[libcxx-commits] [libcxx] 5a3c257 - [libc++][Windows] Enable thread::hardware_concurrency to support more than 64 processors (#168229)

[via libcxx-commits]

Author: Yexuan Xiao
Date: 2025-11-21T18:43:35+08:00
New Revision: 5a3c2573a9644efe3384c4144b119148088b48ef

URL: https://github.com/llvm/llvm-project/commit/5a3c2573a9644efe3384c4144b119148088b48ef
DIFF: https://github.com/llvm/llvm-project/commit/5a3c2573a9644efe3384c4144b119148088b48ef.diff

LOG: [libc++][Windows] Enable thread::hardware_concurrency to support more than 64 processors (#168229)

Starting with Windows 11, processes can utilize more than 64 processors
by default, but GetSystemInfo can only report a maximum of 64. Starting
with Windows Vista, GetActiveProcessorCount accurately retrieves the
total number of processors on the current system. I’ve implemented
similar improvements to Microsoft STL. The following links contain
additional background information:
https://github.com/microsoft/STL/issues/5453,
https://github.com/microsoft/STL/pull/5459 (note: Reason STL uses the
more complex GetLogicalProcessorInformationEx:
https://github.com/microsoft/STL/pull/5459#discussion_r2072242241.).

Added: 
    

Modified: 
    libcxx/src/thread.cpp

Removed: 
    


################################################################################
diff  --git a/libcxx/src/thread.cpp b/libcxx/src/thread.cpp
index 028d36e3bfb37..e494574ec21dd 100644
--- a/libcxx/src/thread.cpp
+++ b/libcxx/src/thread.cpp
@@ -74,9 +74,7 @@ unsigned thread::hardware_concurrency() noexcept {
     return 0;
   return static_cast<unsigned>(result);
 #elif defined(_LIBCPP_WIN32API)
-  SYSTEM_INFO info;
-  GetSystemInfo(&info);
-  return info.dwNumberOfProcessors;
+  return static_cast<unsigned>(GetActiveProcessorCount(ALL_PROCESSOR_GROUPS));
 #else // defined(CTL_HW) && defined(HW_NCPU)
   // TODO: grovel through /proc or check cpuid on x86 and similar
   // instructions on other architectures.