Simplify and speed up string-to-multibyte

* src/character.h (str_to_multibyte):
* src/character.c (str_to_multibyte): Change signature and simplify;
the conversion is no longer done in-place.
* src/fns.c (string_to_multibyte): Drop temporary buffer and memcpy;
adapt to new str_to_multibyte signature.
* src/print.c (print_string): Drop memcpy; adapt call to str_to_multibyte.
* test/src/fns-tests.el (fns--string-to-unibyte): Rename to...
(fns--string-to-unibyte-multibyte): ... this and strengthen, so that
the test covers string-to-multibyte reasonably well.

1 files changed, 7 insertions, 16 deletions

diff --git a/src/fns.c b/src/fns.c
index 61ed01eee4e..7d8f957ef98 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -1237,33 +1237,24 @@ string_make_multibyte (Lisp_Object string)
 
 
 /* Convert STRING (if unibyte) to a multibyte string without changing
-   the number of characters.  Characters 0200 through 0237 are
-   converted to eight-bit characters. */
+   the number of characters.  Characters 0x80..0xff are interpreted as
+   raw bytes. */
 
 Lisp_Object
 string_to_multibyte (Lisp_Object string)
 {
-  unsigned char *buf;
-  ptrdiff_t nbytes;
-  Lisp_Object ret;
-  USE_SAFE_ALLOCA;
-
   if (STRING_MULTIBYTE (string))
     return string;
 
-  nbytes = count_size_as_multibyte (SDATA (string), SBYTES (string));
+  ptrdiff_t nchars = SCHARS (string);
+  ptrdiff_t nbytes = count_size_as_multibyte (SDATA (string), nchars);
   /* If all the chars are ASCII, they won't need any more bytes once
      converted.  */
-  if (nbytes == SBYTES (string))
+  if (nbytes == nchars)
     return make_multibyte_string (SSDATA (string), nbytes, nbytes);
 
-  buf = SAFE_ALLOCA (nbytes);
-  memcpy (buf, SDATA (string), SBYTES (string));
-  str_to_multibyte (buf, nbytes, SBYTES (string));
-
-  ret = make_multibyte_string ((char *) buf, SCHARS (string), nbytes);
-  SAFE_FREE ();
-
+  Lisp_Object ret = make_uninit_multibyte_string (nchars, nbytes);
+  str_to_multibyte (SDATA (ret), SDATA (string), nchars, nbytes);
   return ret;
 }