[WebAssembly] Define __funcref_call_table in generated asm and objects (#180900)

Currently, calls into funcref (`call addrspace(20)`) are lowered into a
store of the ref into a special single-element `__funcref_call_table`, a
call_indirect into said table, then a store of null into it.

It functions just fine when `-filetype=asm` (default), but when emitting
an object file directly from llc it fails with `LLVM ERROR: undefined
table symbol cannot be weak`

This fixes said error by "defining" it during `WebAssemblyAsmPrinter`,
and ensuring it is definitely emitted as weak (so duplicate tables in
multiple objects are merged into a single one).

llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp

@@ -333,6 +333,39 @@ void WebAssemblyAsmPrinter::emitDecls(const Module &M) {
       emitSymbolType(Sym);
   }
 
+  // We handle `__funcref_call_table` specially here.
+  //
+  // Unlike most table symbols, which are attached to a `GlobalVariable`
+  // this one is a directly created, freestanding MCSymbol, much like
+  // `__indirect_function_table`. However, given that the table is always
+  // identical (single element, default initialized), we declare it
+  // weak in each object, and define it here rather than in the linker.
+  //
+  // TODO: consider moving this definition elsewhere, or doing away with
+  // the table entirely (in favor of `call_ref` exclusively).
+  {
+    StringRef Name = "__funcref_call_table";
+    auto *Sym = static_cast<MCSymbolWasm *>(OutContext.lookupSymbol(Name));
+    if (Sym) {
+      if (!Sym->isFunctionTable())
+        OutContext.reportError(SMLoc(), "symbol is not a wasm funcref table");
+
+      // symbol is declared weak in `getOrCreateFuncrefCallTableSymbol`
+      assert(Sym->isWeak());
+      OutStreamer->emitSymbolAttribute(Sym, MCSA_Weak);
+
+      // Make sure we haven't already emitted it for whatever reason.
+      assert(!Sym->isDefined());
+
+      // Actually define the symbol.
+      // Confusingly enough, `emitLabel` is what "defines" a MCSymbol.
+      // Provides it a fragment, so that it `!isUndefined`
+      OutStreamer->emitLabel(Sym);
+      // No initializer needed. Default ref.null is good
+      OutStreamer->addBlankLine();
+    }
+  }
+
   DenseSet<MCSymbol *> InvokeSymbols;
   for (const auto &F : M) {
     if (F.isIntrinsic())

llvm/test/CodeGen/WebAssembly/funcref-call.ll

@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s --mtriple=wasm32-unknown-unknown -fast-isel=0 -mattr=+reference-types | FileCheck %s
+; RUN: llc < %s --mtriple=wasm32-unknown-unknown --filetype=obj
 
 ; Use -fast-isel-abort=3 (never fall back to SelectionDAG) to make sure that the
 ; funcref call is selected by FastISel