Don't compile usage of std::thread

As of the time of this writing it's not actually used anywhere meaningfullly
throughout the LLVM repo that we need, and it unfortunately uses `std::thread`
which isn't available in mingw-w64 toolchains with the win32 threading model
(the one that we use).

The change made to achive this was to just always use the single-threaded
support in `include/llvm/Support/thread.h`, and hopefuly that'll be enough...

For reference, the upstream LLVM bug has been reported [1]

[1]: https://llvm.org/bugs/show_bug.cgi?id=26365

include/llvm/ExecutionEngine/Orc/RPCChannel.h

@@ -40,42 +40,29 @@ class RPCChannel {
 
   /// Flush the stream if possible.
   virtual Error send() = 0;
-
-  /// Get the lock for stream reading.
-  std::mutex &getReadLock() { return readLock; }
-
-  /// Get the lock for stream writing.
-  std::mutex &getWriteLock() { return writeLock; }
-
-private:
-  std::mutex readLock, writeLock;
 };
 
 /// Notify the channel that we're starting a message send.
 /// Locks the channel for writing.
 inline Error startSendMessage(RPCChannel &C) {
-  C.getWriteLock().lock();
   return Error::success();
 }
 
 /// Notify the channel that we're ending a message send.
 /// Unlocks the channel for writing.
 inline Error endSendMessage(RPCChannel &C) {
-  C.getWriteLock().unlock();
   return Error::success();
 }
 
 /// Notify the channel that we're starting a message receive.
 /// Locks the channel for reading.
 inline Error startReceiveMessage(RPCChannel &C) {
-  C.getReadLock().lock();
   return Error::success();
 }
 
 /// Notify the channel that we're ending a message receive.
 /// Unlocks the channel for reading.
 inline Error endReceiveMessage(RPCChannel &C) {
-  C.getReadLock().unlock();
   return Error::success();
 }
 

include/llvm/ExecutionEngine/Orc/RPCUtils.h

@@ -102,6 +102,7 @@ class RPCBase {
 
     template <typename ChannelT>
     static Error readResult(ChannelT &C, std::promise<OptionalReturn> &P) {
+#if 0
       RetT Val;
       auto Err = deserialize(C, Val);
       auto Err2 = endReceiveMessage(C);
@@ -112,11 +113,14 @@ class RPCBase {
         return Err;
       }
       P.set_value(std::move(Val));
+#endif
       return Error::success();
     }
 
     static void abandon(std::promise<OptionalReturn> &P) {
+#if 0
       P.set_value(OptionalReturn());
+#endif
     }
 
     template <typename ChannelT, typename SequenceNumberT>
@@ -159,11 +163,17 @@ class RPCBase {
     template <typename ChannelT>
     static Error readResult(ChannelT &C, std::promise<OptionalReturn> &P) {
       // Void functions don't have anything to deserialize, so we're good.
+#if 0
       P.set_value(true);
+#endif
       return endReceiveMessage(C);
     }
 
-    static void abandon(std::promise<OptionalReturn> &P) { P.set_value(false); }
+    static void abandon(std::promise<OptionalReturn> &P) {
+#if 0
+        P.set_value(false);
+#endif
+    }
 
     template <typename ChannelT, typename SequenceNumberT>
     static Error respond(ChannelT &C, SequenceNumberT SeqNo,
@@ -617,13 +627,11 @@ class RPC : public RPCBase {
     }
 
     void reset() {
-      std::lock_guard<std::mutex> Lock(SeqNoLock);
       NextSequenceNumber = 0;
       FreeSequenceNumbers.clear();
     }
 
     SequenceNumberT getSequenceNumber() {
-      std::lock_guard<std::mutex> Lock(SeqNoLock);
       if (FreeSequenceNumbers.empty())
         return NextSequenceNumber++;
       auto SequenceNumber = FreeSequenceNumbers.back();
@@ -632,12 +640,10 @@ class RPC : public RPCBase {
     }
 
     void releaseSequenceNumber(SequenceNumberT SequenceNumber) {
-      std::lock_guard<std::mutex> Lock(SeqNoLock);
       FreeSequenceNumbers.push_back(SequenceNumber);
     }
 
   private:
-    std::mutex SeqNoLock;
     SequenceNumberT NextSequenceNumber = 0;
     std::vector<SequenceNumberT> FreeSequenceNumbers;
   };

include/llvm/Support/ThreadPool.h

@@ -16,6 +16,8 @@
 
 #include "llvm/Support/thread.h"
 
