Eric Blake wrote:
...
> That discards bits, in the case where non-malloced values are being used. The
> assertion is nice when you know that all values are malloc'd, but is harsh for
> a default. So, how about I squash this slight modification into my patch
> before pushing this commit?
>
> diff --git i/lib/hash.c w/lib/hash.c
> index 52a211e..c9866c5 100644
> --- i/lib/hash.c
> +++ w/lib/hash.c
> @@ -483,7 +483,14 @@ hash_reset_tuning (Hash_tuning *tuning)
> static size_t
> raw_hasher (const void *data, size_t n)
> {
> - return (size_t) data % n;
> + /* When hashing unique pointers, it is often the case that they were
> + generated by malloc and thus have the property that the low-order
> + bits are 0. As this tends to give poorer performance with small
> + tables, we rotate the pointer value before performing division,
> + in an attempt to improve hash quality. */
> + size_t val = data;
> + val = (val >> 3) | (val << (CHAR_BIT * sizeof val - 3));
> + return val % n;
That looks fine.
Though how about adding size_t variants to lib/bitrotate.h?
Then we can use this:
val = rotr<something> (val, 3);
Hmm... now that I look at the existing definitions in that file,
I'll bet they don't all do a final
... & TYPE_MAXIMUM (x)
for nothing.
Then there's the question of the name.
rotr_sz? rotr_size_t is not kosher since it ends in _t,
which is reserved. I don't like rotr_sizet.
Maybe rotrsz, but I prefer with the underscore.
