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Fix power-of-two integer hash collisions #1720

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Merged
bnoordhuis merged 1 commit into from
Sep 9, 2026
Merged

Fix power-of-two integer hash collisions #1720

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23 changes: 23 additions & 0 deletions cutils.h
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Original file line number Diff line number Diff line change
Expand Up @@ -206,6 +206,20 @@ static inline uint32_t hash32(uint32_t a)
return a * 0x61c88647;
}

static inline uint64_t hash64(uint64_t a)
{
return a * 0x61c8864680b583ebULL;

@saghul saghul Sep 8, 2026

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Just curious: how is this number picked? By fair dice roll? 😅

@bnoordhuis bnoordhuis Sep 9, 2026

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There's a comment a few lines up that explains it, it's based on the golden ratio. hash32 uses the same constant, except truncated to 32 bits.

}

static inline uintptr_t hash_uintptr(uintptr_t a)
{
if (sizeof(uintptr_t) == sizeof(uint32_t)) {
return hash32(a);
} else {
return hash64(a);
}
}

/* WARNING: undefined if a = 0 */
static inline int clz32(unsigned int a)
{
Expand Down Expand Up @@ -237,6 +251,15 @@ static inline int clz64(uint64_t a)
#endif
}

static inline uintptr_t clz_uintptr(uintptr_t a)
{
if (sizeof(uintptr_t) == sizeof(uint32_t)) {
return clz32(a);
} else {
return clz64(a);
}
}

/* WARNING: undefined if a = 0 */
static inline int ctz32(unsigned int a)
{
Expand Down
19 changes: 12 additions & 7 deletions quickjs.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5601,7 +5601,12 @@ static int init_shape_hash(JSRuntime *rt)
return 0;
}

/* same magic hash multiplier as the Linux kernel */
static uintptr_t hash_atom(JSAtom atom, uintptr_t mask)
{
int shift = 1 + clz_uintptr(mask+1);
return hash_uintptr(atom) >> shift;
}

static uint32_t shape_hash(uint32_t h, uint32_t val)
{
return hash32(h + val);
Expand Down Expand Up @@ -5881,7 +5886,7 @@ static no_inline int resize_properties(JSContext *ctx, JSShape **psh,
sizeof(prop_hash_end(sh)[0]) * new_hash_size);
for(i = 0, pr = get_shape_prop(sh); i < sh->prop_count; i++, pr++) {
if (pr->atom != JS_ATOM_NULL) {
h = ((uintptr_t)pr->atom & new_hash_mask);
h = hash_atom(pr->atom, new_hash_mask);
pr->hash_next = prop_hash_end(sh)[-h - 1];
prop_hash_end(sh)[-h - 1] = i + 1;
}
Expand Down Expand Up @@ -5956,7 +5961,7 @@ static int compact_properties(JSContext *ctx, JSObject *p)
if (old_pr->atom != JS_ATOM_NULL) {
pr->atom = old_pr->atom;
pr->flags = old_pr->flags;
h = ((uintptr_t)old_pr->atom & new_hash_mask);
h = hash_atom(old_pr->atom, new_hash_mask);
pr->hash_next = prop_hash_end(sh)[-h - 1];
prop_hash_end(sh)[-h - 1] = j + 1;
prop[j] = prop[i];
Expand Down Expand Up @@ -6018,7 +6023,7 @@ static int add_shape_property(JSContext *ctx, JSShape **psh,
pr->flags = prop_flags;
/* add in hash table */
hash_mask = sh->prop_hash_mask;
h = atom & hash_mask;
h = hash_atom(atom, hash_mask);
pr->hash_next = prop_hash_end(sh)[-h - 1];
prop_hash_end(sh)[-h - 1] = sh->prop_count;
return 0;
Expand Down Expand Up @@ -6810,7 +6815,7 @@ static inline JSShapeProperty *find_own_property1(JSObject *p, JSAtom atom)
JSShapeProperty *pr, *prop;
intptr_t h;
sh = p->shape;
h = (uintptr_t)atom & sh->prop_hash_mask;
h = hash_atom(atom, sh->prop_hash_mask);
h = prop_hash_end(sh)[-h - 1];
prop = get_shape_prop(sh);
while (h) {
Expand All @@ -6831,7 +6836,7 @@ static inline JSShapeProperty *find_own_property(JSProperty **ppr,
JSShapeProperty *pr, *prop;
intptr_t h;
sh = p->shape;
h = (uintptr_t)atom & sh->prop_hash_mask;
h = hash_atom(atom, sh->prop_hash_mask);
h = prop_hash_end(sh)[-h - 1];
prop = get_shape_prop(sh);
while (h) {
Expand Down Expand Up @@ -10237,7 +10242,7 @@ static int delete_property(JSContext *ctx, JSObject *p, JSAtom atom)

redo:
sh = p->shape;
h1 = atom & sh->prop_hash_mask;
h1 = hash_atom(atom, sh->prop_hash_mask);
h = prop_hash_end(sh)[-h1 - 1];
prop = get_shape_prop(sh);
lpr = NULL;
Expand Down
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