[WebAssemby] Enable "-mthread-model single" by default, for now.

The WebAssembly standard does not yet have threads, and while it's in the
process of being standardized, it'll take some time for it to make it
through and be available in all popular implementations. With increasing
numbers of people using the LLVM wasm backend through LLVM directly rather
than through Emscripten, it's increasingly important to have friendly
defaults.

See also https://bugs.llvm.org/show_bug.cgi?id=35411


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319101 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 9 insertions, 0 deletions

diff --git a/lib/Driver/ToolChains/WebAssembly.cpp b/lib/Driver/ToolChains/WebAssembly.cpp
index 1464c150c9..d00d833fbb 100644
--- a/lib/Driver/ToolChains/WebAssembly.cpp
+++ b/lib/Driver/ToolChains/WebAssembly.cpp
@@ -135,6 +135,14 @@ void WebAssembly::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                      getDriver().SysRoot + "/include/c++/v1");
 }
 
+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.
+  return "single";
+}
+
 Tool *WebAssembly::buildLinker() const {
   return new tools::wasm::Linker(*this);
 }
diff --git a/lib/Driver/ToolChains/WebAssembly.h b/lib/Driver/ToolChains/WebAssembly.h
index 7524361d51..8784e12dfb 100644
--- a/lib/Driver/ToolChains/WebAssembly.h
+++ b/lib/Driver/ToolChains/WebAssembly.h
@@ -62,6 +62,7 @@ private:
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
+  std::string getThreadModel() const override;
 
   const char *getDefaultLinker() const override {
     return "lld";