[swiftc] Fixed for Cygwin

Fixed for the difference of Cygwin with other Windows variants (MSVC,
Itanium, MinGW).

- The platform name is renamed to "cygwin" from "windows" which is used
  for searching the standard libraries.

- The consideration for DLL storage class (DllExport/DllImport) is not
  required for Cygwin and MinGW. There is no problem when linking in
  these environment.

- Cygwin should use large memory model as default.(This may be changed
  if someone ports to 32bit)

- Cygwin and MinGW should use the autolink feature in the sameway of
  Linux due to the linker's limit.

lib/IRGen/IRGenModule.cpp

@@ -117,6 +117,11 @@ static clang::CodeGenerator *createClangCodeGenerator(ASTContext &Context,
   return ClangCodeGen;
 }
 
+/// A helper for determining if the triple uses the DLL storage
+static bool useDllStorage(const llvm::Triple &Triple) {
+  return Triple.isOSBinFormatCOFF() && !Triple.isOSCygMing();
+}
+
 IRGenModule::IRGenModule(IRGenerator &irgen,
                          std::unique_ptr<llvm::TargetMachine> &&target,
                          SourceFile *SF, llvm::LLVMContext &LLVMContext,
@@ -439,7 +444,7 @@ llvm::Constant *swift::getRuntimeFn(llvm::Module &Module,
   if (auto fn = dyn_cast<llvm::Function>(cache)) {
     fn->setCallingConv(cc);
 
-    if (llvm::Triple(Module.getTargetTriple()).isOSBinFormatCOFF() &&
+    if (::useDllStorage(llvm::Triple(Module.getTargetTriple())) &&
         (fn->getLinkage() == llvm::GlobalValue::ExternalLinkage ||
          fn->getLinkage() == llvm::GlobalValue::AvailableExternallyLinkage))
       fn->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
@@ -519,7 +524,7 @@ llvm::Constant *swift::getWrapperFn(llvm::Module &Module,
     auto *globalFnPtr = new llvm::GlobalVariable(
         Module, fnPtrTy, false, llvm::GlobalValue::ExternalLinkage, nullptr,
         symbol);
-    if (llvm::Triple(Module.getTargetTriple()).isOSBinFormatCOFF())
+    if (::useDllStorage(llvm::Triple(Module.getTargetTriple())))
       globalFnPtr->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
 
     // Forward all arguments.
@@ -638,7 +643,7 @@ llvm::Constant *IRGenModule::getEmptyTupleMetadata() {
   EmptyTupleMetadata = Module.getOrInsertGlobal(
                           MANGLE_AS_STRING(METADATA_SYM(EMPTY_TUPLE_MANGLING)),
                           FullTypeMetadataStructTy);
-  if (Triple.isOSBinFormatCOFF())
+  if (useDllStorage())
     cast<llvm::GlobalVariable>(EmptyTupleMetadata)
         ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
   return EmptyTupleMetadata;
@@ -652,7 +657,7 @@ llvm::Constant *IRGenModule::getObjCEmptyCachePtr() {
     // struct objc_cache _objc_empty_cache;
     ObjCEmptyCachePtr = Module.getOrInsertGlobal("_objc_empty_cache",
                                                  OpaquePtrTy->getElementType());
-    if (Triple.isOSBinFormatCOFF())
+    if (useDllStorage())
       cast<llvm::GlobalVariable>(ObjCEmptyCachePtr)
           ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
   } else {
@@ -685,7 +690,7 @@ Address IRGenModule::getAddrOfObjCISAMask() {
   assert(TargetInfo.hasISAMasking());
   if (!ObjCISAMaskPtr) {
     ObjCISAMaskPtr = Module.getOrInsertGlobal("swift_isaMask", IntPtrTy);
-    if (Triple.isOSBinFormatCOFF())
+    if (useDllStorage())
       cast<llvm::GlobalVariable>(ObjCISAMaskPtr)
           ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
   }
@@ -851,7 +856,7 @@ void IRGenModule::addLinkLibrary(const LinkLibrary &linkLib) {
     llvm::SmallString<64> buf;
     encodeForceLoadSymbolName(buf, linkLib.getName());
     auto symbolAddr = Module.getOrInsertGlobal(buf.str(), Int1Ty);
-    if (Triple.isOSBinFormatCOFF())
+    if (useDllStorage())
       cast<llvm::GlobalVariable>(symbolAddr)
           ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
 
@@ -916,9 +921,9 @@ void IRGenModule::emitAutolinkInfo() {
                                        }),
                         AutolinkEntries.end());
 
-  if (TargetInfo.OutputObjectFormat == llvm::Triple::COFF ||
-      TargetInfo.OutputObjectFormat == llvm::Triple::MachO ||
-      Triple.isPS4()) {
+  if ((TargetInfo.OutputObjectFormat == llvm::Triple::COFF &&
+       !Triple.isOSCygMing()) ||
+      TargetInfo.OutputObjectFormat == llvm::Triple::MachO || Triple.isPS4()) {
     llvm::LLVMContext &ctx = Module.getContext();
 
     if (!LinkerOptions) {
@@ -935,8 +940,9 @@ void IRGenModule::emitAutolinkInfo() {
       assert(FoundOldEntry && "Could not replace old linker options entry?");
     }
   } else {
-    assert(TargetInfo.OutputObjectFormat == llvm::Triple::ELF &&
-           "expected ELF output format");
+    assert((TargetInfo.OutputObjectFormat == llvm::Triple::ELF ||
+            Triple.isOSCygMing()) &&
+           "expected ELF output format or COFF format for Cygwin/MinGW");
 
     // Merge the entries into null-separated string.
     llvm::SmallString<64> EntriesString;
@@ -969,7 +975,7 @@ void IRGenModule::emitAutolinkInfo() {
                                  llvm::GlobalValue::CommonLinkage,
                                  llvm::Constant::getNullValue(Int1Ty),
                                  buf.str());
-    if (Triple.isOSBinFormatCOFF())
+    if (useDllStorage())
       symbol->setDLLStorageClass(llvm::GlobalValue::DLLExportStorageClass);
   }
 }
@@ -1072,6 +1078,8 @@ void IRGenModule::error(SourceLoc loc, const Twine &message) {
                          message.toStringRef(buffer));
 }
 
+bool IRGenModule::useDllStorage() { return ::useDllStorage(Triple); }
+
 void IRGenerator::addGenModule(SourceFile *SF, IRGenModule *IGM) {
   assert(GenModules.count(SF) == 0);
   GenModules[SF] = IGM;