Allow ENCODE_FILE and DECODE_FILE to use no-copy conversion

They already did return their argument under some circumstances;
this change broadens it to further reduce allocation in common cases
(bug#40407).

* src/coding.c (convert_string_nocopy): New function.
(decode_file_name, encode_file_name): Use convert_string_nocopy.
* src/coding.h (ENCODE_FILE, DECODE_FILE): Note the nocopy semantics.

2 files changed, 19 insertions, 14 deletions

diff --git a/src/coding.c b/src/coding.c
index 9848f983a81..0daa390bc85 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -9559,10 +9559,7 @@ code_convert_string (Lisp_Object string, Lisp_Object coding_system,
 
 
 /* Encode or decode STRING according to CODING_SYSTEM.
-   Do not set Vlast_coding_system_used.
-
-   This function is called only from macros DECODE_FILE and
-   ENCODE_FILE, thus we ignore character composition.  */
+   Do not set Vlast_coding_system_used.  */
 
 Lisp_Object
 code_convert_string_norecord (Lisp_Object string, Lisp_Object coding_system,
@@ -10332,6 +10329,16 @@ DEFUN ("internal-decode-string-utf-8", Finternal_decode_string_utf_8,
 
 #endif  /* ENABLE_UTF_8_CONVERTER_TEST */
 
+/* Encode or decode STRING using CODING_SYSTEM, with the possibility of
+   returning STRING itself if it equals the result.
+   Do not set Vlast_coding_system_used.  */
+static Lisp_Object
+convert_string_nocopy (Lisp_Object string, Lisp_Object coding_system,
+                       bool encodep)
+{
+  return code_convert_string (string, coding_system, Qt, encodep, 1, 1);
+}
+
 /* Encode or decode a file name, to or from a unibyte string suitable
    for passing to C library functions.  */
 Lisp_Object
@@ -10342,14 +10349,13 @@ decode_file_name (Lisp_Object fname)
      converts the file names either to UTF-16LE or to the system ANSI
      codepage internally, depending on the underlying OS; see w32.c.  */
   if (! NILP (Fcoding_system_p (Qutf_8)))
-    return code_convert_string_norecord (fname, Qutf_8, 0);
+    return convert_string_nocopy (fname, Qutf_8, 0);
   return fname;
 #else  /* !WINDOWSNT */
   if (! NILP (Vfile_name_coding_system))
-    return code_convert_string_norecord (fname, Vfile_name_coding_system, 0);
+    return convert_string_nocopy (fname, Vfile_name_coding_system, 0);
   else if (! NILP (Vdefault_file_name_coding_system))
-    return code_convert_string_norecord (fname,
-                                         Vdefault_file_name_coding_system, 0);
+    return convert_string_nocopy (fname, Vdefault_file_name_coding_system, 0);
   else
     return fname;
 #endif
@@ -10369,14 +10375,13 @@ encode_file_name (Lisp_Object fname)
      converts the file names either to UTF-16LE or to the system ANSI
      codepage internally, depending on the underlying OS; see w32.c.  */
   if (! NILP (Fcoding_system_p (Qutf_8)))
-    return code_convert_string_norecord (fname, Qutf_8, 1);
+    return convert_string_nocopy (fname, Qutf_8, 1);
   return fname;
 #else  /* !WINDOWSNT */
   if (! NILP (Vfile_name_coding_system))
-    return code_convert_string_norecord (fname, Vfile_name_coding_system, 1);
+    return convert_string_nocopy (fname, Vfile_name_coding_system, 1);
   else if (! NILP (Vdefault_file_name_coding_system))
-    return code_convert_string_norecord (fname,
-                                         Vdefault_file_name_coding_system, 1);
+    return convert_string_nocopy (fname, Vdefault_file_name_coding_system, 1);
   else
     return fname;
 #endif
diff --git a/src/coding.h b/src/coding.h
index 91856c5702b..c2a7b2a00ff 100644
--- a/src/coding.h
+++ b/src/coding.h
@@ -642,11 +642,11 @@ struct coding_system
   } while (false)
 
 /* Encode the file name NAME using the specified coding system
-   for file names, if any.  */
+   for file names, if any.  May return NAME itself.  */
 #define ENCODE_FILE(NAME)  encode_file_name (NAME)
 
 /* Decode the file name NAME using the specified coding system
-   for file names, if any.  */
+   for file names, if any.  May return NAME itself.  */
 #define DECODE_FILE(NAME)  decode_file_name (NAME)
 
 /* Encode the string STR using the specified coding system