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Avoid overexposing fixnums for hash codes

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Following a suggestion by Stefan Monnier in:
https://lists.gnu.org/r/emacs-devel/2019-07/msg00530.html
* doc/lispref/hash.texi (Creating Hash, Defining Hash):
* src/fns.c (Fsxhash_eq, Fsxhash_eql, Fsxhash_equal, Fmake_hash_table):
Don’t insist that hash codes be fixnums, reverting
the recent doc changes to the contrary.
* src/bytecode.c (exec_byte_code): Special-case only the eq case,
as the others aren’t worth tuning now that we treat bignum hashes
like fixnums.
* src/fns.c (hashfn_user_defined): If the hash code is a bignum,
reduce its hash down to a fixnum.
This commit is contained in:
Paul Eggert 2019-07-22 21:27:33 -07:00
parent 97477edaf2
commit f378ed1a0b
3 changed files with 19 additions and 23 deletions
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16
doc/lispref/hash.texi
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@ -132,7 +132,7 @@ When you add an association to a hash table and the table is full,
it grows automatically. This value specifies how to make the hash table
larger, at that time.
If @var{rehash-size} is a fixnum, it should be positive and the hash
If @var{rehash-size} is an integer, it should be positive, and the hash
table grows by adding approximately that much to the nominal size. If
@var{rehash-size} is floating point, it had better be greater
than 1, and the hash table grows by multiplying the old size by
@ -239,8 +239,8 @@ to understand how hash tables work, and what a @dfn{hash code} means.
You can think of a hash table conceptually as a large array of many
slots, each capable of holding one association. To look up a key,
@code{gethash} first computes a fixnum, the hash code, from the key.
It reduces this fixnum modulo the length of the array, to produce an
@code{gethash} first computes an integer, the hash code, from the key.
It can reduce this integer modulo the length of the array, to produce an
index in the array. Then it looks in that slot, and if necessary in
other nearby slots, to see if it has found the key being sought.
@ -265,7 +265,7 @@ The function @var{test-fn} should accept two arguments, two keys, and
return non-@code{nil} if they are considered the same.
The function @var{hash-fn} should accept one argument, a key, and return
a fixnum that is the hash code of that key. For good results, the
an integer that is the hash code of that key. For good results, the
function should use the whole range of fixnums for hash codes,
including negative fixnums.
@ -276,12 +276,12 @@ under the property @code{hash-table-test}; the property value's form is
@defun sxhash-equal obj
This function returns a hash code for Lisp object @var{obj}.
This is a fixnum that reflects the contents of @var{obj}
This is an integer that reflects the contents of @var{obj}
and the other Lisp objects it points to.
If two objects @var{obj1} and @var{obj2} are @code{equal}, then
@code{(sxhash-equal @var{obj1})} and @code{(sxhash-equal @var{obj2})}
are the same fixnum.
are the same integer.
If the two objects are not @code{equal}, the values returned by
@code{sxhash-equal} are usually different, but not always; once in a
@ -299,7 +299,7 @@ result reflects identity of @var{obj}, but not its contents.
If two objects @var{obj1} and @var{obj2} are @code{eq}, then
@code{(sxhash-eq @var{obj1})} and @code{(sxhash-eq @var{obj2})} are
the same fixnum.
the same integer.
@end defun
@defun sxhash-eql obj
@ -310,7 +310,7 @@ in which case a hash code is generated for the value.
If two objects @var{obj1} and @var{obj2} are @code{eql}, then
@code{(sxhash-eql @var{obj1})} and @code{(sxhash-eql @var{obj2})} are
the same fixnum.
the same integer.
@end defun
This example creates a hash table whose keys are strings that are

