Make string-lessp vectorisation safer

Use memcpy for loading unaligned words on platforms where this can be
done efficiently.  This guards against problems arising from future
compiler autovectorisation improvements that might cause instructions
that require aligned addresses to be emitted, and should also work
with an address sanitiser enabled.

* src/fns.c (HAVE_FAST_UNALIGNED_ACCESS): Only define when optimising.
(load_unaligned_size_t): New.
(Fstring_lessp): Use load_unaligned_size_t.
* src/lisp.h (UNALIGNED_LOAD_SIZE): Remove now unused macro.

1 files changed, 25 insertions, 18 deletions

diff --git a/src/fns.c b/src/fns.c
index 0af9b725c7a..94505eda444 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -439,17 +439,29 @@ If string STR1 is greater, the value is a positive number N;
 }
 
 /* Check whether the platform allows access to unaligned addresses for
-   size_t integers without trapping or undue penalty (a few cycles is OK).
+   size_t integers without trapping or undue penalty (a few cycles is OK),
+   and that a word-sized memcpy can be used to generate such an access.
 
    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__
+#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__)		\
+  && defined __OPTIMIZE__
 #define HAVE_FAST_UNALIGNED_ACCESS 1
+
+/* Load a word from a possibly unaligned address.  */
+static inline size_t
+load_unaligned_size_t (const void *p)
+{
+  size_t x;
+  memcpy (&x, p, sizeof x);
+  return x;
+}
+
 #else
 #define HAVE_FAST_UNALIGNED_ACCESS 0
 #endif
@@ -497,17 +509,12 @@ Symbols are also allowed; their print names are used instead.  */)
       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)
-	    {
-	      if (UNALIGNED_LOAD_SIZE (w1, b / ws)
-		  != UNALIGNED_LOAD_SIZE (w2, b / ws))
-		break;
-	      b += ws;
-	    }
+	  int ws = sizeof (size_t);
+	  const char *w1 = SSDATA (string1);
+	  const char *w2 = SSDATA (string2);
+	  while (b < nb - ws + 1 &&    load_unaligned_size_t (w1 + b)
+		                    == load_unaligned_size_t (w2 + b))
+	    b += ws;
 	}
 
       /* Scan forward to the differing byte.  */