From a4bbe8af0b48f9c80ccc2c4619309c4a81c1460a Mon Sep 17 00:00:00 2001 From: Alexander Monakov Date: Mon, 28 Jul 2025 22:05:20 +0300 Subject: [PATCH] xdiff: optimize xdl_hash_record_verbatim xdl_hash_record_verbatim uses modified djb2 hash with XOR instead of ADD for combining. The ADD-based variant is used as the basis of the modern ("GNU") symbol lookup scheme in ELF. Glibc dynamic loader received an optimized version of this hash function thanks to Noah Goldstein [1]. Switch xdl_hash_record_verbatim to additive hashing and implement an optimized loop following the scheme suggested by Noah. Timing 'git log --oneline --shortstat v2.0.0..v2.5.0' under perf, I got version | cycles, bn | instructions, bn --------------------------------------- A 6.38 11.3 B 6.21 10.89 C 5.80 9.95 D 5.83 8.74 --------------------------------------- A: baseline (git master at e4ef0485fd78) B: plus 'xdiff: refactor xdl_hash_record()' C: and plus this patch D: with 'xdiff: use xxhash' by Phillip Wood The resulting speedup for xdl_hash_record_verbatim itself is about 1.5x. [1] https://inbox.sourceware.org/libc-alpha/20220519221803.57957-6-goldstein.w.n@gmail.com/ Signed-off-by: Alexander Monakov Signed-off-by: Junio C Hamano --- xdiff/xutils.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++---- 1 file 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; } -unsigned long xdl_hash_record_verbatim(char const **data, char const *top) { - unsigned long ha = 5381; - char const *ptr = *data; +/* + * 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, 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; }