doc: add v8 fast api contribution guidelines

Author: anonrig

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

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+# Adding V8 Fast API
+
+Node.js uses [V8](https://github.com/v8/v8) as the JavaScript engine.
+In order to provide fast paths for functions that are called quite often,
+V8 proposes the usage of `fast api calls` that does not use any of the V8
+internals, but uses internal C functions.
+
+## Limitations
+
+* Fast api calls can not use `V8` internals inside the fast path.
+* 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) file.
+
+## Requirements
+
+* Each unique fast path function signature should be defined inside
+  [`node_external_reference.h`](/src/node_external_reference.h) file.
+* In order to test fast paths, make sure to run the tests in a loop
+  with more than 1000 iterations to force V8 to optimize and prefer the
+  fast api over slow path.
+
+## Fallback to slow path
+
+Fast apis support fallback to slow path (implementation that uses V8 internals)
+in case logically it is wise to do so, for example when providing a more
+detailed error. Fallback mechanism can be enabled, and changed from both caller
+JavaScript function or from the fast api function declaration.
+
+Every fast api function accessible from JavaScript side, can pass an object
+consisting of fallback key, with a boolean value as the last parameter
+(or the first parameter, if no parameters of the function exist).
+
+In V8 the options fallback is defined as `FastApiCallbackOptions` inside
+[`v8-fast-api-calls.h`](/deps/v8/include/v8-fast-api-calls.h) file.
+
+* JavaScript land
+
+Example of a JavaScript:
+
+```javascript
+// Let calculateX be a function that provides fast api calls.
+const { calculateX } = internalBinding('custom_namespace');
+
+function conditionallyGetX(useSlowPath) {
+  return calculateX({ fallback: useSlowPath })
+}
+```
+
+* C++ land
+
+Example of a conditional fast path on C++
+
+```c++
+// Anywhere in the execution flow, you can set fallback and stop the execution.
+static void FastCalculateX(const v8::FastApiCallbackOptions& options) {
+  if (true) {
+    options.fallback = true;
+  }
+}
+```
+
+## Example
+
+A typical function that communicates between JavaScript and C++ is as follows.
+
+* On the JavaScript side:
+
+```javascript
+const { calculateX } = internalBinding('custom_namespace');
+```
+
+* On the C++ side:
+
+```c++
+namespace node {
+namespace custom_namespace {
+
+#define PREFER_FALLBACK = false;
+
+static void CalculateX(const FunctionCallbackInfo<Value>& args) {
+  int calculated_x = 5;
+  args.GetReturnValue().Set(calculated_x);
+}
+
+static int FastCalculateX(const v8::FastApiCallbackOptions& options) {
+  if (PREFER_FALLBACK) {
+    options.fallback = true;
+    return;
+  }
+  return 5;
+}
+
+CFunction fast_calculate_x_(CFunction::Make(FastCalculateX));
+
+static void Initialize(Local<Object> target,
+                       Local<Value> unused,
+                       Local<Context> context,
+                       void* priv) {
+  SetFastMethod(context, target, "calculateX", CalculateX, &calculate_x_);
+}
+
+void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
+  registry->Register(CalculateX);
+  registry->Register(FastCalculateX);
+  registry->Register(fast_calculate_x_.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 `int(const v8::FastApiCallbackOptions& options)` signature,
+we need to add it to external references.
+
+Example declaration:
+
+```c++
+using CFunctionCallbackReturningInt = int (*)(const v8::FastApiCallbackOptions& options);
+```