doc: add v8 fast api contribution guidelines

doc/contributing/adding-v8-fast-api.md

@@ -0,0 +1,150 @@
+# Adding V8 Fast API
+
+Node.js uses [V8](https://v8.dev/) as its JavaScript engine.
+Embedding functions implemented in C++ incur a high overhead, so V8
+provides an API to implement native functions which may be invoked directly
+from JITted code. These functions also come with additional constraints,
+for example, they may not trigger garbage collection.
+
+## Limitations
+
+* Fast API functions may not trigger garbage collection. This means by proxy
+  that JavaScript execution and heap allocation are also forbidden, including
+  `v8::Array::Get()` or `v8::Number::New()`.
+* Throwing errors is not available from within a fast API call, but can be done
+  through the fallback to the slow API.
+* Not all parameter and return types are supported in fast API calls.
+  For a full list, please look into
+  [`v8-fast-api-calls.h`](../../deps/v8/include/v8-fast-api-calls.h).
+
+## Requirements
+
+* Any function passed to `CFunction::Make`, including fast API function
+  declarations, should have their signature registered in
+  [`node_external_reference.h`](../../src/node_external_reference.h) file.
+  Although, it would not start failing or crashing until the function ends up
+  in a snapshot (either the built-in or a user-land one). Please refer to the
+  [binding functions documentation](../../src#binding-functions) for more
+  information.
+* To test fast APIs, make sure to run the tests in a loop with a decent
+  iterations count to trigger relevant optimizations that prefer the fast API
+  over the slow one.
+* The fast callback must be idempotent up to the point where error and fallback
+  conditions are checked, because otherwise executing the slow callback might
+  produce visible side effects twice.
+
+## Fallback to slow path
+
+Fast API supports fallback to slow path for when it is desirable to do so,
+for example, when throwing a custom error or executing JavaScript code is
+needed. The fallback mechanism can be enabled and changed from the C++
+implementation of the fast API function declaration.
+
+Passing `true` to the `fallback` option will force V8 to run the slow path
+with the same arguments.
+
+In V8, the options fallback is defined as `FastApiCallbackOptions` inside
+[`v8-fast-api-calls.h`](../../deps/v8/include/v8-fast-api-calls.h).
+
+* C++ land
+
+  Example of a conditional fast path on C++
+
+  ```cpp
+  // Anywhere in the execution flow, you can set fallback and stop the execution.
+  static double divide(const int32_t a,
+                       const int32_t b,
+                       v8::FastApiCallbackOptions& options) {
+    if (b == 0) {
+      options.fallback = true;
+      return 0;
+    } else {
+      return a / b;
+    }
+  }
+  ```
+
+## Example
+
+A typical function that communicates between JavaScript and C++ is as follows.
+
+* On the JavaScript side:
+
+  ```js
+  const { divide } = internalBinding('custom_namespace');
+  ```
+
+* On the C++ side:
+
+  ```cpp
+  #include "v8-fast-api-calls.h"
+
+  namespace node {
+  namespace custom_namespace {
+
+  static void SlowDivide(const FunctionCallbackInfo<Value>& args) {
+    Environment* env = Environment::GetCurrent(args);
+    CHECK_GE(args.Length(), 2);
+    CHECK(args[0]->IsInt32());
+    CHECK(args[1]->IsInt32());
+    auto a = args[0].As<v8::Int32>();
+    auto b = args[1].As<v8::Int32>();
+
+    if (b->Value() == 0) {
+      return node::THROW_ERR_INVALID_STATE(env, "Error");
+    }
+
+    double result = a->Value() / b->Value();
+    args.GetReturnValue().Set(v8::Number::New(env->isolate(), result));
+  }
+
+  static double FastDivide(const int32_t a,
+                           const int32_t b,
+                           v8::FastApiCallbackOptions& options) {
+    if (b == 0) {
+      options.fallback = true;
+      return 0;
+    } else {
+      return a / b;
+    }
+  }
+
+  CFunction fast_divide_(CFunction::Make(FastDivide));
+
+  static void Initialize(Local<Object> target,
+                         Local<Value> unused,
+                         Local<Context> context,
+                         void* priv) {
+    SetFastMethod(context, target, "divide", SlowDivide, &fast_divide_);
+  }
+
+  void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
+    registry->Register(SlowDivide);
+    registry->Register(FastDivide);
+    registry->Register(fast_divide_.GetTypeInfo());
+  }
+
+  } // namespace custom_namespace
+  } // namespace node
+
+  NODE_BINDING_CONTEXT_AWARE_INTERNAL(custom_namespace,
+                                      node::custom_namespace::Initialize);
+  NODE_BINDING_EXTERNAL_REFERENCE(
+                        custom_namespace,
+                        node::custom_namespace::RegisterExternalReferences);
+  ```
+
+* Update external references ([`node_external_reference.h`](../../src/node_external_reference.h))
+
+  Since our implementation used
+  `double(const int32_t a, const int32_t b, v8::FastApiCallbackOptions& options)`
+  signature, we need to add it to external references and in
+  `ALLOWED_EXTERNAL_REFERENCE_TYPES`.
+
+  Example declaration:
+
+  ```cpp
+  using CFunctionCallbackReturningDouble = double (*)(const int32_t a,
+                                                      const int32_t b,
+                                                      v8::FastApiCallbackOptions& options);
+  ```

src/README.md

@@ -31,6 +31,9 @@ embedder API.
 Important concepts when using V8 are the ones of [`Isolate`][]s and
 [JavaScript value handles][].
 
+V8 supports [fast API calls][], which can be useful for improving the
+performance in certain cases.
+
 ## libuv API documentation
 
 The other major dependency of Node.js is [libuv][], providing
@@ -1055,6 +1058,7 @@ static void GetUserInfo(const FunctionCallbackInfo<Value>& args) {
 [cleanup hooks]: #cleanup-hooks
 [event loop]: #event-loop
 [exception handling]: #exception-handling
+[fast API calls]: ../doc/contributing/adding-v8-fast-api.md
 [internal field]: #internal-fields
 [introduction for V8 embedders]: https://v8.dev/docs/embed
 [libuv]: https://libuv.org/