vm: add OP_RETNIL for returning nil directly

Add a new opcode that returns nil without requiring LOADNIL + RETURN.
This avoids loading nil into a register by setting the return value (v)
directly. The implementation uses a separate label (L_RETURN_NIL) to
bypass v = regs[a], preserving self in regs[0] for ensure blocks.

Codegen applies peephole optimization to fuse LOADNIL + RETURN -> RETNIL.

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

NEWS.md

@@ -85,6 +85,7 @@ New super-instructions that fuse common opcode sequences to reduce bytecode size
 - `OP_RETSELF`: Single-byte instruction for `return self` pattern ([a71db8c](https://github.com/mruby/mruby/commit/a71db8c))
 - `OP_MATCHERR`: Pattern matching error with conditional execution for `in` patterns ([944168a](https://github.com/mruby/mruby/commit/944168a), [e9a9ba4](https://github.com/mruby/mruby/commit/e9a9ba4))
 - `OP_BLKCALL`: Direct block call for `yield`, bypassing method dispatch (13-17% faster)
+- `OP_RETNIL`: Single-byte instruction for `return nil` pattern ([96641c9](https://github.com/mruby/mruby/commit/96641c9))
 
 # Fixed GitHub Issues
 

include/mruby/ops.h

@@ -74,6 +74,7 @@ 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(RETNIL,     Z)        /* return nil */
 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

@@ -1151,7 +1151,12 @@ gen_return(codegen_scope *s, uint8_t op, uint16_t src)
       rewind_pc(s);
       genop_0(s, OP_RETSELF);
     }
-    else if (data.insn != OP_RETURN && data.insn != OP_RETSELF) {
+    else if (data.insn == OP_LOADNIL && src == data.a && op == OP_RETURN) {
+      /* LOADNIL + RETURN -> RETNIL */
+      rewind_pc(s);
+      genop_0(s, OP_RETNIL);
+    }
+    else if (data.insn != OP_RETURN && data.insn != OP_RETSELF && data.insn != OP_RETNIL) {
       genop_1(s, op, src);
     }
   }

src/codedump.c

@@ -399,6 +399,9 @@ codedump(mrb_state *mrb, const mrb_irep *irep, FILE *out)
     CASE(OP_RETSELF, Z):
       fprintf(out, "RETSELF\n");
       break;
+    CASE(OP_RETNIL, Z):
+      fprintf(out, "RETNIL\n");
+      break;
     CASE(OP_BREAK, B):
       fprintf(out, "BREAK\t\tR%d\t", a);
       print_lv_a(mrb, irep, a, out);

src/vm.c

@@ -2744,6 +2744,10 @@ RETRY_TRY_BLOCK:
       a = 0;
       goto NORMAL_RETURN;
     }
+    CASE(OP_RETNIL, Z) {
+      a = 0;
+      goto L_RETURN_NIL;
+    }
     CASE(OP_RETURN, B) {
       mrb_int acc;
       mrb_value v;
@@ -2751,6 +2755,10 @@ RETRY_TRY_BLOCK:
 
     NORMAL_RETURN:
       v = regs[a];
+      goto L_RETURN;
+    L_RETURN_NIL:
+      v = mrb_nil_value();
+    L_RETURN:
       mrb_gc_protect(mrb, v);
       return_ci = ci;
       CHECKPOINT_RESTORE(RBREAK_TAG_BREAK) {