[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, 9 insertions, 0 deletions

diff --git a/llvm/lib/Support/Unix/Unix.h b/llvm/lib/Support/Unix/Unix.h
index 86309b0..1fc9a41 100644
--- a/llvm/lib/Support/Unix/Unix.h
+++ b/llvm/lib/Support/Unix/Unix.h
@@ -21,6 +21,7 @@
 #include "llvm/Config/config.h"
 #include "llvm/Support/Chrono.h"
 #include "llvm/Support/Errno.h"
+#include "llvm/Support/ErrorHandling.h"
 #include <algorithm>
 #include <assert.h>
 #include <cerrno>
@@ -69,6 +70,14 @@ static inline bool MakeErrMsg(
   return true;
 }
 
+// Include StrError(errnum) in a fatal error message.
+LLVM_ATTRIBUTE_NORETURN static inline void ReportErrnumFatal(const char *Msg,
+                                                             int errnum) {
+  std::string ErrMsg;
+  MakeErrMsg(&ErrMsg, Msg, errnum);
+  llvm::report_fatal_error(ErrMsg);
+}
+
 namespace llvm {
 namespace sys {