8315841: RISC-V: Check for hardware TSO support

Reviewed-by: vkempik, rehn, fyang

Author: luhenry

src/hotspot/cpu/riscv/globals_riscv.hpp

@@ -109,6 +109,7 @@ define_pd_global(intx, InlineSmallCode,          1000);
   product(bool, UseZicbom, false, EXPERIMENTAL, "Use Zicbom instructions")       \
   product(bool, UseZicbop, false, EXPERIMENTAL, "Use Zicbop instructions")       \
   product(bool, UseZicboz, false, EXPERIMENTAL, "Use Zicboz instructions")       \
+  product(bool, UseZtso, false, EXPERIMENTAL, "Assume Ztso memory model")        \
   product(bool, UseZihintpause, false, EXPERIMENTAL,                             \
           "Use Zihintpause instructions")                                        \
   product(bool, UseRVVForBigIntegerShiftIntrinsics, true,                        \

src/hotspot/cpu/riscv/macroAssembler_riscv.hpp

@@ -376,8 +376,24 @@ class MacroAssembler: public Assembler {
     return ((predecessor & 0x3) << 2) | (successor & 0x3);
   }
 
+  void fence(uint32_t predecessor, uint32_t successor) {
+    if (UseZtso) {
+      if ((pred_succ_to_membar_mask(predecessor, successor) & StoreLoad) == StoreLoad) {
+        // TSO allows for stores to be reordered after loads. When the compiler
+        // generates a fence to disallow that, we are required to generate the
+        // fence for correctness.
+        Assembler::fence(predecessor, successor);
+      } else {
+        // TSO guarantees other fences already.
+      }
+    } else {
+      // always generate fence for RVWMO
+      Assembler::fence(predecessor, successor);
+    }
+  }
+
   void pause() {
-    fence(w, 0);
+    Assembler::fence(w, 0);
   }
 
   // prints msg, dumps registers and stops execution

src/hotspot/cpu/riscv/vm_version_riscv.cpp

@@ -210,6 +210,14 @@ void VM_Version::initialize() {
       unaligned_access.value() == MISALIGNED_FAST);
   }
 
+#ifdef __riscv_ztso
+  // Hotspot is compiled with TSO support, it will only run on hardware which
+  // supports Ztso
+  if (FLAG_IS_DEFAULT(UseZtso)) {
+    FLAG_SET_DEFAULT(UseZtso, true);
+  }
+#endif
+
   if (UseZbb) {
     if (FLAG_IS_DEFAULT(UsePopCountInstruction)) {
       FLAG_SET_DEFAULT(UsePopCountInstruction, true);

src/hotspot/cpu/riscv/vm_version_riscv.hpp

@@ -134,6 +134,7 @@ class VM_Version : public Abstract_VM_Version {
   decl(ext_Zicsr       , "Zicsr"       , RV_NO_FLAG_BIT, true , NO_UPDATE_DEFAULT)              \
   decl(ext_Zifencei    , "Zifencei"    , RV_NO_FLAG_BIT, true , NO_UPDATE_DEFAULT)              \
   decl(ext_Zic64b      , "Zic64b"      , RV_NO_FLAG_BIT, true , UPDATE_DEFAULT(UseZic64b))      \
+  decl(ext_Ztso        , "Ztso"        , RV_NO_FLAG_BIT, true , UPDATE_DEFAULT(UseZtso))        \
   decl(ext_Zihintpause , "Zihintpause" , RV_NO_FLAG_BIT, true , UPDATE_DEFAULT(UseZihintpause)) \
   decl(mvendorid       , "VendorId"    , RV_NO_FLAG_BIT, false, NO_UPDATE_DEFAULT)              \
   decl(marchid         , "ArchId"      , RV_NO_FLAG_BIT, false, NO_UPDATE_DEFAULT)              \

src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp

@@ -236,6 +236,7 @@ void VM_Version::rivos_features() {
   ext_Zicsr.enable_feature();
   ext_Zifencei.enable_feature();
   ext_Zic64b.enable_feature();
+  ext_Ztso.enable_feature();
   ext_Zihintpause.enable_feature();
 
   unaligned_access.enable_feature(MISALIGNED_FAST);