Add specialization for String#to_s on plain strings

When calling "to_s" on an instance of String, the method simply returns
self. In this situation most of the work comes from setting up the
method call. It turns out that both railsbench and liquid-render do this
a lot.

When generating code for opt_send_without_block, we already generate a
known class guard, so we can detect when the receiver is a String
instance. Since gen_send_cfunc() is also used for gen_invokesuper(), and
gen_invokesuper() doesn't generate a known class guard, a new nullable
parameter for specialized codegen function is added.

Closes GH-245

1 files changed, 35 insertions, 0 deletions

diff --git a/bootstraptest/test_yjit.rb b/bootstraptest/test_yjit.rb
index 73b87e17fd..91c39323e0 100644
--- a/bootstraptest/test_yjit.rb
+++ b/bootstraptest/test_yjit.rb
@@ -2038,3 +2038,38 @@ assert_equal '42',  %q{
 
   Ractor.new { A.foo }.take
 }
+
+assert_equal '["plain", "special", "sub", "plain"]', %q{
+  def foo(arg)
+    arg.to_s
+  end
+
+  class Sub < String
+  end
+
+  special = String.new("special")
+  special.singleton_class
+
+  [
+    foo("plain"),
+    foo(special),
+    foo(Sub.new("sub")),
+    foo("plain")
+  ]
+}
+
+assert_equal '["sub", "sub"]', %q{
+  def foo(arg)
+    arg.to_s
+  end
+
+  class Sub < String
+    def to_s
+      super
+    end
+  end
+
+  sub = Sub.new("sub")
+
+  [foo(sub), foo(sub)]
+}