Re: HAVE_FAST_UNALIGNED_ACCESS

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

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1 apr. 2023 kl. 08.39 skrev Eli Zaretskii <eliz@gnu.org>:

> I don't want to make changes on the release branch without a positive
> evidence that the existing code causes verifiable problems.  And even
> then I prefer to make changes only for platforms where those problems
> happen.

I'm not overly worried about the code, but to put this matter to rest, what about simply dropping the vectorisation altogether, on all platforms, in Emacs 29? That should be very safe.


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diff --git a/src/fns.c b/src/fns.c
index ff364c65e26..95c4d2c028c 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -438,22 +438,6 @@ DEFUN ("compare-strings", Fcompare_strings, Scompare_strings, 6, 7, 0,
   return Qt;
 }
 
-/* Check whether the platform allows access to unaligned addresses for
-   size_t integers without trapping or undue penalty (a few cycles is OK).
-
-   This whitelist is incomplete but since it is only used to improve
-   performance, omitting cases is safe.  */
-#if defined __x86_64__|| defined __amd64__	\
-    || defined __i386__ || defined __i386	\
-    || defined __arm64__ || defined __aarch64__	\
-    || defined __powerpc__ || defined __powerpc	\
-    || defined __ppc__ || defined __ppc		\
-    || defined __s390__ || defined __s390x__
-#define HAVE_FAST_UNALIGNED_ACCESS 1
-#else
-#define HAVE_FAST_UNALIGNED_ACCESS 0
-#endif
-
 DEFUN ("string-lessp", Fstring_lessp, Sstring_lessp, 2, 2, 0,
        doc: /* Return non-nil if STRING1 is less than STRING2 in lexicographic order.
 Case is significant.
@@ -491,20 +475,6 @@ DEFUN ("string-lessp", Fstring_lessp, Sstring_lessp, 2, 2, 0,
       ptrdiff_t nb = min (nb1, nb2);
       ptrdiff_t b = 0;
 
-      /* String data is normally allocated with word alignment, but
-	 there are exceptions (notably pure strings) so we restrict the
-	 wordwise skipping to safe architectures.  */
-      if (HAVE_FAST_UNALIGNED_ACCESS)
-	{
-	  /* First compare entire machine words.  */
-	  typedef size_t word_t;
-	  int ws = sizeof (word_t);
-	  const word_t *w1 = (const word_t *) SDATA (string1);
-	  const word_t *w2 = (const word_t *) SDATA (string2);
-	  while (b < nb - ws + 1 && w1[b / ws] == w2[b / ws])
-	    b += ws;
-	}
-
       /* Scan forward to the differing byte.  */
       while (b < nb && SREF (string1, b) == SREF (string2, b))
 	b++;