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::component::translate::*;
119use crate::fact;
120use crate::EntityType;
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/// Configuration options which can be specified as part of the canonical ABI
146/// in the component model.
147#[derive(Debug, Clone, Hash, Eq, PartialEq)]
148pub struct AdapterOptions {
149    /// The Wasmtime-assigned component instance index where the options were
150    /// originally specified.
151    pub instance: RuntimeComponentInstanceIndex,
152    /// How strings are encoded.
153    pub string_encoding: StringEncoding,
154    /// An optional memory definition supplied.
155    pub memory: Option<dfg::CoreExport<MemoryIndex>>,
156    /// If `memory` is specified, whether it's a 64-bit memory.
157    pub memory64: bool,
158    /// An optional definition of `realloc` to used.
159    pub realloc: Option<dfg::CoreDef>,
160    /// The async callback function used by these options, if specified.
161    pub callback: Option<dfg::CoreDef>,
162    /// An optional definition of a `post-return` to use.
163    pub post_return: Option<dfg::CoreDef>,
164    /// Whether to use the async ABI for lifting or lowering.
165    pub async_: bool,
166}
167
168impl<'data> Translator<'_, 'data> {
169    /// This is the entrypoint of functionality within this module which
170    /// performs all the work of identifying adapter usages and organizing
171    /// everything into adapter modules.
172    ///
173    /// This will mutate the provided `component` in-place and fill out the dfg
174    /// metadata for adapter modules.
175    pub(super) fn partition_adapter_modules(&mut self, component: &mut dfg::ComponentDfg) {
176        // Visit each adapter, in order of its original definition, during the
177        // partitioning. This allows for the guarantee that dependencies are
178        // visited in a topological fashion ideally.
179        let mut state = PartitionAdapterModules::default();
180        for (id, adapter) in component.adapters.iter() {
181            state.adapter(component, id, adapter);
182        }
183        state.finish_adapter_module();
184
185        // Now that all adapters have been partitioned into modules this loop
186        // generates a core wasm module for each adapter module, translates
187        // the module using standard core wasm translation, and then fills out
188        // the dfg metadata for each adapter.
189        for (module_id, adapter_module) in state.adapter_modules.iter() {
190            let mut module =
191                fact::Module::new(self.types.types(), self.tunables.debug_adapter_modules);
192            let mut names = Vec::with_capacity(adapter_module.adapters.len());
193            for adapter in adapter_module.adapters.iter() {
194                let name = format!("adapter{}", adapter.as_u32());
195                module.adapt(&name, &component.adapters[*adapter]);
196                names.push(name);
197            }
198            let wasm = module.encode();
199            let imports = module.imports().to_vec();
200
201            // Extend the lifetime of the owned `wasm: Vec<u8>` on the stack to
202            // a higher scope defined by our original caller. That allows to
203            // transform `wasm` into `&'data [u8]` which is much easier to work
204            // with here.
205            let wasm = &*self.scope_vec.push(wasm);
206            if log::log_enabled!(log::Level::Trace) {
207                match wasmprinter::print_bytes(wasm) {
208                    Ok(s) => log::trace!("generated adapter module:\n{}", s),
209                    Err(e) => log::trace!("failed to print adapter module: {}", e),
210                }
211            }
212
213            // With the wasm binary this is then pushed through general
214            // translation, validation, etc. Note that multi-memory is
215            // specifically enabled here since the adapter module is highly
216            // likely to use that if anything is actually indirected through
217            // memory.
218            self.validator.reset();
219            let translation = ModuleEnvironment::new(
220                self.tunables,
221                &mut self.validator,
222                self.types.module_types_builder(),
223            )
224            .translate(Parser::new(0), wasm)
225            .expect("invalid adapter module generated");
226
227            // Record, for each adapter in this adapter module, the module that
228            // the adapter was placed within as well as the function index of
229            // the adapter in the wasm module generated. Note that adapters are
230            // paritioned in-order so we're guaranteed to push the adapters
231            // in-order here as well. (with an assert to double-check)
232            for (adapter, name) in adapter_module.adapters.iter().zip(&names) {
233                let index = translation.module.exports[name];
234                let i = component.adapter_partitionings.push((module_id, index));
235                assert_eq!(i, *adapter);
236            }
237
238            // Finally the metadata necessary to instantiate this adapter
239            // module is also recorded in the dfg. This metadata will be used
240            // to generate `GlobalInitializer` entries during the linearization
241            // final phase.
