Add custom asyncify section(s)

Names of all the asyncified export functions are now added as data of custom "asyncify" sections.

This allows to detect them in a JS wrapper and wrap into async JS functions only if necessary.

We could come up with a more condensed representation that doesn't duplicate export function names or name of "asyncify" section itself, but this would add unnecessary complexity to implementation with little benefit after Gzip / Brotli, which already take care of duplicated strings quite well.

Fixes WebAssembly#2322.

src/passes/Asyncify.cpp

@@ -1150,6 +1150,32 @@ struct Asyncify : public Pass {
       runner.setValidateGlobally(false);
       runner.run();
     }
+    // Mark exported asyncified functions.
+    //
+    // This creates a custom section for each function that:
+    // 1) Is exported and, thus, directly accessible by user.
+    // 2) Might directly or indirectly start an async operation (and unwind).
+    //
+    // Each such section will have a name "asyncify" and name of the export as
+    // its data.
+    //
+    // This gives the JavaScript side an opportunity to wrap functions into an
+    // `async` variant only if it's necessary, and leave others intact.
+    auto& userSections = module->userSections;
+    for (auto& exp : module->exports) {
+      if (exp->kind != ExternalKind::Function) {
+        continue;
+      }
+      auto* func = module->getFunction(exp->value);
+      if (!analyzer.needsInstrumentation(func)) {
+        continue;
+      }
+      userSections.resize(userSections.size() + 1);
+      auto& section = userSections.back();
+      section.name = BinaryConsts::UserSections::Asyncify;
+      auto& name = exp->name;
+      section.data.assign(name.c_str(), name.c_str() + name.size());
+    }
     // Finally, add function support (that should not have been seen by
     // the previous passes).
     addFunctions(module);

src/wasm-binary.h

@@ -398,6 +398,7 @@ extern const char* Dylink;
 extern const char* Linking;
 extern const char* Producers;
 extern const char* TargetFeatures;
+extern const char* Asyncify;
 
 extern const char* AtomicsFeature;
 extern const char* BulkMemoryFeature;

src/wasm/wasm.cpp

@@ -33,6 +33,7 @@ const char* Dylink = "dylink";
 const char* Linking = "linking";
 const char* Producers = "producers";
 const char* TargetFeatures = "target_features";
+const char* Asyncify = "asyncify";
 const char* AtomicsFeature = "atomics";
 const char* BulkMemoryFeature = "bulk-memory";
 const char* ExceptionHandlingFeature = "exception-handling";