Provide internal nop

For the safe of return address, nop has to be part of IR, so that pc can
correctly advance. The internal nop can be used as the placeholder along
with super-instructions.

src/decode.c

@@ -450,8 +450,10 @@ static inline bool op_op_imm(rv_insn_t *ir, const uint32_t insn)
     decode_itype(ir, insn);
 
     /* nop can be implemented as "addi x0, x0, 0" */
-    if (unlikely(ir->rd == rv_reg_zero))
+    if (unlikely(ir->rd == rv_reg_zero)) {
+        ir->opcode = rv_insn_nop;
         return true;
+    }
 
     /* dispatch from funct3 field */
     switch (decode_funct3(insn)) {
@@ -566,8 +568,10 @@ static inline bool op_op(rv_insn_t *ir, const uint32_t insn)
     decode_rtype(ir, insn);
 
     /* nop can be implemented as "add x0, x1, x2" */
-    if (unlikely(ir->rd == rv_reg_zero))
+    if (unlikely(ir->rd == rv_reg_zero)) {
+        ir->opcode = rv_insn_nop;
         return true;
+    }
 
     uint8_t funct3 = decode_funct3(insn);
 

src/decode.h

@@ -11,6 +11,7 @@
 /* RISC-V instruction list */
 /* clang-format off */
 #define RISCV_INSN_LIST                    \
+    _(nop)                                 \
     /* RV32I Base Instruction Set */       \
     _(lui)                                 \
     _(auipc)                               \

src/emulate.c

@@ -319,6 +319,9 @@ static bool emulate(riscv_t *rv, const block_t *block)
     /* main loop */
     DISPATCH()
 
+    /* Internal */
+    _(nop, /* no operation */)
+
     /* LUI (Load Upper Immediate) is used to build 32-bit constants and uses the
      * U-type format. LUI places the U-immediate value in the top 20 bits of the
      * destination register rd, filling in the lowest 12 bits with zeros. The