[llvm] Windows: use EcoQoS for ThreadPriority::Background (PR #148797)

[Tim Blechmann via llvm-commits]

https://github.com/timblechmann updated https://github.com/llvm/llvm-project/pull/148797

>From 5c586bf7478b7041dda53bfb921f4bdd514f1cf5 Mon Sep 17 00:00:00 2001
From: Tim Blechmann <tim at klingt.org>
Date: Tue, 15 Jul 2025 15:32:15 +0800
Subject: [PATCH] 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.
---
 llvm/lib/Support/Windows/Threading.inc | 29 ++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/llvm/lib/Support/Windows/Threading.inc b/llvm/lib/Support/Windows/Threading.inc
index d862dbd7f71c9..571e0960a6888 100644
--- a/llvm/lib/Support/Windows/Threading.inc
+++ b/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 = ControlMaskAndStateMask;
+      state.StateMask = ControlMaskAndStateMask;
+      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