14
src/bytecode.c
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@ -1406,18 +1406,12 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
/* h->count is a faster approximation for HASH_TABLE_SIZE (h)
here. */
if (h->count <= 5)
if (h->count <= 5 && !h->test.cmpfn)
{ /* Do a linear search if there are not many cases
FIXME: 5 is arbitrarily chosen. */
Lisp_Object hash_code
= h->test.cmpfn ? h->test.hashfn (v1, h) : Qnil;
for (i = h->count; 0 <= --i; )
if (EQ (v1, HASH_KEY (h, i))
|| (h->test.cmpfn
&& EQ (hash_code, HASH_HASH (h, i))
&& !NILP (h->test.cmpfn (v1, HASH_KEY (h, i), h))))
break;
for (i = h->count; 0 <= --i; )
if (EQ (v1, HASH_KEY (h, i)))
break;
}
else
i = hash_lookup (h, v1, NULL);

12
src/fns.c
View File

@ -47,6 +47,7 @@ static void sort_vector_copy (Lisp_Object, ptrdiff_t,
enum equal_kind { EQUAL_NO_QUIT, EQUAL_PLAIN, EQUAL_INCLUDING_PROPERTIES };
static bool internal_equal (Lisp_Object, Lisp_Object,
enum equal_kind, int, Lisp_Object);
static EMACS_UINT sxhash_bignum (struct Lisp_Bignum *);
DEFUN ("identity", Fidentity, Sidentity, 1, 1, 0,
doc: /* Return the argument unchanged. */
@ -4021,7 +4022,8 @@ Lisp_Object
hashfn_user_defined (Lisp_Object key, struct Lisp_Hash_Table *h)
{
Lisp_Object args[] = { h->test.user_hash_function, key };
return hash_table_user_defined_call (ARRAYELTS (args), args, h);
Lisp_Object hash = hash_table_user_defined_call (ARRAYELTS (args), args, h);
return BIGNUMP (hash) ? make_fixnum (sxhash_bignum (XBIGNUM (hash))) : hash;
}
struct hash_table_test const
@ -4707,7 +4709,7 @@ sxhash (Lisp_Object obj, int depth)
***********************************************************************/
DEFUN ("sxhash-eq", Fsxhash_eq, Ssxhash_eq, 1, 1, 0,
doc: /* Return a fixnum hash code for OBJ suitable for `eq'.
doc: /* Return an integer hash code for OBJ suitable for `eq'.
If (eq A B), then (= (sxhash-eq A) (sxhash-eq B)).
Hash codes are not guaranteed to be preserved across Emacs sessions. */)
@ -4717,7 +4719,7 @@ Hash codes are not guaranteed to be preserved across Emacs sessions. */)
}
DEFUN ("sxhash-eql", Fsxhash_eql, Ssxhash_eql, 1, 1, 0,
doc: /* Return a fixnum hash code for OBJ suitable for `eql'.
doc: /* Return an integer hash code for OBJ suitable for `eql'.
If (eql A B), then (= (sxhash-eql A) (sxhash-eql B)).
Hash codes are not guaranteed to be preserved across Emacs sessions. */)
@ -4727,7 +4729,7 @@ Hash codes are not guaranteed to be preserved across Emacs sessions. */)
}
DEFUN ("sxhash-equal", Fsxhash_equal, Ssxhash_equal, 1, 1, 0,
doc: /* Return a fixnum hash code for OBJ suitable for `equal'.
doc: /* Return an integer hash code for OBJ suitable for `equal'.
If (equal A B), then (= (sxhash-equal A) (sxhash-equal B)).
Hash codes are not guaranteed to be preserved across Emacs sessions. */)
@ -4751,7 +4753,7 @@ keys. Default is `eql'. Predefined are the tests `eq', `eql', and
Default is 65.
:rehash-size REHASH-SIZE - Indicates how to expand the table when it
fills up. If REHASH-SIZE is a fixnum, increase the size by that
fills up. If REHASH-SIZE is an integer, increase the size by that
amount. If it is a float, it must be > 1.0, and the new size is the
old size multiplied by that factor. Default is 1.5.