`Primitive.mandatory_only?` for fast path

Compare with the C methods, A built-in methods written in Ruby is
slower if only mandatory parameters are given because it needs to
check the argumens and fill default values for optional and keyword
parameters (C methods can check the number of parameters with `argc`,
so there are no overhead). Passing mandatory arguments are common
(optional arguments are exceptional, in many cases) so it is important
to provide the fast path for such common cases.

`Primitive.mandatory_only?` is a special builtin function used with
`if` expression like that:

```ruby
  def self.at(time, subsec = false, unit = :microsecond, in: nil)
    if Primitive.mandatory_only?
      Primitive.time_s_at1(time)
    else
      Primitive.time_s_at(time, subsec, unit, Primitive.arg!(:in))
    end
  end
```

and it makes two ISeq,

```
  def self.at(time, subsec = false, unit = :microsecond, in: nil)
    Primitive.time_s_at(time, subsec, unit, Primitive.arg!(:in))
  end

  def self.at(time)
    Primitive.time_s_at1(time)
  end
```

and (2) is pointed by (1). Note that `Primitive.mandatory_only?`
should be used only in a condition of an `if` statement and the
`if` statement should be equal to the methdo body (you can not
put any expression before and after the `if` statement).

A method entry with `mandatory_only?` (`Time.at` on the above case)
is marked as `iseq_overload`. When the method will be dispatch only
with mandatory arguments (`Time.at(0)` for example), make another
method entry with ISeq (2) as mandatory only method entry and it
will be cached in an inline method cache.

The idea is similar discussed in https://bugs.ruby-lang.org/issues/16254
but it only checks mandatory parameters or more, because many cases
only mandatory parameters are given. If we find other cases (optional
or keyword parameters are used frequently and it hurts performance),
we can extend the feature.

1 files changed, 15 insertions, 4 deletions

diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index b9950f4fe2..8e0793e070 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -678,7 +678,7 @@ rb_vm_frame_method_entry(const rb_control_frame_t *cfp)
     return check_method_entry(ep[VM_ENV_DATA_INDEX_ME_CREF], TRUE);
 }
 
-static rb_iseq_t *
+static const rb_iseq_t *
 method_entry_iseqptr(const rb_callable_method_entry_t *me)
 {
     switch (me->def->type) {
@@ -1754,13 +1754,16 @@ vm_ccs_verify(struct rb_class_cc_entries *ccs, ID mid, VALUE klass)
         VM_ASSERT(vm_ci_mid(ci) == mid);
         VM_ASSERT(IMEMO_TYPE_P(cc, imemo_callcache));
         VM_ASSERT(vm_cc_class_check(cc, klass));
-        VM_ASSERT(vm_cc_cme(cc) == ccs->cme);
+        VM_ASSERT(vm_cc_check_cme(cc, ccs->cme));
     }
     return TRUE;
 }
 #endif
 
 #ifndef MJIT_HEADER
+
+static const rb_callable_method_entry_t *overloaded_cme(const rb_callable_method_entry_t *cme);
+
 static const struct rb_callcache *
 vm_search_cc(const VALUE klass, const struct rb_callinfo * const ci)
 {
@@ -1829,7 +1832,6 @@ vm_search_cc(const VALUE klass, const struct rb_callinfo * const ci)
 
     VM_ASSERT(cme == rb_callable_method_entry(klass, mid));
 
-    const struct rb_callcache *cc = vm_cc_new(klass, cme, vm_call_general);
     METHOD_ENTRY_CACHED_SET((struct rb_callable_method_entry_struct *)cme);
 
     if (ccs == NULL) {
@@ -1846,6 +1848,14 @@ vm_search_cc(const VALUE klass, const struct rb_callinfo * const ci)
         }
     }
 
+    if ((cme->def->iseq_overload &&
+         (int)vm_ci_argc(ci) == method_entry_iseqptr(cme)->body->param.lead_num)) {
+        // use alternative
+        cme = overloaded_cme(cme);
+        METHOD_ENTRY_CACHED_SET((struct rb_callable_method_entry_struct *)cme);
+        // rp(cme);
+    }
+    const struct rb_callcache *cc = vm_cc_new(klass, cme, vm_call_general);
     vm_ccs_push(klass, ccs, ci, cc);
 
     VM_ASSERT(vm_cc_cme(cc) != NULL);
@@ -3529,9 +3539,10 @@ vm_call_method_each_type(rb_execution_context_t *ec, rb_control_frame_t *cfp, st
 {
     const struct rb_callinfo *ci = calling->ci;
     const struct rb_callcache *cc = calling->cc;
+    const rb_callable_method_entry_t *cme = vm_cc_cme(cc);
     VALUE v;
 
-    switch (vm_cc_cme(cc)->def->type) {
+    switch (cme->def->type) {
       case VM_METHOD_TYPE_ISEQ:
         CC_SET_FASTPATH(cc, vm_call_iseq_setup, TRUE);
         return vm_call_iseq_setup(ec, cfp, calling);