Hello!

While profiling a Scheme program, I noticed that ‘string=?’ was
surprisingly high.  I ran OProfile on this Scheme program:

--8<---------------cut here---------------start------------->8---
(define s (make-string 123 #\a))
(let loop ()
  (string= s s)
  (loop))
--8<---------------cut here---------------end--------------->8---

The flat profile was like this:

--8<---------------cut here---------------start------------->8---
samples  %        symbol name
13683    24.6367  scm_i_string_ref
13447    24.2118  compare_strings
8652     15.5782  scm_i_string_chars
4801      8.6444  vm_debug_engine
4535      8.1654  scm_i_str2symbol
2123      3.8225  scm_ihashq
1338      2.4091  scm_fluid_ref
993       1.7879  scm_i_string_hash
750       1.3504  scm_hash_fn_get_handle
616       1.1091  scm_module_variable
445       0.8012  scm_from_locale_stringn
--8<---------------cut here---------------end--------------->8---

I came up with the following patch, which adds a shortcut for the most
common case: