8324734: Relax too-strict assert(VM_Version::supports_evex()) in Assembler::locate_operand()

Co-authored-by: Vladimir Kozlov <kvn@openjdk.org>
Reviewed-by: kvn, shade

src/hotspot/cpu/x86/assembler_x86.cpp

@@ -1089,7 +1089,7 @@ address Assembler::locate_operand(address inst, WhichOperand which) {
     break;
 
   case 0x62: // EVEX_4bytes
-    assert(VM_Version::supports_evex(), "shouldn't have EVEX prefix");
+    assert(VM_Version::cpu_supports_evex(), "shouldn't have EVEX prefix");
     assert(ip == inst+1, "no prefixes allowed");
     // no EVEX collisions, all instructions that have 0x62 opcodes
     // have EVEX versions and are subopcodes of 0x66

src/hotspot/cpu/x86/vm_version_x86.cpp

@@ -809,7 +809,8 @@ void VM_Version::get_processor_features() {
   _stepping = cpu_stepping();
 
   if (cpu_family() > 4) { // it supports CPUID
-    _features = feature_flags();
+    _features = feature_flags(); // These can be changed by VM settings
+    _cpu_features = _features;   // Preserve features
     // Logical processors are only available on P4s and above,
     // and only if hyperthreading is available.
     _logical_processors_per_package = logical_processor_count();

src/hotspot/cpu/x86/vm_version_x86.hpp

@@ -640,7 +640,7 @@ public:
   }
 
   //
-  // Feature identification
+  // Feature identification which can be affected by VM settings
   //
   static bool supports_cpuid()        { return _features  != 0; }
   static bool supports_cmov()         { return (_features & CPU_CMOV) != 0; }
@@ -703,6 +703,11 @@ public:
   static bool supports_cet_ss()       { return (_features & CPU_CET_SS) != 0; }
   static bool supports_cet_ibt()      { return (_features & CPU_CET_IBT) != 0; }
 
+  //
+  // Feature identification not affected by VM flags
+  //
+  static bool cpu_supports_evex()     { return (_cpu_features & CPU_AVX512F) != 0; }
+
   // Intel features
   static bool is_intel_family_core() { return is_intel() &&
                                        extended_cpu_family() == CPU_FAMILY_INTEL_CORE; }

src/hotspot/share/runtime/abstract_vm_version.cpp

@@ -34,6 +34,7 @@ const char* Abstract_VM_Version::_s_internal_vm_info_string = Abstract_VM_Versio
 
 uint64_t Abstract_VM_Version::_features = 0;
 const char* Abstract_VM_Version::_features_string = "";
+uint64_t Abstract_VM_Version::_cpu_features = 0;
 
 #ifndef SUPPORTS_NATIVE_CX8
 bool Abstract_VM_Version::_supports_cx8 = false;

src/hotspot/share/runtime/abstract_vm_version.hpp

@@ -54,10 +54,13 @@ class Abstract_VM_Version: AllStatic {
   static const char*  _s_vm_release;
   static const char*  _s_internal_vm_info_string;
 
-  // CPU feature flags.
+  // CPU feature flags, can be affected by VM settings.
   static uint64_t _features;
   static const char* _features_string;
 
+  // Original CPU feature flags, not affected by VM settings.
+  static uint64_t _cpu_features;
+
   // These are set by machine-dependent initializations
 #ifndef SUPPORTS_NATIVE_CX8
   static bool         _supports_cx8;