diff options
| -rw-r--r-- | xdiff/xutils.c | 59 |
1 files changed, 55 insertions, 4 deletions
diff --git a/xdiff/xutils.c b/xdiff/xutils.c index e070ed649f..78d1cf74b1 100644 --- a/xdiff/xutils.c +++ b/xdiff/xutils.c @@ -294,16 +294,67 @@ unsigned long xdl_hash_record_with_whitespace(char const **data, return ha; } +/* + * Compiler reassociation barrier: pretend to modify X and Y to disallow + * changing evaluation order with respect to following uses of X and Y. + */ +#ifdef __GNUC__ +#define REASSOC_FENCE(x, y) __asm__("" : "+r"(x), "+r"(y)) +#else +#define REASSOC_FENCE(x, y) +#endif + unsigned long xdl_hash_record_verbatim(char const **data, char const *top) { - unsigned long ha = 5381; + unsigned long ha = 5381, c0, c1; char const *ptr = *data; - +#if 0 + /* + * The baseline form of the optimized loop below. This is the djb2 + * hash (the above function uses a variant with XOR instead of ADD). + */ for (; ptr < top && *ptr != '\n'; ptr++) { ha += (ha << 5); - ha ^= (unsigned long) *ptr; + ha += (unsigned long) *ptr; } *data = ptr < top ? ptr + 1: ptr; - +#else + /* Process two characters per iteration. */ + if (top - ptr >= 2) do { + if ((c0 = ptr[0]) == '\n') { + *data = ptr + 1; + return ha; + } + if ((c1 = ptr[1]) == '\n') { + *data = ptr + 2; + c0 += ha; + REASSOC_FENCE(c0, ha); + ha = ha * 32 + c0; + return ha; + } + /* + * Combine characters C0 and C1 into the hash HA. We have + * HA = (HA * 33 + C0) * 33 + C1, and we want to ensure + * that dependency chain over HA is just one multiplication + * and one addition, i.e. we want to evaluate this as + * HA = HA * 33 * 33 + (C0 * 33 + C1), and likewise prefer + * (C0 * 32 + (C0 + C1)) for the expression in parenthesis. + */ + ha *= 33 * 33; + c1 += c0; + REASSOC_FENCE(c1, c0); + c1 += c0 * 32; + REASSOC_FENCE(c1, ha); + ha += c1; + + ptr += 2; + } while (ptr < top - 1); + *data = top; + if (ptr < top && (c0 = ptr[0]) != '\n') { + c0 += ha; + REASSOC_FENCE(c0, ha); + ha = ha * 32 + c0; + } +#endif return ha; } |