Remove support for runtime multi-threading.

This patch removes the functions llvm_start_multithreaded() and
llvm_stop_multithreaded(), and changes llvm_is_multithreaded()
to return a constant value based on the value of the compile-time
definition LLVM_ENABLE_THREADS.

Previously, it was possible to have compile-time support for
threads on, and runtime support for threads off, in which case
certain mutexes were not allocated or ever acquired.  Now, if the
build is created with threads enabled, mutexes are always acquired.

A test before/after patch of compiling a very large TU showed no
noticeable performance impact of this change.

Reviewers: rnk

Differential Revision: http://reviews.llvm.org/D4076

llvm-svn: 210600

llvm/lib/Support/ErrorHandling.cpp

@@ -23,6 +23,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <cstdlib>
+#include <mutex>
 
 #if defined(HAVE_UNISTD_H)
 # include <unistd.h>
@@ -37,16 +38,18 @@ using namespace llvm;
 static fatal_error_handler_t ErrorHandler = nullptr;
 static void *ErrorHandlerUserData = nullptr;
 
+static llvm::recursive_mutex ErrorHandlerMutex;
+
 void llvm::install_fatal_error_handler(fatal_error_handler_t handler,
                                        void *user_data) {
-  assert(!llvm_is_multithreaded() &&
-         "Cannot register error handlers after starting multithreaded mode!\n");
+  std::lock_guard<llvm::recursive_mutex> Lock(ErrorHandlerMutex);
   assert(!ErrorHandler && "Error handler already registered!\n");
   ErrorHandler = handler;
   ErrorHandlerUserData = user_data;
 }
 
 void llvm::remove_fatal_error_handler() {
+  std::lock_guard<llvm::recursive_mutex> Lock(ErrorHandlerMutex);
   ErrorHandler = nullptr;
 }
 
@@ -63,6 +66,7 @@ void llvm::report_fatal_error(StringRef Reason, bool GenCrashDiag) {
 }
 
 void llvm::report_fatal_error(const Twine &Reason, bool GenCrashDiag) {
+  std::lock_guard<llvm::recursive_mutex> Lock(ErrorHandlerMutex);
   if (ErrorHandler) {
     ErrorHandler(ErrorHandlerUserData, Reason.str(), GenCrashDiag);
   } else {