242            assert_eq!(imports.len(), translation.module.imports().len());
243            let args = imports
244                .iter()
245                .zip(translation.module.imports())
246                .map(|(arg, (_, _, ty))| fact_import_to_core_def(component, arg, ty))
247                .collect::<Vec<_>>();
248            let static_index = self.static_modules.push(translation);
249            let id = component.adapter_modules.push((static_index, args.into()));
250            assert_eq!(id, module_id);
251        }
252    }
253}
254
255fn fact_import_to_core_def(
256    dfg: &mut dfg::ComponentDfg,
257    import: &fact::Import,
258    ty: EntityType,
259) -> dfg::CoreDef {
260    let mut simple_intrinsic = |trampoline: dfg::Trampoline| {
261        let signature = ty.unwrap_func();
262        let index = dfg
263            .trampolines
264            .push((signature.unwrap_module_type_index(), trampoline));
265        dfg::CoreDef::Trampoline(index)
266    };
267    match import {
268        fact::Import::CoreDef(def) => def.clone(),
269        fact::Import::Transcode {
270            op,
271            from,
272            from64,
273            to,
274            to64,
275        } => {
276            fn unwrap_memory(def: &dfg::CoreDef) -> dfg::CoreExport<MemoryIndex> {
277                match def {
278                    dfg::CoreDef::Export(e) => e.clone().map_index(|i| match i {
279                        EntityIndex::Memory(i) => i,
280                        _ => unreachable!(),
281                    }),
282                    _ => unreachable!(),
283                }
284            }
285
286            let from = dfg.memories.push(unwrap_memory(from));
287            let to = dfg.memories.push(unwrap_memory(to));
288            let signature = ty.unwrap_func();
289            let index = dfg.trampolines.push((
290                signature.unwrap_module_type_index(),
291                dfg::Trampoline::Transcoder {
292                    op: *op,
293                    from,
294                    from64: *from64,
295                    to,
296                    to64: *to64,
297                },
298            ));
299            dfg::CoreDef::Trampoline(index)
300        }
301        fact::Import::ResourceTransferOwn => simple_intrinsic(dfg::Trampoline::ResourceTransferOwn),
302        fact::Import::ResourceTransferBorrow => {
303            simple_intrinsic(dfg::Trampoline::ResourceTransferBorrow)
304        }
305        fact::Import::ResourceEnterCall => simple_intrinsic(dfg::Trampoline::ResourceEnterCall),
306        fact::Import::ResourceExitCall => simple_intrinsic(dfg::Trampoline::ResourceExitCall),
307        fact::Import::AsyncEnterCall => simple_intrinsic(dfg::Trampoline::AsyncEnterCall),
308        fact::Import::AsyncExitCall {
309            callback,
310            post_return,
311        } => simple_intrinsic(dfg::Trampoline::AsyncExitCall {
312            callback: callback.clone().map(|v| dfg.callbacks.push(v)),
313            post_return: post_return.clone().map(|v| dfg.post_returns.push(v)),
314        }),
315        fact::Import::FutureTransfer => simple_intrinsic(dfg::Trampoline::FutureTransfer),
316        fact::Import::StreamTransfer => simple_intrinsic(dfg::Trampoline::StreamTransfer),
317        fact::Import::ErrorContextTransfer => {
318            simple_intrinsic(dfg::Trampoline::ErrorContextTransfer)
319        }
320    }
321}
322
323#[derive(Default)]
324struct PartitionAdapterModules {
325    /// The next adapter module that's being created. This may be empty.
326    next_module: AdapterModuleInProgress,
327
328    /// The set of items which are known to be defined which the adapter module
329    /// in progress is allowed to depend on.
330    defined_items: HashSet<Def>,
331
332    /// Finished adapter modules that won't be added to.
333    ///
334    /// In theory items could be added to preexisting modules here but to keep
335    /// this pass linear this is never modified after insertion.
336    adapter_modules: PrimaryMap<dfg::AdapterModuleId, AdapterModuleInProgress>,
337}
338
339#[derive(Default)]
340struct AdapterModuleInProgress {
341    /// The adapters which have been placed into this module.
342    adapters: Vec<dfg::AdapterId>,
343}
344
345/// Items that adapters can depend on.
346///
347/// Note that this is somewhat of a flat list and is intended to mostly model
348/// core wasm instances which are side-effectful unlike other host items like
349/// lowerings or always-trapping functions.
