[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, 37 insertions, 26 deletions

diff --git a/llvm/lib/Support/Unix/Threading.inc b/llvm/lib/Support/Unix/Threading.inc
index ed9a965..afb887f 100644
--- a/llvm/lib/Support/Unix/Threading.inc
+++ b/llvm/lib/Support/Unix/Threading.inc
@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "Unix.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/Twine.h"
 
@@ -40,47 +42,56 @@
 #include <unistd.h>      // For syscall()
 #endif
 
-namespace {
-  struct ThreadInfo {
-    void(*UserFn)(void *);
-    void *UserData;
-  };
+static void *threadFuncSync(void *Arg) {
+  SyncThreadInfo *TI = static_cast<SyncThreadInfo *>(Arg);
+  TI->UserFn(TI->UserData);
+  return nullptr;
 }
 
-static void *ExecuteOnThread_Dispatch(void *Arg) {
-  ThreadInfo *TI = reinterpret_cast<ThreadInfo*>(Arg);
-  TI->UserFn(TI->UserData);
+static void *threadFuncAsync(void *Arg) {
+  std::unique_ptr<AsyncThreadInfo> Info(static_cast<AsyncThreadInfo *>(Arg));
+  (*Info)();
   return nullptr;
 }
 
-void llvm::llvm_execute_on_thread(void(*Fn)(void*), void *UserData,
-  unsigned RequestedStackSize) {
-  ThreadInfo Info = { Fn, UserData };
-  pthread_attr_t Attr;
-  pthread_t Thread;
+static void
+llvm_execute_on_thread_impl(void *(*ThreadFunc)(void *), void *Arg,
+                            llvm::Optional<unsigned> StackSizeInBytes,
+                            JoiningPolicy JP) {
+  int errnum;
 
   // Construct the attributes object.
-  if (::pthread_attr_init(&Attr) != 0)
-    return;
+  pthread_attr_t Attr;
+  if ((errnum = ::pthread_attr_init(&Attr)) != 0) {
+    ReportErrnumFatal("pthread_attr_init failed", errnum);
+  }
+
+  auto AttrGuard = llvm::make_scope_exit([&] {
+    if ((errnum = ::pthread_attr_destroy(&Attr)) != 0) {
+      ReportErrnumFatal("pthread_attr_destroy failed", errnum);
+    }
+  });
 
   // Set the requested stack size, if given.
-  if (RequestedStackSize != 0) {
-    if (::pthread_attr_setstacksize(&Attr, RequestedStackSize) != 0)
-      goto error;
+  if (StackSizeInBytes) {
+    if ((errnum = ::pthread_attr_setstacksize(&Attr, *StackSizeInBytes)) != 0) {
+      ReportErrnumFatal("pthread_attr_setstacksize failed", errnum);
+    }
   }
 
   // Construct and execute the thread.
-  if (::pthread_create(&Thread, &Attr, ExecuteOnThread_Dispatch, &Info) != 0)
-    goto error;
-
-  // Wait for the thread and clean up.
-  ::pthread_join(Thread, nullptr);
+  pthread_t Thread;
+  if ((errnum = ::pthread_create(&Thread, &Attr, ThreadFunc, Arg)) != 0)
+    ReportErrnumFatal("pthread_create failed", errnum);
 
-error:
-  ::pthread_attr_destroy(&Attr);
+  if (JP == JoiningPolicy::Join) {
+    // Wait for the thread
+    if ((errnum = ::pthread_join(Thread, nullptr)) != 0) {
+      ReportErrnumFatal("pthread_join failed", errnum);
+    }
+  }
 }
 
-
 uint64_t llvm::get_threadid() {
 #if defined(__APPLE__)
   // Calling "mach_thread_self()" bumps the reference count on the thread