Generate thunks for address-taken imports (#554)

Under emscripten, C code can take the address of a function implemented
in Javascript (which is exposed via an import in wasm). Because imports
do not have linear memory address in wasm, we need to generate a thunk
to be the target of the indirect call; it call the import directly.

This is facilited by a new .s directive (.functype) which declares the
types of functions which are declared but not defined.

Fixes https://github.com/WebAssembly/binaryen/issues/392

1 files changed, 16 insertions, 0 deletions

diff --git a/src/wasm-linker.h b/src/wasm-linker.h
index 5fe01b393..a0684d02b 100644
--- a/src/wasm-linker.h
+++ b/src/wasm-linker.h
@@ -119,6 +119,15 @@ class LinkerObject {
     undefinedFunctionCalls[call->target].push_back(call);
   }
 
+  void addExternType(Name name, FunctionType* ty) {
+    externTypesMap[name] = ty;
+  }
+  FunctionType* getExternType(Name name) {
+    auto f = externTypesMap.find(name);
+    if (f == externTypesMap.end()) return nullptr;
+    return f->second;
+  }
+
   bool isEmpty() {
     return wasm.functions.empty();
   }
@@ -146,6 +155,9 @@ class LinkerObject {
   using CallList = std::vector<Call*>;
   std::map<Name, CallList> undefinedFunctionCalls;
 
+  // Types of functions which are declared but not defined.
+  std::unordered_map<cashew::IString, FunctionType*> externTypesMap;
+
   std::map<Name, Address> segments; // name => segment index (in wasm module)
 
   std::vector<Name> initializerFunctions;
@@ -244,6 +256,8 @@ class Linker {
     o << "]";
   }
 
+  void ensureImport(Name target, std::string signature);
+
   // Create thunks for use with emscripten Runtime.dynCall. Creates one for each
   // signature in the indirect function table.
   void makeDynCallThunks();
@@ -263,6 +277,8 @@ class Linker {
     out.wasm.addExport(exp);
   }
 
+  Function* generateImportThunk(Name name, const FunctionType* t);
+
   // The output module (linked executable)
   LinkerObject out;