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

…isabled

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 emscripten-core/emscripten#16152

Differential Revision: https://reviews.llvm.org/D118571

clang/lib/Basic/Targets/WebAssembly.cpp

@@ -260,6 +260,7 @@ void WebAssemblyTargetInfo::adjust(DiagnosticsEngine &Diags,
   if (!HasAtomics) {
     Opts.POSIXThreads = false;
     Opts.setThreadModel(LangOptions::ThreadModelKind::Single);
+    Opts.ThreadsafeStatics = false;
   }
 }
 

clang/test/CodeGenCXX/static-init-wasm.cpp

@@ -1,6 +1,6 @@
-// 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.
@@ -52,3 +52,17 @@ A theA;
 // 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