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Date: Sun, 28 Jun 2015 02:48:34 +0300
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: [PATCH v2 5/6] dynlink.c: compute modulus via magic multiplication
Based on http://ridiculousfish.com/blog/posts/labor-of-division-episode-iii.html
Do a little hand-holding for the compiler and fold magic post-shift into
32-bit high word -> low word shift on 64-bit platforms.
---
src/ldso/dynlink.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 64 insertions(+), 2 deletions(-)
diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
index 1c62efe..f181209 100644
--- a/src/ldso/dynlink.c
+++ b/src/ldso/dynlink.c
@@ -41,6 +41,11 @@ struct td_index {
struct td_index *next;
};
+struct udiv {
+ uint32_t mul;
+ unsigned char s1, s2, inc;
+};
+
struct dso {
unsigned char *base;
char *name;
@@ -54,6 +59,7 @@ struct dso {
Sym *syms;
uint32_t *hashtab;
uint32_t *ghashtab;
+ struct udiv gudiv;
int16_t *versym;
char *strings;
unsigned char *map;
@@ -140,6 +146,60 @@ static int search_vec(size_t *v, size_t *r, size_t key)
return 1;
}
+static void precompute_udiv(uint32_t div, struct udiv *p)
+{
+ if (!(div&(div-1)))
+ return;
+ int bits = 0, s2adj = (sizeof(long) == 8) ? 32 : 0;
+again:
+ p->s1 = bits;
+ uint32_t tmp = 1u<<31, quo = tmp/div, rem = tmp%div;
+
+ int log=0;
+ tmp=div; do log++; while (tmp>>=1);
+
+ int exp, rdshf;
+ uint32_t pow, rdmul=0;
+ for (exp=0, pow=1u<<bits; ; exp++, pow<<=1) {
+ int ovf = rem >= div - rem;
+ quo *= 2; rem *= 2;
+ if (ovf) quo++, rem -= div;
+
+ if (exp >= log-bits || div-rem <= pow)
+ break;
+
+ if (!rdmul && rem <= pow)
+ rdmul = quo, rdshf = exp;
+ }
+ if (exp < log) {
+ p->mul = quo+1;
+ p->s2 = exp + s2adj;
+ p->inc = 0;
+ } else if (div & 1) {
+ p->mul = rdmul;
+ p->s2 = rdshf + s2adj;
+ p->inc = 1;
+ } else {
+ do bits++; while (!((div >>= 1) & 1));
+ goto again;
+ }
+}
+
+static uint32_t umod(uint32_t x, uint32_t div, struct udiv *p)
+{
+ if (!(div&(div-1)))
+ return x&(div-1);
+ uint32_t v = x;
+ v >>= p->s1;
+ if (v + p->inc) v += p->inc;
+ int s32=32, s2=p->s2;
+ if (sizeof(long) == 8)
+ s32=s2, s2=0;
+ v = (1ull * v * p->mul) >> s32;
+ v >>= s2;
+ return x-v*div;
+}
+
static uint32_t sysv_hash(const char *s0)
{
const unsigned char *s = (void *)s0;
@@ -178,7 +238,7 @@ static Sym *gnu_lookup(uint32_t h1, uint32_t *hashtab, struct dso *dso, const ch
{
uint32_t nbuckets = hashtab[0];
uint32_t *buckets = hashtab + 4 + hashtab[2]*(sizeof(size_t)/4);
- uint32_t i = buckets[h1 % nbuckets];
+ uint32_t i = buckets[umod(h1, nbuckets, &dso->gudiv)];
if (!i) return 0;
@@ -696,8 +756,10 @@ static void decode_dyn(struct dso *p)
p->rpath_orig = (void *)(p->strings + dyn[DT_RPATH]);
if (dyn[0]&(1<<DT_RUNPATH))
p->rpath_orig = (void *)(p->strings + dyn[DT_RUNPATH]);
- if (search_vec(p->dynv, dyn, DT_GNU_HASH))
+ if (search_vec(p->dynv, dyn, DT_GNU_HASH)) {
p->ghashtab = (void *)(p->base + *dyn);
+ precompute_udiv(p->ghashtab[0], &p->gudiv);
+ }
if (search_vec(p->dynv, dyn, DT_VERSYM))
p->versym = (void *)(p->base + *dyn);
}
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