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further tighten eq-hash code generation

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Avoids including the bit thet indicates whether the object
is GCable in the eq hash code (in a configuration where
bits are available in the GC header for hashing).
This commit is contained in:
Matthew Flatt 2013-10-12 08:33:57 -06:00
parent 8b7b96215b
commit 2a79377c60
1 changed files with 22 additions and 12 deletions
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34
racket/src/racket/src/hash.c
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@ -51,6 +51,22 @@ static void register_traversers(void);
#define to_signed_hash(v) ((intptr_t)v)
#define to_unsigned_hash(v) ((uintptr_t)v)
#ifdef OBJHEAD_HAS_HASH_BITS
/* In 3m mode, we only have 14 bits of hash code in the
Scheme_Object header. But the GC-level object header has some
leftover bits (currently 9, 11, 41, or 43, depending on the
platform), so use those, too. That only works for GCable
objects, so we use 1 of our 14 bits to indicate whether the
other bits are present. */
# define GCABLE_OBJ_HASH_BIT 0x0004
# define OBJ_HASH_USELESS_BITS 3
#else
# define GCABLE_OBJ_HASH_BIT 0
# define OBJ_HASH_USELESS_BITS 2
#endif
#define OBJ_HASH_USEFUL_BITS (16 - OBJ_HASH_USELESS_BITS)
#define OBJ_HASH_USEFUL_MASK ((1 << OBJ_HASH_USEFUL_BITS)-1)
#ifdef MZ_PRECISE_GC
/* keygen race conditions below are "ok", because keygen is randomness
used to create a hashkey. Technically, a race condition allows
@ -59,7 +75,7 @@ static void register_traversers(void);
that only one thread at a time sets a hash code in a specific
object, though, and watch out for a race with JIT-generated code
running in a future and setting flags on pairs. */
SHARED_OK static uintptr_t keygen;
SHARED_OK static uintptr_t keygen = GCABLE_OBJ_HASH_BIT;
XFORM_NONGCING static MZ_INLINE
uintptr_t PTR_TO_LONG(Scheme_Object *o)
@ -76,17 +92,11 @@ uintptr_t PTR_TO_LONG(Scheme_Object *o)
uintptr_t local_keygen = keygen;
v |= (short)local_keygen;
#ifdef OBJHEAD_HAS_HASH_BITS
/* In 3m mode, we only have 14 bits of hash code in the
Scheme_Object header. But the GC-level object header has some
leftover bits (currently 9, 11, 41, or 43, depending on the
platform), so use those, too. That only works for GCable
objects, so we use 1 of our 14 bits to indicate whether the
other bit are present. */
if (GC_is_allocated(o)) {
OBJHEAD_HASH_BITS(o) = (local_keygen >> 16);
v |= 0x4000;
v |= GCABLE_OBJ_HASH_BIT;
} else
v &= ~0x4000;
v &= ~GCABLE_OBJ_HASH_BIT;
#endif
if (!v) v = 0x1AD0;
#ifdef MZ_USE_FUTURES
@ -99,11 +109,11 @@ uintptr_t PTR_TO_LONG(Scheme_Object *o)
} else
#endif
o->keyex = v;
keygen += 4;
keygen += (1 << OBJ_HASH_USELESS_BITS);
}
#ifdef OBJHEAD_HAS_HASH_BITS
if (v & 0x4000)
if (v & GCABLE_OBJ_HASH_BIT)
bits = OBJHEAD_HASH_BITS(o);
else
#endif
@ -112,7 +122,7 @@ uintptr_t PTR_TO_LONG(Scheme_Object *o)
/* We need to drop the low two bits of `v', which
are used for non-hashing purposes in some types. */
return (bits << 14) | ((v >> 2) & 0x3FFF);
return (bits << OBJ_HASH_USEFUL_BITS) | ((v >> OBJ_HASH_USELESS_BITS) & OBJ_HASH_USEFUL_MASK);
}
#else
# define PTR_TO_LONG(p) ((uintptr_t)(p)>>2)