[WebAssembly] Explicily disable comdat support for wasm output

[sbc100]

Summary

Disably comdat support in the same way we do the Mach-O. This
also causes clang not to generated comdats.

The hope is that we can support C++ without full comdat
support in the same that Mach-O is able to do this.

[dschuff]

I'm fine with making this explicit for now. I'm not sure I'd yet go so far as to say that the goal is not to support comdats. We should definitely investigate how C++ and its debug info are supported in mach-o, but it may be that comdats are better than whatever they do. In general in the absence of good reasons (e.g. our 2-level namespace) I'd probably have a slight bias toward doing things the ELF way rather than the macho way; most people I've talked to who've worked a lot with macho don't speak too highly of it :)

[sunfish]

If this is a temporary workaround, it's fine though we should add comments saying that. If this is meant as a permanent change, I don't currently know enough about MachO to know what to think about it.

include/llvm/ADT/Triple.h

public:
   }
 
   /// Tests whether the target is AArch64 (little and big endian).
   bool isAArch64() const {
     return getArch() == Triple::aarch64 || getArch() == Triple::aarch64_be;
   }
 
   /// Tests wether the target supports comdat
-  bool supportsCOMDAT() const { return !isOSBinFormatMachO(); }
+  bool supportsCOMDAT() const {
+    return !isOSBinFormatMachO() && !isOSBinFormatWasm();
+  }
 
   /// @}
   /// @name Mutators
   /// @{
 
   /// setArch - Set the architecture (first) component of the triple
   /// to a known type.
   void setArch(ArchType Kind);

lib/Bitcode/Reader/BitcodeReader.cpp

Expected<Value *> BitcodeReader::recordValue(SmallVectorImpl<uint64_t> &Record,
 
   StringRef NameStr(ValueName.data(), ValueName.size());
   if (NameStr.find_first_of(0) != StringRef::npos)
     return error("Invalid value name");
   V->setName(NameStr);
   auto *GO = dyn_cast<GlobalObject>(V);
   if (GO) {
     if (GO->getComdat() == reinterpret_cast<Comdat *>(1)) {
-      if (TT.isOSBinFormatMachO())
-        GO->setComdat(nullptr);
-      else
+      if (TT.supportsCOMDAT())
         GO->setComdat(TheModule->getOrInsertComdat(V->getName()));
+      else
+        GO->setComdat(nullptr);
     }
   }
   return V;
 }
 
 /// Helper to note and return the current location, and jump to the given
 /// offset.
 static uint64_t jumpToValueSymbolTable(uint64_t Offset,

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

void TargetLoweringObjectFileCOFF::emitLinkerFlagsForGlobal(
     raw_ostream &OS, const GlobalValue *GV) const {
   emitLinkerFlagsForGlobalCOFF(OS, GV, getTargetTriple(), getMangler());
 }
 
 //===----------------------------------------------------------------------===//
 //                                  Wasm
 //===----------------------------------------------------------------------===//
 
-static const Comdat *getWasmComdat(const GlobalValue *GV) {
+static void checkWasmComdat(const GlobalValue *GV) {
   const Comdat *C = GV->getComdat();
   if (!C)
-    return nullptr;
-
-  if (C->getSelectionKind() != Comdat::Any)
-    report_fatal_error("Wasm COMDATs only support SelectionKind::Any, '" +
-                       C->getName() + "' cannot be lowered.");
+    return;
 
-  return C;
+  report_fatal_error("wasm doesn't support COMDATs, '" + C->getName() +
+                     "' cannot be lowered.");
 }
 
 MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   StringRef Name = GO->getSection();
+  checkWasmComdat(GO);
   return getContext().getWasmSection(Name, SectionKind::getData());
 }
 
 static MCSectionWasm *selectWasmSectionForGlobal(
     MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
     const TargetMachine &TM, bool EmitUniqueSection, unsigned *NextUniqueID) {
   StringRef Group = "";
-  if (getWasmComdat(GO))
-    llvm_unreachable("comdat not yet supported for wasm");
+  checkWasmComdat(GO);
 
   bool UniqueSectionNames = TM.getUniqueSectionNames();
   SmallString<128> Name = getSectionPrefixForGlobal(Kind);
 
   if (const auto *F = dyn_cast<Function>(GO)) {
     const auto &OptionalPrefix = F->getSectionPrefix();
     if (OptionalPrefix)
       Name += *OptionalPrefix;

test/CodeGen/WebAssembly/comdat.ll

+; RUN: not llc -mtriple wasm32-unknown-unknown-wasm %s 2>&1 | FileCheck %s
+
+$f = comdat any
+@f = global i32 0, comdat
+; CHECK: LLVM ERROR: wasm doesn't support COMDATs, 'f' cannot be lowered.