Create a table import if there is none in GenerateDynCalls::generateDynCallThunk (#3580)

This fixes LLVM=>emscripten autoroller breakage due to llvm/llvm-project@f48923e

commit f48923e884611e6271a8da821a58aedd24d91cf7 (HEAD)
Author: Andy Wingo <wingo@igalia.com>
Date:   Wed Feb 17 17:20:28 2021 +0100

    [WebAssembly][lld] --importTable flag only imports table if needed
    
    Before, --importTable forced the creation of an indirect function table,
    whether it was needed or not.  Now it only imports a table if needed.
    
    Differential Revision: https://reviews.llvm.org/D96872

1 files changed, 7 insertions, 3 deletions

diff --git a/src/passes/GenerateDynCalls.cpp b/src/passes/GenerateDynCalls.cpp
index 9669dcdb8..827dccea0 100644
--- a/src/passes/GenerateDynCalls.cpp
+++ b/src/passes/GenerateDynCalls.cpp
@@ -127,9 +127,13 @@ void GenerateDynCalls::generateDynCallThunk(Signature sig) {
   for (const auto& param : sig.params) {
     args.push_back(builder.makeLocalGet(++i, param));
   }
-  // FIXME: Should the existence of a table be ensured here? i.e. create one if
-  // there is none?
-  assert(wasm->tables.size() > 0);
+  if (wasm->tables.empty()) {
+    // Add an imported table in exactly the same manner as the LLVM wasm backend
+    // would add one.
+    auto* table = wasm->addTable(Builder::makeTable(Name::fromInt(0)));
+    table->module = ENV;
+    table->base = "__indirect_function_table";
+  }
   f->body = builder.makeCallIndirect(wasm->tables[0]->name, fptr, args, sig);
 
   wasm->addFunction(std::move(f));