+# if 0
+
 #ifdef _MSC_VER
 // concrt.h depends on eh.h for __uncaught_exception declaration
 // even if we disable exceptions.
@@ -134,4 +136,6 @@ class ThreadPool {
 };
 }
 
+# endif
+
 #endif // LLVM_SUPPORT_THREAD_POOL_H

include/llvm/Support/thread.h

@@ -19,7 +19,7 @@
 
 #include "llvm/Config/llvm-config.h"
 
-#if LLVM_ENABLE_THREADS
+#if LLVM_ENABLE_THREADS && 0
 
 #ifdef _MSC_VER
 // concrt.h depends on eh.h for __uncaught_exception declaration

lib/CodeGen/ParallelCG.cpp

@@ -49,6 +49,7 @@ std::unique_ptr<Module> llvm::splitCodeGen(
     return M;
   }
 
+#if 0
   // Create ThreadPool in nested scope so that threads will be joined
   // on destruction.
   {
@@ -95,5 +96,6 @@ std::unique_ptr<Module> llvm::splitCodeGen(
         PreserveLocals);
   }
 
+#endif
   return {};
 }

lib/LTO/ThinLTOCodeGenerator.cpp

@@ -64,7 +64,7 @@ extern cl::opt<bool> LTODiscardValueNames;
 namespace {
 
 static cl::opt<int> ThreadCount("threads",
-                                cl::init(std::thread::hardware_concurrency()));
+                                cl::init(1));
 
 static void diagnosticHandler(const DiagnosticInfo &DI) {
   DiagnosticPrinterRawOStream DP(errs());
@@ -667,6 +667,7 @@ std::unique_ptr<MemoryBuffer> ThinLTOCodeGenerator::codegen(Module &TheModule) {
 
 // Main entry point for the ThinLTO processing
 void ThinLTOCodeGenerator::run() {
+#if 0
   if (CodeGenOnly) {
     // Perform only parallel codegen and return.
     ThreadPool Pool;
@@ -832,4 +833,5 @@ void ThinLTOCodeGenerator::run() {
   // If statistics were requested, print them out now.
   if (llvm::AreStatisticsEnabled())
     llvm::PrintStatistics();
+#endif
 }

lib/Support/ThreadPool.cpp

@@ -11,14 +11,16 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if 0
+
 #include "llvm/Support/ThreadPool.h"
 
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 
-#if LLVM_ENABLE_THREADS
+#if LLVM_ENABLE_THREADS && 0
 
 // Default to std::thread::hardware_concurrency
 ThreadPool::ThreadPool() : ThreadPool(std::thread::hardware_concurrency()) {}
@@ -156,3 +158,5 @@ ThreadPool::~ThreadPool() {
 }
 
 #endif
+
+#endif

test/CMakeLists.txt

@@ -27,7 +27,6 @@ set(LLVM_TEST_DEPENDS
           count
           llc
           lli
-          lli-child-target
           llvm-ar
           llvm-as
           llvm-bcanalyzer

tools/lli/CMakeLists.txt

@@ -1,7 +1,3 @@
-if ( LLVM_INCLUDE_UTILS )
-  add_subdirectory(ChildTarget)
-endif()
-
 set(LLVM_LINK_COMPONENTS
   CodeGen
   Core
@@ -39,6 +35,5 @@ endif( LLVM_USE_INTEL_JITEVENTS )
 
 add_llvm_tool(lli
   lli.cpp
-  OrcLazyJIT.cpp
   )
 export_executable_symbols(lli)

tools/lli/LLVMBuild.txt

@@ -15,9 +15,6 @@
 ;
 ;===------------------------------------------------------------------------===;
 
-[common]
-subdirectories = ChildTarget
-
 [component_0]
 type = Tool
 name = lli