8388926: RISC-V: Skip redundant zext.w in set_narrow_klass when bit 31 is clear

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp

@@ -5325,7 +5325,12 @@ void  MacroAssembler::set_narrow_klass(Register dst, Klass* k) {
   relocate(metadata_Relocation::spec(index), [&] {
     li32(dst, nk);
   });
-  zext(dst, dst, 32);
+  // li32 uses addiw which sign-extends; only need zext when bit 31 is set.
+  // Safe: narrow klass immediates are never patched at runtime on RISC-V
+  // (see metadata_Relocation::pd_fix_value).
+  if ((int32_t)nk < 0) {
+    zext(dst, dst, 32);
+  }
 }
 
 address MacroAssembler::reloc_call(Address entry, Register tmp) {

src/hotspot/cpu/riscv/relocInfo_riscv.cpp

@@ -117,5 +117,8 @@ void poll_Relocation::fix_relocation_after_move(const CodeBuffer* src, CodeBuffe
   }
 }
 
+// Currently a no-op: no immediate patching is performed.
+// If this changes, update MacroAssembler::set_narrow_klass accordingly
+// (its conditional zext relies on the narrow klass value being immutable).
 void metadata_Relocation::pd_fix_value(address x) {
 }