.post_return {
403            self.core_def(dfg, def);
404        }
405        match &options.data_model {
406            DataModel::Gc {} => {
407                // Nothing to do here yet.
408            }
409            DataModel::LinearMemory {
410                memory,
411                memory64: _,
412                realloc,
413            } => {
414                if let Some(memory) = memory {
415                    self.core_export(dfg, memory);
416                }
417                if let Some(def) = realloc {
418                    self.core_def(dfg, def);
419                }
420            }
421        }
422    }
423
424    fn core_def(&mut self, dfg: &dfg::ComponentDfg, def: &dfg::CoreDef) {
425        match def {
426            dfg::CoreDef::Export(e) => self.core_export(dfg, e),
427            dfg::CoreDef::Adapter(id) => {
428                // If this adapter is already defined then we can safely depend
429                // on it with no consequences.
430                if self.defined_items.contains(&Def::Adapter(*id)) {
431                    log::debug!("using existing adapter {id:?} ");
432                    return;
433                }
434
435                log::debug!("splitting module needing {id:?} ");
436
437                // .. otherwise we found a case of an adapter depending on an
438                // adapter-module-in-progress meaning that the current adapter
439                // module must be completed and then a new one is started.
440                self.finish_adapter_module();
441                assert!(self.defined_items.contains(&Def::Adapter(*id)));
442            }
443
444            // These items can't transitively depend on an adapter
445            dfg::CoreDef::Trampoline(_) | dfg::CoreDef::InstanceFlags(_) => {}
446        }
447    }
448
449    fn core_export<T>(&mut self, dfg: &dfg::ComponentDfg, export: &dfg::CoreExport<T>) {
450        // When an adapter depends on an exported item it actually depends on
451        // the instance of that exported item. The caveat here is that the
452        // adapter not only depends on that particular instance, but also all
453        // prior instances to that instance as well because instance
454        // instantiation order is fixed and cannot change.
455        //
456        // To model this the instance index space is looped over here and while
457        // an instance hasn't been visited it's visited. Note that if an
458        // instance has already been visited then all prior instances have
459        // already been visited so there's no need to continue.
460        let mut instance = export.instance;
461        while self.defined_items.insert(Def::Instance(instance)) {
462            self.instance(dfg, instance);
463            if instance.as_u32() == 0 {
464                break;
465            }
466            instance = dfg::InstanceId::from_u32(instance.as_u32() - 1);
467        }
468    }
469
470    fn instance(&mut self, dfg: &dfg::ComponentDfg, instance: dfg::InstanceId) {
471        log::debug!("visiting instance {instance:?}");
472
473        // ... otherwise if this is the first timet he instance has been seen
474        // then the instances own arguments are recursively visited to find
475        // transitive dependencies on adapters.
476        match &dfg.instances[instance] {
477            dfg::Instance::Static(_, args) => {
478                for arg in args.iter() {
479                    self.core_def(dfg, arg);
480                }
481            }
482            dfg::Instance::Import(_, args) => {
483                for (_, values) in args {
484                    for (_, def) in values {
485                        self.core_def(dfg, def);
486                    }
487                }
488            }
489        }
490    }
491
492    fn finish_adapter_module(&mut self) {
493        if self.next_module.adapters.is_empty() {
494            return;
495        }
496
497        // Reset the state of the current module-in-progress and then flag all
498        // pending adapters as now defined since the current module is being
499        // committed.
500        let module = mem::take(&mut self.next_module);
501        for adapter in module.adapters.iter() {
502            let inserted = self.defined_items.insert(Def::Adapter(*adapter));
503            assert!(inserted);
504        }
505        let idx = self.adapter_modules.push(module);
506        log::debug!("finishing adapter module {idx:?}");
507    }
508}