Index: include/clang/Driver/ToolChain.h
===================================================================
--- include/clang/Driver/ToolChain.h
+++ include/clang/Driver/ToolChain.h
@@ -453,7 +453,9 @@
   virtual bool SupportsEmbeddedBitcode() const { return false; }
 
   /// getThreadModel() - Which thread model does this target use?
-  virtual std::string getThreadModel() const { return "posix"; }
+  virtual std::string getThreadModel(const llvm::opt::ArgList &) const {
+    return "posix";
+  }
 
   /// isThreadModelSupported() - Does this target support a thread model?
   virtual bool isThreadModelSupported(const StringRef Model) const;
Index: lib/Driver/Driver.cpp
===================================================================
--- lib/Driver/Driver.cpp
+++ lib/Driver/Driver.cpp
@@ -1528,7 +1528,7 @@
     if (TC.isThreadModelSupported(A->getValue()))
       OS << "Thread model: " << A->getValue();
   } else
-    OS << "Thread model: " << TC.getThreadModel();
+    OS << "Thread model: " << TC.getThreadModel(C.getArgs());
   OS << '\n';
 
   // Print out the install directory.
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -3823,7 +3823,7 @@
     CmdArgs.push_back(A->getValue());
   }
   else
-    CmdArgs.push_back(Args.MakeArgString(TC.getThreadModel()));
+    CmdArgs.push_back(Args.MakeArgString(TC.getThreadModel(Args)));
 
   Args.AddLastArg(CmdArgs, options::OPT_fveclib);
 
Index: lib/Driver/ToolChains/WebAssembly.h
===================================================================
--- lib/Driver/ToolChains/WebAssembly.h
+++ lib/Driver/ToolChains/WebAssembly.h
@@ -64,7 +64,7 @@
       llvm::opt::ArgStringList &CC1Args) const override;
   void AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
                            llvm::opt::ArgStringList &CmdArgs) const override;
-  std::string getThreadModel() const override;
+  std::string getThreadModel(const llvm::opt::ArgList &) const override;
 
   const char *getDefaultLinker() const override { return "wasm-ld"; }
 
Index: lib/Driver/ToolChains/WebAssembly.cpp
===================================================================
--- lib/Driver/ToolChains/WebAssembly.cpp
+++ lib/Driver/ToolChains/WebAssembly.cpp
@@ -62,7 +62,9 @@
     if (ToolChain.ShouldLinkCXXStdlib(Args))
       ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
 
-    if (Args.hasArg(options::OPT_pthread))
+    if (Args.hasFlag(clang::driver::options::OPT_pthread,
+                     clang::driver::options::OPT_no_pthread),
+        false)
       CmdArgs.push_back("-lpthread");
 
     CmdArgs.push_back("-lc");
@@ -123,6 +125,13 @@
   if (DriverArgs.hasFlag(clang::driver::options::OPT_fuse_init_array,
                          options::OPT_fno_use_init_array, true))
     CC1Args.push_back("-fuse-init-array");
+  bool HasAtomics =
+      DriverArgs.hasFlag(clang::driver::options::OPT_matomics,
+                         clang::driver::options::OPT_mno_atomics, false);
+  bool HasPthread = DriverArgs.hasFlag(clang::driver::options::OPT_pthread,
+                                       clang::driver::options::OPT_no_pthread);
+  if (HasAtomics || HasPthread)
+    CC1Args.push_back("-target-feature +atomics");
 }
 
 ToolChain::RuntimeLibType WebAssembly::GetDefaultRuntimeLibType() const {
@@ -180,11 +189,16 @@
   }
 }
 
-std::string WebAssembly::getThreadModel() const {
-  // The WebAssembly MVP does not yet support threads; for now, use the
-  // "single" threading model, which lowers atomics to non-atomic operations.
-  // When threading support is standardized and implemented in popular engines,
-  // this override should be removed.
+std::string WebAssembly::getThreadModel(const ArgList &DriverArgs) const {
+  // The WebAssembly MVP does not yet support threads. We set this to "posix"
+  // only when we have atomics target feature on or '-pthread' is set.
+  bool HasAtomics =
+      DriverArgs.hasFlag(clang::driver::options::OPT_matomics,
+                         clang::driver::options::OPT_mno_atomics, false);
+  bool HasPthread = DriverArgs.hasFlag(clang::driver::options::OPT_pthread,
+                                       clang::driver::options::OPT_no_pthread);
+  if (HasAtomics || HasPthread)
+    return "posix";
   return "single";
 }
 
Index: test/Driver/wasm-toolchain.c
===================================================================
--- test/Driver/wasm-toolchain.c
+++ test/Driver/wasm-toolchain.c
@@ -38,3 +38,17 @@
 
 // RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-wasi-musl --sysroot=/foo %s 2>&1 | FileCheck -check-prefix=COMPILE %s
 // COMPILE: clang{{.*}}" "-cc1" {{.*}} "-internal-isystem" "/foo/include/wasm32-wasi-musl" "-internal-isystem" "/foo/include"
+
+// Thread-related command line tests.
+// - '-matomics' sets '-mthread-model posix'
+// - '-mthread-model' sets '-matomics'
+// - '-phread' sets both '-matomics' and '-mthread-model posix'
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 -matomics | FileCheck -check-prefix=ATOMICS %s
+// ATOMICS: clang{{.*}}" "-cc1" {{.*}} "-mthread-model" "posix"
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 -mthread-model posix | FileCheck -check-prefix=POSIX %s
+// POSIX: clang{{.*}}" "-cc1" {{.*}} "-target-feature +atomics"
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 -pthread | FileCheck -check-prefix=PTHREAD %s
+// PTHREAD: clang{{.*}}" "-cc1" {{.*}} "-mthread-model" "posix" {{.*}} "-target-feature +atomics"
Index: test/Driver/wasm-toolchain.cpp
===================================================================
--- test/Driver/wasm-toolchain.cpp
+++ test/Driver/wasm-toolchain.cpp
@@ -38,3 +38,17 @@
 
 // RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-wasi-musl --sysroot=/foo --stdlib=c++ %s 2>&1 | FileCheck -check-prefix=COMPILE %s
 // COMPILE: clang{{.*}}" "-cc1" {{.*}} "-internal-isystem" "/foo/include/wasm32-wasi-musl/c++/v1" "-internal-isystem" "/foo/include/c++/v1" "-internal-isystem" "/foo/include/wasm32-wasi-musl" "-internal-isystem" "/foo/include"
+
+// Thread-related command line tests.
+// - '-matomics' sets '-mthread-model posix'
+// - '-mthread-model' sets '-matomics'
+// - '-phread' sets both '-matomics' and '-mthread-model posix'
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 -matomics | FileCheck -check-prefix=ATOMICS %s
+// ATOMICS: clang{{.*}}" "-cc1" {{.*}} "-mthread-model" "posix"
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 -mthread-model posix | FileCheck -check-prefix=POSIX %s
+// POSIX: clang{{.*}}" "-cc1" {{.*}} "-target-feature +atomics"
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 -pthread | FileCheck -check-prefix=PTHREAD %s
+// PTHREAD: clang{{.*}}" "-cc1" {{.*}} "-mthread-model" "posix" {{.*}} "-target-feature +atomics"