Save block argument position in e->cioff; fix #3593

src/kernel.c

@@ -157,10 +157,16 @@ mrb_f_block_given_p_m(mrb_state *mrb, mrb_value self)
         /* top-level does not have block slot (alway false) */
         if (sp == mrb->c->stbase)
           return mrb_false_value();
+        if (e->cioff < 0) {
+          /* use saved block arg position */
+          bp = &e->stack[-e->cioff];
+        }
+        else {
           ci = mrb->c->cibase + e->cioff;
           bp = ci[1].stackent + 1;
         }
       }
+    }
     if (ci->argc > 0) {
       bp += ci->argc;
     }

src/vm.c

@@ -257,8 +257,13 @@ mrb_env_unshare(mrb_state *mrb, struct REnv *e)
 {
   size_t len = (size_t)MRB_ENV_STACK_LEN(e);
   mrb_value *p = (mrb_value *)mrb_malloc(mrb, sizeof(mrb_value)*len);
+  ptrdiff_t cioff = e->cioff;
 
   MRB_ENV_UNSHARE_STACK(e);
+  if (!e->c) {
+    /* save block argument position (negated) */
+    e->cioff = -mrb->c->cibase[cioff].argc-1;
+  }
   if (len > 0) {
     stack_copy(p, e->stack, len);
   }