`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, 77 insertions, 0 deletions

diff --git a/compile.c b/compile.c
index 9d3b91beb7..9f31bcfd1b 100644
--- a/compile.c
+++ b/compile.c
@@ -791,8 +791,10 @@ rb_iseq_compile_node(rb_iseq_t *iseq, const NODE *node)
 	    }
 	  case ISEQ_TYPE_METHOD:
 	    {
+                ISEQ_COMPILE_DATA(iseq)->root_node = node->nd_body;
 		ADD_TRACE(ret, RUBY_EVENT_CALL);
 		CHECK(COMPILE(ret, "scoped node", node->nd_body));
+                ISEQ_COMPILE_DATA(iseq)->root_node = node->nd_body;
 		ADD_TRACE(ret, RUBY_EVENT_RETURN);
 		ISEQ_COMPILE_DATA(iseq)->last_line = nd_line(node);
 		break;
@@ -7884,6 +7886,65 @@ compile_builtin_arg(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, c
     UNKNOWN_NODE("arg!", node, COMPILE_NG);
 }
 
+static NODE *
+mandatory_node(const rb_iseq_t *iseq, const NODE *cond_node)
+{
+    const NODE *node = ISEQ_COMPILE_DATA(iseq)->root_node;
+    if (nd_type(node) == NODE_IF && node->nd_cond == cond_node) {
+        return node->nd_body;
+    }
+    else {
+        rb_bug("mandatory_node: can't find mandatory node");
+    }
+}
+
+static int
+compile_builtin_mandatory_only_method(rb_iseq_t *iseq, const NODE *node, const NODE *line_node)
+{
+    // argumens
+    struct rb_args_info args = {
+        .pre_args_num = iseq->body->param.lead_num,
+    };
+    NODE args_node;
+    rb_node_init(&args_node, NODE_ARGS, 0, 0, (VALUE)&args);
+
+    // local table without non-mandatory parameters
+    const int skip_local_size = iseq->body->param.size - iseq->body->param.lead_num;
+    const int table_size = iseq->body->local_table_size - skip_local_size;
+    ID *tbl = ALLOCA_N(ID, table_size + 1);
+    tbl[0] = table_size;
+    int i;
+
+    // lead parameters
+    for (i=0; i<iseq->body->param.lead_num; i++) {
+        tbl[i+1] = iseq->body->local_table[i];
+    }
+    // local variables
+    for (; i<table_size; i++) {
+        tbl[i+1] = iseq->body->local_table[i + skip_local_size];
+    }
+
+    NODE scope_node;
+    rb_node_init(&scope_node, NODE_SCOPE, (VALUE)tbl, (VALUE)mandatory_node(iseq, node), (VALUE)&args_node);
+
+    rb_ast_body_t ast = {
+        .root = &scope_node,
+        .compile_option = 0,
+        .script_lines = iseq->body->variable.script_lines,
+    };
+
+    int prev_inline_index = GET_VM()->builtin_inline_index;
+
+    iseq->body->mandatory_only_iseq =
+      rb_iseq_new_with_opt(&ast, rb_iseq_base_label(iseq),
+                           rb_iseq_path(iseq), rb_iseq_realpath(iseq),
+                           INT2FIX(nd_line(line_node)), NULL, 0,
+                           ISEQ_TYPE_METHOD, ISEQ_COMPILE_DATA(iseq)->option);
+
+    GET_VM()->builtin_inline_index = prev_inline_index;
+    return COMPILE_OK;
+}
+
 static int
 compile_builtin_function_call(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const node, const NODE *line_node, int popped,
                               const rb_iseq_t *parent_block, LINK_ANCHOR *args, const char *builtin_func)
@@ -7922,6 +7983,18 @@ compile_builtin_function_call(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NOD
             else if (strcmp("arg!", builtin_func) == 0) {
                 return compile_builtin_arg(iseq, ret, args_node, line_node, popped);
             }
+            else if (strcmp("mandatory_only?", builtin_func) == 0) {
+                if (popped) {
+                    rb_bug("mandatory_only? should be in if condition");
+                }
+                else if (!LIST_INSN_SIZE_ZERO(ret)) {
+                    rb_bug("mandatory_only? should be put on top");
+                }
+
+                ADD_INSN1(ret, line_node, putobject, Qfalse);
+                return compile_builtin_mandatory_only_method(iseq, node, line_node);
+                return COMPILE_OK;
+            }
             else if (1) {
                 rb_bug("can't find builtin function:%s", builtin_func);
             }
@@ -11628,6 +11701,7 @@ ibf_dump_iseq_each(struct ibf_dump *dump, const rb_iseq_t *iseq)
     const ibf_offset_t catch_table_offset = ibf_dump_catch_table(dump, iseq);
     const int parent_iseq_index =           ibf_dump_iseq(dump, iseq->body->parent_iseq);
     const int local_iseq_index =            ibf_dump_iseq(dump, iseq->body->local_iseq);
+    const int mandatory_only_iseq_index =   ibf_dump_iseq(dump, iseq->body->mandatory_only_iseq);
     const ibf_offset_t ci_entries_offset =  ibf_dump_ci_entries(dump, iseq);
     const ibf_offset_t outer_variables_offset = ibf_dump_outer_variables(dump, iseq);
 
