vm_args.c (rb_warn_check): Use iseq_unique_id instead of its pointer

(This is the second try of 036bc1da6c6c9b0fa9b7f5968d897a9554dd770e.)

If iseq is GC'ed, the pointer of iseq may be reused, which may hide a
deprecation warning of keyword argument change.

http://ci.rvm.jp/results/trunk-test1@phosphorus-docker/2474221

```
1) Failure:
TestKeywordArguments#test_explicit_super_kwsplat [/tmp/ruby/v2/src/trunk-test1/test/ruby/test_keyword.rb:549]:
--- expected
+++ actual
@@ -1 +1 @@
-/The keyword argument is passed as the last hash parameter.* for `m'/m
+""
```

This change ad-hocly adds iseq_unique_id for each iseq, and use it
instead of iseq pointer.  This covers the case where caller is GC'ed.
Still, the case where callee is GC'ed, is not covered.

But anyway, it is very rare that iseq is GC'ed.  Even when it occurs, it
just hides some warnings.  It's no big deal.

compile.c

@@ -10490,7 +10490,7 @@ ibf_load_location_str(const struct ibf_load *load, VALUE str_index)
 static void
 ibf_load_iseq_each(struct ibf_load *load, rb_iseq_t *iseq, ibf_offset_t offset)
 {
-    struct rb_iseq_constant_body *load_body = iseq->body = ZALLOC(struct rb_iseq_constant_body);
+    struct rb_iseq_constant_body *load_body = iseq->body = rb_iseq_constant_body_alloc();
 
     ibf_offset_t reading_pos = offset;
 

iseq.c

@@ -427,11 +427,22 @@ rb_iseq_memsize(const rb_iseq_t *iseq)
     return size;
 }
 
+static unsigned long fresh_iseq_unique_id = 0; /* -- Remove In 3.0 -- */
+
+struct rb_iseq_constant_body *
+rb_iseq_constant_body_alloc(void)
+{
+    struct rb_iseq_constant_body *iseq_body;
+    iseq_body = ZALLOC(struct rb_iseq_constant_body);
+    iseq_body->iseq_unique_id = fresh_iseq_unique_id++; /* -- Remove In 3.0 -- */
+    return iseq_body;
+}
+
 static rb_iseq_t *
 iseq_alloc(void)
 {
     rb_iseq_t *iseq = iseq_imemo_alloc();
-    iseq->body = ZALLOC(struct rb_iseq_constant_body);
+    iseq->body = rb_iseq_constant_body_alloc();
     return iseq;
 }
 

iseq.h

@@ -186,6 +186,7 @@ void rb_iseq_trace_set(const rb_iseq_t *iseq, rb_event_flag_t turnon_events);
 void rb_iseq_trace_set_all(rb_event_flag_t turnon_events);
 void rb_iseq_insns_info_encode_positions(const rb_iseq_t *iseq);
 
+struct rb_iseq_constant_body *rb_iseq_constant_body_alloc(void);
 VALUE rb_iseqw_new(const rb_iseq_t *iseq);
 const rb_iseq_t *rb_iseqw_to_iseq(VALUE iseqw);
 

vm_args.c

@@ -593,8 +593,12 @@ VALUE rb_iseq_location(const rb_iseq_t *iseq);
  */
 static st_table *caller_to_callees = 0;
 
-static VALUE rb_warn_check(const rb_execution_context_t * const ec, const void *const callee)
+static VALUE rb_warn_check(const rb_execution_context_t * const ec, const rb_iseq_t *const iseq)
 {
+    if (!iseq) return 0;
+
+    const void *const callee = (void *)(iseq->body->iseq_unique_id * 2);
+
     const rb_control_frame_t * const cfp = rb_vm_get_ruby_level_next_cfp(ec, ec->cfp);
 
     if (!cfp) return 0;

vm_core.h

@@ -447,6 +447,8 @@ struct rb_iseq_constant_body {
     long unsigned total_calls; /* number of total calls with `mjit_exec()` */
     struct rb_mjit_unit *jit_unit;
 #endif
+
+    unsigned long iseq_unique_id; /* -- Remove In 3.0 -- */
 };
 
 /* T_IMEMO/iseq */