More conservative CPU name based feature detection (#37482)

It seems that it's actually possible to pair cores with different feature set
using big.LITTLE or DynamIQ so we at least need to constrain ourselves to the intersection
of the features for all the cores.

Author: yuyichao

src/processor_arm.cpp

@@ -1195,17 +1195,41 @@ static NOINLINE std::pair<uint32_t,FeatureList<feature_sz>> _get_host_cpu()
 
     std::set<uint32_t> cpus;
     std::vector<std::pair<uint32_t,CPUID>> list;
+    // Ideally the feature detection above should be enough.
+    // However depending on the kernel version not all features are available
+    // and it's also impossible to detect the ISA version which contains
+    // some features not yet exposed by the kernel.
+    // We therefore try to get a more complete feature list from the CPU name.
+    // Since it is possible to pair cores that have different feature set
+    // (Observed for exynos 9810 with exynos-m3 + cortex-a55) we'll compute
+    // an intersection of the known features from each core.
+    // If there's a core that we don't recognize, treat it as generic.
+    bool extra_initialized = false;
+    FeatureList<feature_sz> extra_features = {};
     for (auto info: cpuinfo) {
         auto name = (uint32_t)get_cpu_name(info);
-        if (name == 0)
+        if (name == 0) {
+            // no need to clear the feature set if it wasn't initialized
+            if (extra_initialized)
+                extra_features = FeatureList<feature_sz>{};
+            extra_initialized = true;
             continue;
+        }
         if (!check_cpu_arch_ver(name, arch))
             continue;
         if (cpus.insert(name).second) {
-            features = features | find_cpu(name)->features;
+            if (extra_initialized) {
+                extra_features = extra_features & find_cpu(name)->features;
+            }
+            else {
+                extra_initialized = true;
+                extra_features = find_cpu(name)->features;
+            }
             list.emplace_back(name, info);
         }
     }
+    features = features | extra_features;
+
     // Not all elements/pairs are valid
     static constexpr CPU v8order[] = {
         CPU::arm_cortex_a35,