[clang][WebAssembly] Imply -fno-threadsafe-static when threading is disabled

[sbc100]

Summary

When we don't enable atomics we completely disabled threading in
which case there is no point in generating thread safe code for
static initialization.

This should always be safe because, in WebAssembly, it is not
possible to link object compiled without the atomics feature into a
mutli-threaded program.

See https://github.com/emscripten-core/emscripten/pull/16152

[sbc100]

• revert unrelated line

clang/lib/Basic/Targets/WebAssembly.cpp

 
 void WebAssemblyTargetInfo::adjust(DiagnosticsEngine &Diags,
                                    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);
+    Opts.ThreadsafeStatics = false;
   }
 }
 
 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/CodeGenCXX/static-init-wasm.cpp

-// RUN: %clang_cc1 -emit-llvm -triple=wasm32-unknown-unknown -o - %s \
+// RUN: %clang_cc1 -emit-llvm -triple=wasm32-unknown-unknown -target-feature +atomics -o - %s \
 // RUN:   | FileCheck %s -check-prefix=WEBASSEMBLY32
-// RUN: %clang_cc1 -emit-llvm -triple=wasm64-unknown-unknown -o - %s \
+// RUN: %clang_cc1 -emit-llvm -triple=wasm64-unknown-unknown -target-feature +atomics -o - %s \
 // RUN:   | FileCheck %s -check-prefix=WEBASSEMBLY64
 
 // Test that we don't create common blocks.
 int tentative;
 // WEBASSEMBLY32: @tentative = global i32 0, align 4
 // WEBASSEMBLY64: @tentative = global i32 0, align 4
 
 // Test that WebAssembly uses the ARM-style ABI in which the static

 // WEBASSEMBLY32: call noundef %struct.A* @_ZN1AC1Ev(%struct.A* {{[^,]*}} @theA)
 // WEBASSEMBLY32: define internal void @_GLOBAL__sub_I_static_init_wasm.cpp() #3 {
 // WEBASSEMBLY32: call void @__cxx_global_var_init()
 //
 // WEBASSEMBLY64: define internal void @__cxx_global_var_init() #3 {
 // WEBASSEMBLY64: call noundef %struct.A* @_ZN1AC1Ev(%struct.A* {{[^,]*}} @theA)
 // WEBASSEMBLY64: define internal void @_GLOBAL__sub_I_static_init_wasm.cpp() #3 {
 // WEBASSEMBLY64: call void @__cxx_global_var_init()
+
+// RUN: %clang_cc1 -emit-llvm -triple=wasm32-unknown-unknown -o - %s \
+// RUN:   | FileCheck %s -check-prefix=NOATOMICS
+// RUN: %clang_cc1 -emit-llvm -triple=wasm64-unknown-unknown -o - %s \
+// RUN:   | FileCheck %s -check-prefix=NOATOMICS
+
+// NOATOMICS-LABEL: @_Z1gv()
+// NOATOMICS:       %[[R0:.+]] = load i8, i8* @_ZGVZ1gvE1a, align 1
+// NOATOMICS-NEXT:  %guard.uninitialized = icmp eq i8 %[[R0]], 0
+// NOATOMICS-NEXT:  br i1 %guard.uninitialized, label %[[CHECK:.+]], label %[[END:.+]],
+// NOATOMICS:       [[CHECK]]:
+// NOATOMICS-NOT:   __cxa_guard_acquire
+// NOATOMICS:       [[END]]:
+// NOATOMICS-NEXT:  ret void