[Support] Add a way to run a function on a detached thread

This roughly mimics `std::thread(...).detach()` except it allows to
customize the stack size. Required for https://reviews.llvm.org/D50993.

I've decided against reusing the existing `llvm_execute_on_thread` because
it's not obvious what to do with the ownership of the passed
function/arguments:

1. If we pass possibly owning functions data to `llvm_execute_on_thread`,
   we'll lose the ability to pass small non-owning non-allocating functions
   for the joining case (as it's used now). Is it important enough?
2. If we use the non-owning interface in the new use case, we'll force
   clients to transfer ownership to the spawned thread manually, but
   similar code would still have to exist inside
   `llvm_execute_on_thread(_async)` anyway (as we can't just pass the same
   non-owning pointer to pthreads and Windows implementations, and would be
   forced to wrap it in some structure, and deal with its ownership.

Patch by Dmitry Kozhevnikov!

Differential Revision: https://reviews.llvm.org/D51103

1 files changed, 49 insertions, 2 deletions

diff --git a/llvm/lib/Support/Threading.cpp b/llvm/lib/Support/Threading.cpp
index e5899a60f4d..48750cef5ec 100644
--- a/llvm/lib/Support/Threading.cpp
+++ b/llvm/lib/Support/Threading.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/Threading.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/Host.h"
 
@@ -39,8 +40,8 @@ bool llvm::llvm_is_multithreaded() {
     (!defined(_WIN32) && !defined(HAVE_PTHREAD_H))
 // Support for non-Win32, non-pthread implementation.
 void llvm::llvm_execute_on_thread(void (*Fn)(void *), void *UserData,
-                                  unsigned RequestedStackSize) {
-  (void)RequestedStackSize;
+                                  llvm::Optional<unsigned> StackSizeInBytes) {
+  (void)StackSizeInBytes;
   Fn(UserData);
 }
 
@@ -56,6 +57,25 @@ void llvm::set_thread_name(const Twine &Name) {}
 
 void llvm::get_thread_name(SmallVectorImpl<char> &Name) { Name.clear(); }
 
+#if LLVM_ENABLE_THREADS == 0
+void llvm::llvm_execute_on_thread_async(
+    llvm::unique_function<void()> Func,
+    llvm::Optional<unsigned> StackSizeInBytes) {
+  (void)Func;
+  (void)StackSizeInBytes;
+  report_fatal_error("Spawning a detached thread doesn't make sense with no "
+                     "threading support");
+}
+#else
+// Support for non-Win32, non-pthread implementation.
+void llvm::llvm_execute_on_thread_async(
+    llvm::unique_function<void()> Func,
+    llvm::Optional<unsigned> StackSizeInBytes) {
+  (void)StackSizeInBytes;
+  std::thread(std::move(Func)).detach();
+}
+#endif
+
 #else
 
 #include <thread>
@@ -84,6 +104,17 @@ unsigned llvm::hardware_concurrency() {
   return 1;
 }
 
+namespace {
+struct SyncThreadInfo {
+  void (*UserFn)(void *);
+  void *UserData;
+};
+
+using AsyncThreadInfo = llvm::unique_function<void()>;
+
+enum class JoiningPolicy { Join, Detach };
+} // namespace
+
 // Include the platform-specific parts of this class.
 #ifdef LLVM_ON_UNIX
 #include "Unix/Threading.inc"
@@ -92,4 +123,20 @@ unsigned llvm::hardware_concurrency() {
 #include "Windows/Threading.inc"
 #endif
 
+void llvm::llvm_execute_on_thread(void (*Fn)(void *), void *UserData,
+                                  llvm::Optional<unsigned> StackSizeInBytes) {
+
+  SyncThreadInfo Info = {Fn, UserData};
+  llvm_execute_on_thread_impl(threadFuncSync, &Info, StackSizeInBytes,
+                              JoiningPolicy::Join);
+}
+
+void llvm::llvm_execute_on_thread_async(
+    llvm::unique_function<void()> Func,
+    llvm::Optional<unsigned> StackSizeInBytes) {
+  llvm_execute_on_thread_impl(&threadFuncAsync,
+                              new AsyncThreadInfo(std::move(Func)),
+                              StackSizeInBytes, JoiningPolicy::Detach);
+}
+
 #endif