mruby-compiler: add no_return_value context flag for script optimization

when running scripts via mruby -e or file, return values are unused.
this adds a no_return_value flag to skip generating unnecessary code.

for parallel assignment like a,b = 1,2:
- before: 18 bytes, 5 registers, creates temporary array
- after: 5 bytes, 3 registers, direct register assignment, no RETURN

the flag is set only for the main program, not for libraries loaded
with -r option. eval() and mirb continue returning values correctly.

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

include/mruby/compile.h

@@ -33,6 +33,7 @@ typedef struct mrb_ccontext {
   mrb_bool keep_lv:1;
   mrb_bool no_optimize:1;
   mrb_bool no_ext_ops:1;
+  mrb_bool no_return_value:1;
   const struct RProc *upper;
 
   size_t parser_nerr;
@@ -125,6 +126,7 @@ struct mrb_parser_state {
   mrb_bool no_optimize:1;
   mrb_bool capture_errors:1;
   mrb_bool no_ext_ops:1;
+  mrb_bool no_return_value:1;
   const struct RProc *upper;
   struct mrb_parser_message error_buffer[10];
   struct mrb_parser_message warn_buffer[10];

mrbgems/mruby-bin-mruby/tools/mruby/mruby.c

@@ -345,6 +345,7 @@ main(int argc, char **argv)
 
     /* set program filename */
     mrb_ccontext_filename(mrb, c, cmdline);
+    c->no_return_value = TRUE;  /* main program doesn't need return value */
 
     /* Load program */
     if (args.mrbfile || mrb_extension_p(cmdline)) {

mrbgems/mruby-compiler/core/codegen.c

@@ -2603,14 +2603,25 @@ scope_body(codegen_scope *s, node *locals, node *body, int val)
   codegen_scope *scope = scope_new(s->mrb, s, locals);
 
   /* Generate code for the body of the scope. */
-  codegen(scope, body, VAL);
-  /* Ensure the scope returns the value of its last expression. */
-  gen_return(scope, OP_RETURN, scope->sp-1);
+  codegen(scope, body, val);
 
   /* If this is the outermost scope (e.g., top-level script), add OP_STOP. */
   if (!s->iseq) { /* s->iseq would be NULL for the initial dummy scope. */
+    if (val) {
+      gen_return(scope, OP_RETURN, scope->sp-1);
+    }
+    /* skip RETURN when no_return_value; STOP will terminate VM */
     genop_0(scope, OP_STOP);
   }
+  else {
+    /* Ensure the scope returns the value of its last expression. */
+    if (val) {
+      gen_return(scope, OP_RETURN, scope->sp-1);
+    }
+    else {
+      gen_return(scope, OP_RETURN, 0);  /* return nil */
+    }
+  }
 
   /* Finalize the IREP for this scope. */
   scope_finish(scope);
@@ -6120,7 +6131,7 @@ codegen_scope_node(codegen_scope *s, const node *varnode, int val)
   struct mrb_ast_scope_node *scope = scope_node(varnode);
 
   /* Pass locals and body directly to scope_body() */
-  scope_body(s, scope->locals, scope->body, NOVAL);
+  scope_body(s, scope->locals, scope->body, val);
 }
 
 static void
@@ -6937,5 +6948,5 @@ generate_code(mrb_state *mrb, parser_state *p, int val)
 MRB_API struct RProc*
 mrb_generate_code(mrb_state *mrb, parser_state *p)
 {
-  return generate_code(mrb, p, VAL);
+  return generate_code(mrb, p, p->no_return_value ? NOVAL : VAL);
 }

mrbgems/mruby-compiler/core/parse.y

@@ -7459,6 +7459,7 @@ parser_init_cxt(parser_state *p, mrb_ccontext *cxt)
   p->capture_errors = cxt->capture_errors;
   p->no_optimize = cxt->no_optimize;
   p->no_ext_ops = cxt->no_ext_ops;
+  p->no_return_value = cxt->no_return_value;
   p->upper = cxt->upper;
   if (cxt->partial_hook) {
     p->cxt = cxt;