[WebAssembly] Use single-threaded mode when -matomics isn't enabled.

[sunfish]

Summary

When the -matomics feature is not enabled, disable POSIXThreads
mode and set the thread model to Single, so that we don't predefine
macros like __STDCPP_THREADS__.

This will help save code size (a thing I care about!) in programs which
use __STDCPP_THREADS__ to determine whether to include support
for threads.

[tlively]

LGTM, thanks!

[sbc100]

clang/lib/Basic/Targets/WebAssembly.cpp

260

POSIXThreads already defaults to false I think. It gets set if the user specified -pthread on the command line I think. I think there should be no need to override it.

clang/lib/Basic/Targets/WebAssembly.h

CallingConvCheckResult checkCallingConvention(CallingConv CC) const override {
     default:
       return CCCR_Warning;
     }
   }
 
   bool hasExtIntType() const override { return true; }
 
   bool hasProtectedVisibility() const override { return false; }
+
+  void adjust(LangOptions &Opts) override;
 };
 
 class LLVM_LIBRARY_VISIBILITY WebAssembly32TargetInfo
     : public WebAssemblyTargetInfo {
 public:
   explicit WebAssembly32TargetInfo(const llvm::Triple &T,
                                    const TargetOptions &Opts)
       : WebAssemblyTargetInfo(T, Opts) {

clang/lib/Basic/Targets/WebAssembly.cpp

bool WebAssemblyTargetInfo::handleTargetFeatures(
   return true;
 }
 
 ArrayRef<Builtin::Info> WebAssemblyTargetInfo::getTargetBuiltins() const {
   return llvm::makeArrayRef(BuiltinInfo, clang::WebAssembly::LastTSBuiltin -
                                              Builtin::FirstTSBuiltin);
 }
 
+void WebAssemblyTargetInfo::adjust(LangOptions &Opts) {
+  // If the Atomics feature isn't available, turn off POSIXThreads and
+  // ThreadModel, so that we don't predefine _REENTRANT or __STDCPP_THREADS__.
+  if (!HasAtomics) {
+    Opts.POSIXThreads = false;
+    Opts.setThreadModel(LangOptions::ThreadModelKind::Single);
+  }
+}
+
 void WebAssembly32TargetInfo::getTargetDefines(const LangOptions &Opts,
                                                MacroBuilder &Builder) const {
   WebAssemblyTargetInfo::getTargetDefines(Opts, Builder);
   defineCPUMacros(Builder, "wasm32", /*Tuning=*/false);
 }
 
 void WebAssembly64TargetInfo::getTargetDefines(const LangOptions &Opts,
                                                MacroBuilder &Builder) const {
   WebAssemblyTargetInfo::getTargetDefines(Opts, Builder);
   defineCPUMacros(Builder, "wasm64", /*Tuning=*/false);
 }

clang/test/Preprocessor/init.c

 // RUN:   < /dev/null \
 // RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY,WEBASSEMBLY64 %s
 // RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=wasm32-wasi \
 // RUN:   < /dev/null \
 // RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY,WEBASSEMBLY32,WEBASSEMBLY-WASI %s
 // RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=wasm64-wasi \
 // RUN:   < /dev/null \
 // RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY,WEBASSEMBLY64,WEBASSEMBLY-WASI %s
+// RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=wasm32-unknown-unknown -x c++ \
+// RUN:   < /dev/null \
+// RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY-CXX %s
+// RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=wasm32-unknown-unknown -x c++ -pthread -target-feature +atomics \
+// RUN:   < /dev/null \
+// RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY-CXX-ATOMICS %s
 //
 // WEBASSEMBLY32:#define _ILP32 1
 // WEBASSEMBLY32-NOT:#define _LP64
 // WEBASSEMBLY64-NOT:#define _ILP32
 // WEBASSEMBLY64:#define _LP64 1
 // WEBASSEMBLY-NEXT:#define __ATOMIC_ACQUIRE 2
 // WEBASSEMBLY-NEXT:#define __ATOMIC_ACQ_REL 4
 // WEBASSEMBLY-NEXT:#define __ATOMIC_CONSUME 1

 // WEBASSEMBLY64-NOT:#define __wasm32
 // WEBASSEMBLY32-NEXT:#define __wasm32__ 1
 // WEBASSEMBLY64-NOT:#define __wasm32__
 // WEBASSEMBLY32-NOT:#define __wasm64__
 // WEBASSEMBLY32-NOT:#define __wasm64
 // WEBASSEMBLY64-NEXT:#define __wasm64 1
 // WEBASSEMBLY64-NEXT:#define __wasm64__ 1
 // WEBASSEMBLY-NEXT:#define __wasm__ 1
+// WEBASSEMBLY-CXX-NOT:_REENTRANT
+// WEBASSEMBLY-CXX-NOT:__STDCPP_THREADS__
+// WEBASSEMBLY-CXX-ATOMICS:#define _REENTRANT 1
+// WEBASSEMBLY-CXX-ATOMICS:#define __STDCPP_THREADS__ 1
 
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple i686-windows-cygnus < /dev/null | FileCheck -match-full-lines -check-prefix CYGWIN-X32 %s
 // CYGWIN-X32: #define __USER_LABEL_PREFIX__ _
 
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple x86_64-windows-cygnus < /dev/null | FileCheck -match-full-lines -check-prefix CYGWIN-X64 %s
 // CYGWIN-X64: #define __USER_LABEL_PREFIX__
 
 // RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=avr \