`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, 4 insertions, 2 deletions

diff --git a/method.h b/method.h
index 5b6fe2d800..a5fa187df1 100644
--- a/method.h
+++ b/method.h
@@ -132,8 +132,9 @@ typedef struct rb_iseq_struct rb_iseq_t;
 #endif
 
 typedef struct rb_method_iseq_struct {
-    rb_iseq_t * iseqptr; /*!< iseq pointer, should be separated from iseqval */
+    const rb_iseq_t * iseqptr; /*!< iseq pointer, should be separated from iseqval */
     rb_cref_t * cref;          /*!< class reference, should be marked */
+    const rb_callable_method_entry_t *mandatory_only_cme;
 } rb_method_iseq_t; /* check rb_add_method_iseq() when modify the fields */
 
 typedef struct rb_method_cfunc_struct {
@@ -171,7 +172,8 @@ enum method_optimized_type {
 
 struct rb_method_definition_struct {
     BITFIELD(rb_method_type_t, type, VM_METHOD_TYPE_MINIMUM_BITS);
-    int alias_count : 28;
+    unsigned int iseq_overload: 1;
+    int alias_count : 27;
     int complemented_count : 28;
 
     union {