8321282: RISC-V: SpinPause() not implemented

Reviewed-by: luhenry, fbredberg, fyang

Author: robehn

src/hotspot/os_cpu/linux_riscv/os_linux_riscv.cpp

@@ -38,6 +38,7 @@
 #include "prims/jvm_misc.hpp"
 #include "runtime/arguments.hpp"
 #include "runtime/frame.inline.hpp"
+#include "runtime/globals.hpp"
 #include "runtime/interfaceSupport.inline.hpp"
 #include "runtime/java.hpp"
 #include "runtime/javaCalls.hpp"
@@ -405,6 +406,14 @@ static inline void atomic_copy64(const volatile void *src, volatile void *dst) {
 
 extern "C" {
   int SpinPause() {
+    if (UseZihintpause) {
+      // PAUSE is encoded as a FENCE instruction with pred=W, succ=0, fm=0, rd=x0, and rs1=x0.
+      // To do: __asm__ volatile("pause " : : : );
+      // Since we're currently not passing '-march=..._zihintpause' to the compiler,
+      // it will not recognize the "pause" instruction, hence the hard-coded instruction.
+      __asm__ volatile(".word 0x0100000f  " : : : );
+      return 1;
+    }
     return 0;
   }