From eb5a65aeb08d787d595044299a41b0226d73b569 Mon Sep 17 00:00:00 2001
From: Timothy Herchen <timothy.herchen@gmail.com>
Date: Sat, 10 Jan 2026 10:18:48 -0800
Subject: [PATCH] Fix RelationCache on Windows 10 compiles

Windows 10 is missing the GroupMasks and GroupCount members, this breaks compiles on Windows 10.
Windows 11 builds, including the official ones, run fine on Windows 10/11.
To support developers/testers on Windows 10, fallback conditionally to the Windows 10 struct definition.

closes https://github.com/official-stockfish/Stockfish/pull/6538

No functional change
---
 src/numa.h | 64 ++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 48 insertions(+), 16 deletions(-)

diff --git a/src/numa.h b/src/numa.h
index df11f0286..b3d1c34d9 100644
--- a/src/numa.h
+++ b/src/numa.h
@@ -371,6 +371,50 @@ inline WindowsAffinity get_process_affinity() {
     return affinity;
 }
 
+// Type machinery used to emulate Cache->GroupCount
+
+template<typename T, typename = void>
+struct HasGroupCount: std::false_type {};
+
+template<typename T>
+struct HasGroupCount<T, std::void_t<decltype(std::declval<T>().Cache.GroupCount)>>: std::true_type {
+};
+
+template<typename T, typename Pred, std::enable_if_t<HasGroupCount<T>::value, bool> = true>
+std::set<CpuIndex> readCacheMembers(const T* info, Pred&& is_cpu_allowed) {
+    std::set<CpuIndex> cpus;
+    // On Windows 10 this will read a 0 because GroupCount doesn't exist
+    int groupCount = std::max(info->Cache.GroupCount, WORD(1));
+    for (WORD procGroup = 0; procGroup < groupCount; ++procGroup)
+    {
+        for (BYTE number = 0; number < WIN_PROCESSOR_GROUP_SIZE; ++number)
+        {
+            WORD           groupNumber = info->Cache.GroupMasks[procGroup].Group;
+            const CpuIndex c = static_cast<CpuIndex>(groupNumber) * WIN_PROCESSOR_GROUP_SIZE
+                             + static_cast<CpuIndex>(number);
+            if (!(info->Cache.GroupMasks[procGroup].Mask & (1ULL << number)) || !is_cpu_allowed(c))
+                continue;
+            cpus.insert(c);
+        }
+    }
+    return cpus;
+}
+
+template<typename T, typename Pred, std::enable_if_t<!HasGroupCount<T>::value, bool> = true>
+std::set<CpuIndex> readCacheMembers(const T* info, Pred&& is_cpu_allowed) {
+    std::set<CpuIndex> cpus;
+    for (BYTE number = 0; number < WIN_PROCESSOR_GROUP_SIZE; ++number)
+    {
+        WORD           groupNumber = info->Cache.GroupMask.Group;
+        const CpuIndex c           = static_cast<CpuIndex>(groupNumber) * WIN_PROCESSOR_GROUP_SIZE
+                         + static_cast<CpuIndex>(number);
+        if (!(info->Cache.GroupMask.Mask & (1ULL << number)) || !is_cpu_allowed(c))
+            continue;
+        cpus.insert(c);
+    }
+    return cpus;
+}
+
 #endif
 
 #if defined(__linux__) && !defined(__ANDROID__)
@@ -1174,29 +1218,17 @@ class NumaConfig {
             if (info->Relationship == RelationCache && info->Cache.Level == 3)
             {
                 L3Domain domain{};
-                for (WORD procGroup = 0; procGroup < info->Cache.GroupCount; ++procGroup)
-                {
-                    for (BYTE number = 0; number < WIN_PROCESSOR_GROUP_SIZE; ++number)
-                    {
-                        WORD           groupNumber = info->Cache.GroupMasks[procGroup].Group;
-                        const CpuIndex c =
-                          static_cast<CpuIndex>(groupNumber) * WIN_PROCESSOR_GROUP_SIZE
-                          + static_cast<CpuIndex>(number);
-                        if (!(info->Cache.GroupMasks[procGroup].Mask & (1ULL << number))
-                            || !is_cpu_allowed(c))
-                            continue;
-                        domain.systemNumaIndex = systemConfig.nodeByCpu.at(c);
-                        domain.cpus.insert(c);
-                    }
-                }
+                domain.cpus = readCacheMembers(info, is_cpu_allowed);
                 if (!domain.cpus.empty())
+                {
+                    domain.systemNumaIndex = systemConfig.nodeByCpu.at(*domain.cpus.begin());
                     l3Domains.push_back(std::move(domain));
+                }
             }
             // Variable length data structure, advance to next
             info = reinterpret_cast<PSYSTEM_LOGICAL_PROCESSOR_INFORMATION_EX>(
               reinterpret_cast<char*>(info) + info->Size);
         }
-
 #endif
 
         if (!l3Domains.empty())