file.c: Optimize `rb_file_dirname_n` fixed costs

- `str_null_check` was performed twice, once by `FilePathStringValue`
  and a second time by `StringValueCStr`.
- `StringValueCStr` was checking for the terminator presence, but we
  don't care about that.
- `FilePathStringValue` calls `rb_str_new_frozen` to ensure `fname`
  isn't mutated, but that's costly for such a check. Instead we
  can do it in debug mode only.
- `rb_enc_get` is slow because it accepts arbitrary objects, even immediates,
  so it has to do numerous type checks. Add a much faster `rb_str_enc_get`
  when we know we're dealing with a string.
- `rb_enc_copy` is slow for the same reasons, since we already have the
  encoding, we can use `rb_enc_str_new` instead.

1 files changed, 7 insertions, 0 deletions

diff --git a/internal/string.h b/internal/string.h
index cd1e8d7929..dd5e20c0c6 100644
--- a/internal/string.h
+++ b/internal/string.h
@@ -50,6 +50,13 @@ rb_str_enc_fastpath(VALUE str)
     return rb_str_encindex_fastpath(ENCODING_GET_INLINED(str));
 }
 
+static inline rb_encoding *
+rb_str_enc_get(VALUE str)
+{
+    RUBY_ASSERT(RB_TYPE_P(str, T_STRING));
+    return rb_enc_from_index(ENCODING_GET(str));
+}
+
 /* string.c */
 VALUE rb_str_dup_m(VALUE str);
 VALUE rb_fstring(VALUE);