vm: add OP_RETSELF instruction for returning self

Fuse LOADSELF + RETURN sequence into single RETSELF instruction.
Saves 2 bytes per occurrence (3 bytes -> 1 byte).

Found 25 occurrences in mrblib, saving 50 bytes total.

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby/ops.h

@@ -71,6 +71,7 @@ OPCODE(KEYEND,     Z)        /* raise unless kdict.empty? */
 OPCODE(KARG,       BB)       /* R[a] = kdict[Syms[b]]; kdict.delete(Syms[b]) */
 OPCODE(RETURN,     B)        /* return R[a] (normal) */
 OPCODE(RETURN_BLK, B)        /* return R[a] (in-block return) */
+OPCODE(RETSELF,    Z)        /* return self */
 OPCODE(BREAK,      B)        /* break R[a] */
 OPCODE(BLKPUSH,    BS)       /* R[a] = block (16=m5:r1:m5:d1:lv4) */
 OPCODE(ADD,        B)        /* R[a] = R[a]+R[a+1] */

mrbgems/mruby-compiler/core/codegen.c

@@ -1141,7 +1141,12 @@ gen_return(codegen_scope *s, uint8_t op, uint16_t src)
       rewind_pc(s);
       genop_1(s, op, data.b);
     }
-    else if (data.insn != OP_RETURN) {
+    else if (data.insn == OP_LOADSELF && src == data.a && op == OP_RETURN) {
+      /* LOADSELF + RETURN -> RETSELF */
+      rewind_pc(s);
+      genop_0(s, OP_RETSELF);
+    }
+    else if (data.insn != OP_RETURN && data.insn != OP_RETSELF) {
       genop_1(s, op, src);
     }
   }

src/codedump.c

@@ -390,6 +390,9 @@ codedump(mrb_state *mrb, const mrb_irep *irep, FILE *out)
       fprintf(out, "RETURN_BLK\tR%d\t", a);
       print_lv_a(mrb, irep, a, out);
       break;
+    CASE(OP_RETSELF, Z):
+      fprintf(out, "RETSELF\n");
+      break;
     CASE(OP_BREAK, B):
       fprintf(out, "BREAK\t\tR%d\t", a);
       print_lv_a(mrb, irep, a, out);

src/vm.c

@@ -2661,6 +2661,10 @@ RETRY_TRY_BLOCK:
       RAISE_LIT(mrb, E_LOCALJUMP_ERROR, "unexpected return");
       /* not reached */
     }
+    CASE(OP_RETSELF, Z) {
+      a = 0;
+      goto NORMAL_RETURN;
+    }
     CASE(OP_RETURN, B) {
       mrb_int acc;
       mrb_value v;