350#[derive(Copy, Clone, Hash, Eq, PartialEq)]
351enum Def {
352    Adapter(dfg::AdapterId),
353    Instance(dfg::InstanceId),
354}
355
356impl PartitionAdapterModules {
357    fn adapter(&mut self, dfg: &dfg::ComponentDfg, id: dfg::AdapterId, adapter: &Adapter) {
358        // Visit all dependencies of this adapter and if anything depends on
359        // the current adapter module in progress then a new adapter module is
360        // started.
361        self.adapter_options(dfg, &adapter.lift_options);
362        self.adapter_options(dfg, &adapter.lower_options);
363        self.core_def(dfg, &adapter.func);
364
365        // With all dependencies visited this adapter is added to the next
366        // module.
367        //
368        // This will either get added the preexisting module if this adapter
369        // didn't depend on anything in that module itself or it will be added
370        // to a fresh module if this adapter depended on something that the
371        // current adapter module created.
372        log::debug!("adding {id:?} to adapter module");
373        self.next_module.adapters.push(id);
374    }
375
376    fn adapter_options(&mut self, dfg: &dfg::ComponentDfg, options: &AdapterOptions) {
377        if let Some(memory) = &options.memory {
378            self.core_export(dfg, memory);
379        }
380        if let Some(def) = &options.realloc {
381            self.core_def(dfg, def);
382        }
383        if let Some(def) = &options.callback {
384            self.core_def(dfg, def);
385        }
386        if let Some(def) = &options.post_return {
387            self.core_def(dfg, def);
388        }
389    }
390
391    fn core_def(&mut self, dfg: &dfg::ComponentDfg, def: &dfg::CoreDef) {
392        match def {
393            dfg::CoreDef::Export(e) => self.core_export(dfg, e),
394            dfg::CoreDef::Adapter(id) => {
395                // If this adapter is already defined then we can safely depend
396                // on it with no consequences.
397                if self.defined_items.contains(&Def::Adapter(*id)) {
398                    log::debug!("using existing adapter {id:?} ");
399                    return;
400                }
401
402                log::debug!("splitting module needing {id:?} ");
403
404                // .. otherwise we found a case of an adapter depending on an
405                // adapter-module-in-progress meaning that the current adapter
406                // module must be completed and then a new one is started.
407                self.finish_adapter_module();
408                assert!(self.defined_items.contains(&Def::Adapter(*id)));
409            }
410
411            // These items can't transitively depend on an adapter
412            dfg::CoreDef::Trampoline(_) | dfg::CoreDef::InstanceFlags(_) => {}
413        }
414    }
415
416    fn core_export<T>(&mut self, dfg: &dfg::ComponentDfg, export: &dfg::CoreExport<T>) {
417        // When an adapter depends on an exported item it actually depends on
418        // the instance of that exported item. The caveat here is that the
419        // adapter not only depends on that particular instance, but also all
420        // prior instances to that instance as well because instance
421        // instantiation order is fixed and cannot change.
422        //
423        // To model this the instance index space is looped over here and while
424        // an instance hasn't been visited it's visited. Note that if an
425        // instance has already been visited then all prior instances have
426        // already been visited so there's no need to continue.
427        let mut instance = export.instance;
428        while self.defined_items.insert(Def::Instance(instance)) {
429            self.instance(dfg, instance);
430            if instance.as_u32() == 0 {
431                break;
432            }
433            instance = dfg::InstanceId::from_u32(instance.as_u32() - 1);
434        }
435    }
436
437    fn instance(&mut self, dfg: &dfg::ComponentDfg, instance: dfg::InstanceId) {
438        log::debug!("visiting instance {instance:?}");
439
440        // ... otherwise if this is the first timet he instance has been seen
441        // then the instances own arguments are recursively visited to find
442        // transitive dependencies on adapters.
443        match &dfg.instances[instance] {
444            dfg::Instance::Static(_, args) => {
445                for arg in args.iter() {
446                    self.core_def(dfg, arg);
447                }
448            }
449            dfg::Instance::Import(_, args) => {
450                for (_, values) in args {
451                    for (_, def) in values {
452                        self.core_def(dfg, def);
453                    }
454                }
455            }
456        }
457    }
458
459    fn finish_adapter_module(&mut self) {
460        if self.next_module.adapters.is_empty() {
461            return;
462        }
463
464        // Reset the state of the current module-in-progress and then flag all
465        // pending adapters as now defined since the current module is being
466        // committed.
467        let module = mem::take(&mut self.next_module);
468        for adapter in module.adapters.iter() {
469            let inserted = self.defined_items.insert(Def::Adapter(*adapter));
470            assert!(inserted);
471        }
472        let idx = self.adapter_modules.push(module);
473        log::debug!("finishing adapter module {idx:?}");
474    }
475}