[clangd] Use pthread instead of thread_local to support more runtimes.

Summary:
thread_local has nice syntax and semantics, but requires __cxa_thread_atexit,
and some not-ancient runtime libraries don't provide it.
The clang-x86_64-linux-selfhost-modules buildbot is one example :-)

It works on windows, and the other platforms clang-tools-extra supports should
all have the relevant pthread API. So we just use that if it's available,
falling back to thread_local (so if a platform has neither, we'll fail to link).

The fallback should really be the other way, that would require cmake changes.

Reviewers: ilya-biryukov, bkramer

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D42742

llvm-svn: 323949

clang-tools-extra/clangd/Context.cpp

@@ -8,8 +8,48 @@
 //===---------------------------------------------------------------------===//
 
 #include "Context.h"
+#include "llvm/Config/config.h"
 #include <cassert>
 
+// The thread-local Context is scoped in a function to avoid init-order issues.
+// It's created by currentContext() when first needed.
+
+#ifdef HAVE_PTHREAD_GETSPECIFIC
+// We'd love to use thread_local everywhere.
+// It requires support from the runtime: __cxa_thread_atexit.
+// Rather than detect this, we use the pthread API where available.
+#include <pthread.h>
+static clang::clangd::Context &currentContext() {
+  using clang::clangd::Context;
+  static pthread_key_t CtxKey;
+
+  // Once (across threads), set up pthread TLS for Context, and its destructor.
+  static int Dummy = [] { // Create key only once, for all threads.
+    if (auto Err = pthread_key_create(&CtxKey, /*destructor=*/+[](void *Ctx) {
+          delete reinterpret_cast<Context *>(Ctx);
+        }))
+      llvm_unreachable(strerror(Err));
+    return 0;
+  }();
+  (void)Dummy;
+
+  // Now grab the current context from TLS, and create it if it doesn't exist.
+  void *Ctx = pthread_getspecific(CtxKey);
+  if (!Ctx) {
+    Ctx = new Context();
+    if (auto Err = pthread_setspecific(CtxKey, Ctx))
+      llvm_unreachable(strerror(Err));
+  }
+  return *reinterpret_cast<Context *>(Ctx);
+}
+#else
+// Only supported platform without pthread is windows, and thread_local works.
+static clang::clangd::Context &currentContext() {
+  static thread_local auto C = clang::clangd::Context::empty();
+  return C;
+}
+#endif
+
 namespace clang {
 namespace clangd {
 
@@ -20,13 +60,6 @@ Context::Context(std::shared_ptr<const Data> DataPtr)
 
 Context Context::clone() const { return Context(DataPtr); }
 
-// The thread-local Context is scoped in a function to avoid
-// initialization-order issues. It's created when first needed.
-static Context &currentContext() {
-  static thread_local Context C = Context::empty();
-  return C;
-}
-
 const Context &Context::current() { return currentContext(); }
 
 Context Context::swapCurrent(Context Replacement) {