diff options
Diffstat (limited to 'src/fns.c')
| -rw-r--r-- | src/fns.c | 41 |
1 files changed, 31 insertions, 10 deletions
diff --git a/src/fns.c b/src/fns.c index bc4915eb25b..40557923827 100644 --- a/src/fns.c +++ b/src/fns.c @@ -433,6 +433,22 @@ If string STR1 is greater, the value is a positive number N; 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. @@ -468,18 +484,23 @@ Symbols are also allowed; their print names are used instead. */) ptrdiff_t nb1 = SBYTES (string1); ptrdiff_t nb2 = SBYTES (string2); ptrdiff_t nb = min (nb1, nb2); - - /* First compare entire machine words. (String data is allocated - with word alignment.) */ - 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); ptrdiff_t b = 0; - while (b < nb - ws + 1 && w1[b / ws] == w2[b / ws]) - b += ws; - /* Scan forward to the differing byte (at most ws-1 bytes). */ + /* 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++; |