@@ -11690,6 +11764,7 @@ ibf_dump_iseq_each(struct ibf_dump *dump, const rb_iseq_t *iseq)
     ibf_dump_write_small_value(dump, IBF_BODY_OFFSET(catch_table_offset));
     ibf_dump_write_small_value(dump, parent_iseq_index);
     ibf_dump_write_small_value(dump, local_iseq_index);
+    ibf_dump_write_small_value(dump, mandatory_only_iseq_index);
     ibf_dump_write_small_value(dump, IBF_BODY_OFFSET(ci_entries_offset));
     ibf_dump_write_small_value(dump, IBF_BODY_OFFSET(outer_variables_offset));
     ibf_dump_write_small_value(dump, body->variable.flip_count);
@@ -11797,6 +11872,7 @@ ibf_load_iseq_each(struct ibf_load *load, rb_iseq_t *iseq, ibf_offset_t offset)
     const ibf_offset_t catch_table_offset = (ibf_offset_t)IBF_BODY_OFFSET(ibf_load_small_value(load, &reading_pos));
     const int parent_iseq_index = (int)ibf_load_small_value(load, &reading_pos);
     const int local_iseq_index = (int)ibf_load_small_value(load, &reading_pos);
+    const int mandatory_only_iseq_index = (int)ibf_load_small_value(load, &reading_pos);
     const ibf_offset_t ci_entries_offset = (ibf_offset_t)IBF_BODY_OFFSET(ibf_load_small_value(load, &reading_pos));
     const ibf_offset_t outer_variables_offset = (ibf_offset_t)IBF_BODY_OFFSET(ibf_load_small_value(load, &reading_pos));
     const rb_snum_t variable_flip_count = (rb_snum_t)ibf_load_small_value(load, &reading_pos);
@@ -11859,6 +11935,7 @@ ibf_load_iseq_each(struct ibf_load *load, rb_iseq_t *iseq, ibf_offset_t offset)
     load_body->catch_table          = ibf_load_catch_table(load, catch_table_offset, catch_table_size);
     load_body->parent_iseq          = ibf_load_iseq(load, (const rb_iseq_t *)(VALUE)parent_iseq_index);
     load_body->local_iseq           = ibf_load_iseq(load, (const rb_iseq_t *)(VALUE)local_iseq_index);
+    load_body->mandatory_only_iseq  = ibf_load_iseq(load, (const rb_iseq_t *)(VALUE)mandatory_only_iseq_index);
 
     ibf_load_code(load, iseq, bytecode_offset, bytecode_size, iseq_size);
 #if VM_INSN_INFO_TABLE_IMPL == 2