MergeSimilarFunctions: do a return_call when possible

This patch makes MergeSimilarFunctions do a return_call when the
module has tail-call enabled. This is not merely an optimization, but
is crucial for correctness when optimizing wasm modules produced by
GHC that relies on tail-call to do control flow transfers. Previously,
-Oz would break tail-call enabled modules by making the control stack
grow where it shouldn't.

Author: TerrorJack

src/passes/MergeSimilarFunctions.cpp

@@ -165,7 +165,8 @@ struct EquivalentClass {
                              Function* target,
                              Function* shared,
                              const std::vector<ParamInfo>& params,
-                             const std::vector<Expression*>& extraArgs);
+                             const std::vector<Expression*>& extraArgs,
+                             bool isReturn);
 
   bool deriveParams(Module* module,
                     std::vector<ParamInfo>& params,
@@ -480,7 +481,7 @@ void EquivalentClass::merge(Module* module,
     for (auto& param : params) {
       extraArgs.push_back(param.lowerToExpression(builder, module, i));
     }
-    replaceWithThunk(builder, func, sharedFn, params, extraArgs);
+    replaceWithThunk(builder, func, sharedFn, params, extraArgs, module->features.hasTailCall());
   }
   return;
 }
@@ -617,7 +618,8 @@ EquivalentClass::replaceWithThunk(Builder& builder,
                                   Function* target,
                                   Function* shared,
                                   const std::vector<ParamInfo>& params,
-                                  const std::vector<Expression*>& extraArgs) {
+                                  const std::vector<Expression*>& extraArgs,
+                                  bool isReturn) {
   std::vector<Expression*> callOperands;
   Type targetParams = target->getParams();
   for (Index i = 0; i < targetParams.size(); i++) {
@@ -628,8 +630,7 @@ EquivalentClass::replaceWithThunk(Builder& builder,
     callOperands.push_back(value);
   }
 
-  // TODO: make a return_call when possible?
-  auto ret = builder.makeCall(shared->name, callOperands, target->getResults());
+  auto ret = builder.makeCall(shared->name, callOperands, target->getResults(), isReturn);
   target->vars.clear();
   target->body = ret;
   return target;