wasm2c: implement the tail-call proposal

Author: keithw

include/wabt/ir.h

@@ -893,6 +893,13 @@ struct Func {
   BindingHash bindings;
   ExprList exprs;
   Location loc;
+
+  // For a subset of features, the BinaryReaderIR tracks whether they are
+  // actually used by the function. wasm2c (CWriter) uses this information to
+  // limit its output in some cases.
+  struct {
+    bool tailcall = false;
+  } features_used;
 };
 
 struct Global {

src/binary-reader-ir.cc

@@ -892,12 +892,22 @@ Result BinaryReaderIR::OnCallRefExpr() {
 }
 
 Result BinaryReaderIR::OnReturnCallExpr(Index func_index) {
+  if (current_func_) {
+    // syntactically, a return_call expr can occur in an init expression
+    // (outside a function)
+    current_func_->features_used.tailcall = true;
+  }
   return AppendExpr(
       std::make_unique<ReturnCallExpr>(Var(func_index, GetLocation())));
 }
 
 Result BinaryReaderIR::OnReturnCallIndirectExpr(Index sig_index,
                                                 Index table_index) {
+  if (current_func_) {
+    // syntactically, a return_call_indirect expr can occur in an init
+    // expression (outside a function)
+    current_func_->features_used.tailcall = true;
+  }
   auto expr = std::make_unique<ReturnCallIndirectExpr>();
   SetFuncDeclaration(&expr->decl, Var(sig_index, GetLocation()));
   expr->table = Var(table_index, GetLocation());