Update Environment.ProcessorCount on Windows to take into account the processor affinity mask (#45943)

- Similarly to cases on Unixes where sched_getaffinity is available
- If `GCCpuGroup` and `Thread_UseAllCpuGroups` are both enabled, I'm not sure if the `CPUGroupInfo` count of active processors takes affinity into account as the docs are not clear, for now I'm not modifying that path until I can verify it
- Otherwise, a process that is started with a specific processor affinity mask still shows full CPU count
- This is one of the differences in the portable managed thread pool implementation, which relies on Environment.ProcessorCount, as opposed to the native thread pool, which uses the affinity mask
- After this change, in affinitized cases on Windows the behavior is consistent perf-wise with Linux in similar situations:
  - The portable thread pool uses the same worker thread count as the native thread pool
  - `Environment.ProcessorCount` returns the number of processors the the process is affinitized to, which may be less than it would have returned before

1 files changed, 0 insertions, 13 deletions

diff --git a/src/coreclr/utilcode/util.cpp b/src/coreclr/utilcode/util.cpp
index c034ae19318..cabe535e03c 100644
--- a/src/coreclr/utilcode/util.cpp
+++ b/src/coreclr/utilcode/util.cpp
@@ -856,7 +856,6 @@ BYTE * ClrVirtualAllocWithinRange(const BYTE *pMinAddr,
 /*static*/ WORD CPUGroupInfo::m_initialGroup = 0;
 /*static*/ CPU_Group_Info *CPUGroupInfo::m_CPUGroupInfoArray = NULL;
 /*static*/ LONG CPUGroupInfo::m_initialization = 0;
-/*static*/ bool CPUGroupInfo::s_hadSingleProcessorAtStartup = false;
 
 #if !defined(FEATURE_REDHAWK) && (defined(TARGET_AMD64) || defined(TARGET_ARM64))
 // Calculate greatest common divisor
@@ -1014,18 +1013,6 @@ DWORD LCM(DWORD u, DWORD v)
     m_threadUseAllCpuGroups = threadUseAllCpuGroups && hasMultipleGroups;
     m_threadAssignCpuGroups = threadAssignCpuGroups && hasMultipleGroups;
 #endif // TARGET_AMD64 || TARGET_ARM64
-
-    // Determine if the process is affinitized to a single processor (or if the system has a single processor)
-    DWORD_PTR processAffinityMask, systemAffinityMask;
-    if (GetProcessAffinityMask(GetCurrentProcess(), &processAffinityMask, &systemAffinityMask))
-    {
-        processAffinityMask &= systemAffinityMask;
-        if (processAffinityMask != 0 && // only one CPU group is involved
-            (processAffinityMask & (processAffinityMask - 1)) == 0) // only one bit is set
-        {
-            s_hadSingleProcessorAtStartup = true;
-        }
-    }
 }
 
 /*static*/ BOOL CPUGroupInfo::IsInitialized()