bug#40407: [PATCH] slow ENCODE_FILE and DECODE_FILE

["Mattias Engdegård" <mattiase@acm.org>]

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4 apr. 2020 kl. 19.22 skrev Eli Zaretskii <eliz@gnu.org>:

>> This does not mean that the remaining 179 calls require a copy; they just use the default value of the parameter.
> 
> And IMO the default must stay that a copy is returned, except when the
> caller says otherwise.

Yes, those can be dealt with piecemeal, and we are in no hurry to do so.

> I think in the use case where we return a copy, we should make sure
> the return value is unibyte when encoding and multibyte when decoding.

I'm not necessarily opposed to the suggestion, but why not return a unibyte string in both cases, simplifying the code? In addition, some operations (aref) are faster on unibyte. Either way, it's nothing that a caller could rely on, is there? (In particular when taking NOCOPY into account.)

> Otherwise, I think this is OK (for the master branch, obviously).

Indeed the intention, thanks.

Here is what I would commit, unless you think the string copy should really be multibyte when decoding.


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From 63400cc62506d2c3d9d5f2f27e7bb3bfe7f8f877 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <mattiase@acm.org>
Date: Fri, 3 Apr 2020 16:01:01 +0200
Subject: [PATCH] Avoid expensive recoding for ASCII identity cases (bug#40407)

Optimise for the common case of encoding or decoding an ASCII-only
string using an ASCII-compatible coding, for file names in particular.

* src/coding.c (string_ascii_p): New function.
(code_convert_string): Return the input string for ASCII-only inputs
and ASCII-compatible codings.
* test/src/coding-tests.el (coding-nocopy-ascii): New test.
---
 src/coding.c             | 23 ++++++++++++++++++++++-
 test/src/coding-tests.el | 11 +++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/src/coding.c b/src/coding.c
index 1049f1b755..2425f5952f 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -9471,6 +9471,17 @@ used (which may be different from CODING-SYSTEM if CODING-SYSTEM is
   return code_convert_region (start, end, coding_system, destination, 1, 0);
 }
 
+/* Whether a (unibyte) string only contains chars in the 0..127 range.  */
+static bool
+string_ascii_p (Lisp_Object str)
+{
+  ptrdiff_t nbytes = SBYTES (str);
+  for (ptrdiff_t i = 0; i < nbytes; i++)
+    if (SREF (str, i) > 127)
+      return false;
+  return true;
+}
+
 Lisp_Object
 code_convert_string (Lisp_Object string, Lisp_Object coding_system,
 		     Lisp_Object dst_object, bool encodep, bool nocopy,
@@ -9502,7 +9513,17 @@ code_convert_string (Lisp_Object string, Lisp_Object coding_system,
   chars = SCHARS (string);
   bytes = SBYTES (string);
 
-  if (BUFFERP (dst_object))
+  if (EQ (dst_object, Qt))
+    {
+      /* Fast path for ASCII-only input and an ASCII-compatible coding:
+         act as identity.  */
+      Lisp_Object attrs = CODING_ID_ATTRS (coding.id);
+      if (! NILP (CODING_ATTR_ASCII_COMPAT (attrs))
+          && (STRING_MULTIBYTE (string)
+              ? (chars == bytes) : string_ascii_p (string)))
+	return nocopy ? string : make_unibyte_string (SDATA (string), bytes);
+    }
+  else if (BUFFERP (dst_object))
     {
       struct buffer *buf = XBUFFER (dst_object);
       ptrdiff_t buf_pt = BUF_PT (buf);
diff --git a/test/src/coding-tests.el b/test/src/coding-tests.el
index 110ff12696..93e6709d44 100644
--- a/test/src/coding-tests.el
+++ b/test/src/coding-tests.el
@@ -383,6 +383,17 @@ coding-nocopy-trivial
     (should-not (eq (encode-coding-string s nil nil) s))
     (should (eq (encode-coding-string s nil t) s))))
 
+(ert-deftest coding-nocopy-ascii ()
+  "Check that the NOCOPY parameter works for ASCII-only strings."
+  (let* ((uni (apply #'string (number-sequence 0 127)))
+         (multi (string-to-multibyte uni)))
+    (dolist (s (list uni multi))
+      (dolist (coding '(us-ascii iso-latin-1 utf-8))
+        (should-not (eq (decode-coding-string s coding nil) s))
+        (should-not (eq (encode-coding-string s coding nil) s))
+        (should (eq (decode-coding-string s coding t) s))
+        (should (eq (encode-coding-string s coding t) s))))))
+
 ;; Local Variables:
 ;; byte-compile-warnings: (not obsolete)
 ;; End:
-- 
2.21.1 (Apple Git-122.3)