Windows: use EcoQoS for ThreadPriority::Background

The SetThreadInformation API allows threads to be scheduled on the most
efficient cores on the most efficient frequency.
Using this API for ThreadPriority::Background should make clangd-based
IDEs a little less CPU hungry.

Author: timblechmann

llvm/lib/Support/Windows/Threading.inc

@@ -107,6 +107,35 @@ void llvm::get_thread_name(SmallVectorImpl<char> &Name) {
 }
 
 SetThreadPriorityResult llvm::set_thread_priority(ThreadPriority Priority) {
+
+  // SetThreadInformation is only available on Windows 8 and later. Since we still
+  // support compilation on Windows 7, we load the function dynamically.
+  typedef BOOL(WINAPI * SetThreadInformation_t)(
+      HANDLE hThread, THREAD_INFORMATION_CLASS ThreadInformationClass,
+      _In_reads_bytes_(ThreadInformationSize) PVOID ThreadInformation,
+      ULONG ThreadInformationSize);
+  static const auto pfnSetThreadInformation =
+      (SetThreadInformation_t)GetProcAddress(
+          GetModuleHandle(TEXT("kernel32.dll")), "SetThreadInformation");
+
+  if (pfnSetThreadInformation) {
+    auto setThreadInformation = [](ULONG ControlMaskAndStateMask) {
+      THREAD_POWER_THROTTLING_STATE state{};
+      state.Version = THREAD_POWER_THROTTLING_CURRENT_VERSION;
+      state.ControlMask = ControlMask;
+      state.StateMask = StateMask;
+      return pfnSetThreadInformation(GetCurrentThread(), ThreadPowerThrottling,
+                                     &state, sizeof(state));
+    };
+
+    // Use EcoQoS for ThreadPriority::Background available (running on most
+    // efficent cores at the most efficient cpu frequency):
+    // https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-setthreadinformation
+    // https://learn.microsoft.com/en-us/windows/win32/procthread/quality-of-service
+    setThreadInformation(Priority == ThreadPriority::Background ? THREAD_POWER_THROTTLING_EXECUTION_SPEED
+                                                                : 0);
+  }
+
   // https://docs.microsoft.com/en-us/windows/desktop/api/processthreadsapi/nf-processthreadsapi-setthreadpriority
   // Begin background processing mode. The system lowers the resource scheduling
   // priorities of the thread so that it can perform background work without