8329823: RISC-V: Need to sync CPU features with related JVM flags

Backport-of: b9331cd25ca88b07ce079405f5e3031cf8c13ea6

Author: zifeihan

src/hotspot/cpu/riscv/vm_version_riscv.hpp

@@ -61,6 +61,10 @@ class VM_Version : public Abstract_VM_Version {
       _enabled = true;
       _value = value;
     }
+    void disable_feature() {
+      _enabled = false;
+      _value = -1;
+    }
     const char* pretty()         { return _pretty; }
     uint64_t feature_bit()       { return _feature_bit; }
     bool feature_string()        { return _feature_string; }
@@ -69,16 +73,21 @@ class VM_Version : public Abstract_VM_Version {
     virtual void update_flag() = 0;
   };
 
-  #define UPDATE_DEFAULT(flag)        \
-  void update_flag() {                \
-      assert(enabled(), "Must be.");  \
-      if (FLAG_IS_DEFAULT(flag)) {    \
-        FLAG_SET_DEFAULT(flag, true); \
-      }                               \
-  }                                   \
-
-  #define NO_UPDATE_DEFAULT           \
-  void update_flag() {}               \
+  #define UPDATE_DEFAULT(flag)             \
+  void update_flag() {                     \
+      assert(enabled(), "Must be.");       \
+      if (FLAG_IS_DEFAULT(flag)) {         \
+        FLAG_SET_DEFAULT(flag, true);      \
+      } else {                             \
+        /* Sync CPU features with flags */ \
+        if (!flag) {                       \
+          disable_feature();               \
+        }                                  \
+      }                                    \
+  }                                        \
+
+  #define NO_UPDATE_DEFAULT                \
+  void update_flag() {}                    \
 
   // Frozen standard extensions
   // I RV64I

src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp

@@ -115,6 +115,15 @@ void VM_Version::setup_cpu_available_features() {
   int i = 0;
   while (_feature_list[i] != nullptr) {
     if (_feature_list[i]->enabled()) {
+      // Change flag default
+      _feature_list[i]->update_flag();
+
+      // Feature will be disabled by update_flag() if flag
+      // is set to false by the user on the command line.
+      if (!_feature_list[i]->enabled()) {
+        continue;
+      }
+
       log_debug(os, cpu)("Enabled RV64 feature \"%s\" (%ld)",
              _feature_list[i]->pretty(),
              _feature_list[i]->value());
@@ -139,8 +148,6 @@ void VM_Version::setup_cpu_available_features() {
       if (_feature_list[i]->feature_bit() != 0) {
         _features |= _feature_list[i]->feature_bit();
       }
-      // Change flag default
-      _feature_list[i]->update_flag();
     }
     i++;
   }