hash: bug: initial hash mask miscalculation.

Hash tables are supposed to start with a mask of 255,
which is the 256 modulus less one. However, due to a
coding mistake, they get a modulus of 252. Now this still
works. It just has a few zero bits in it: 1111 1100
rather than 11111111. When this grows by doubling in size,
we get 1 1111 1001, 11 1111 0011 and so on.
Now this still all works, because hash values AND-ed
with such a mask are no greater than that mask. The
table isn't accessed out of bounds. It's just inefficiently
used.

* hash.c (do_make_hash, make_similar_hash, clearhash):
When converting the power-of-two modulus, which is a Lisp
word, to the integer mask, subtract 1 from the integer
value, not the original Lisp word.

1 files changed, 3 insertions, 3 deletions

diff --git a/hash.c b/hash.c
index cb8ec3b9..c803f845 100644
--- a/hash.c
+++ b/hash.c
@@ -924,7 +924,7 @@ static val do_make_hash(hash_weak_opt_t wkopt, hash_type_t type, val seed)
     h->seed = c_unum(default_arg(seed, if3(hash_seed_s, hash_seed, zero)),
                      self);
     h->wkopt = wkopt;
-    h->mask = c_unum(mod - 1, self);
+    h->mask = c_unum(mod, self) - 1;
     h->count = 0;
     h->table = table;
     h->userdata = nil;
@@ -996,7 +996,7 @@ val make_similar_hash(val existing)
   val table = vector(mod, nil);
   val hash = cobj(coerce(mem_t *, h), hash_cls, &hash_ops);
 
-  h->mask = c_unum(mod - 1, self);
+  h->mask = c_unum(mod, self) - 1;
   h->count = 0;
   h->table = table;
   h->userdata = ex->userdata;
@@ -1147,7 +1147,7 @@ val clearhash(val hash)
   val mod = num_fast(256);
   val table = vector(mod, nil);
   ucnum oldcount = h->count;
-  h->mask = c_unum(mod - 1, self);
+  h->mask = c_unum(mod, self) - 1;
   h->count = 0;
   h->table = table;
   setcheck(hash, table);