Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism

[George Spelvin]

On Fri, Apr 29, 2016 at 9:32 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
wrote:
> For example, that _long_ range of bits set ("7fffffffc" in the middle)
> is effectively just one bit set with a subtraction. And it's *right*
> in that bit area that is supposed to shuffle bits 14-40 to the high bits
> (which is what we actually *use*. So it effectively shuffles none of those
> bits around at all, and if you have a stride of 4096, your'e pretty much
> done for.

Gee, I recall saying something a lot like that.
> 64 bits is just as bad...  0x9e37fffffffc0001 becomes
> 0x7fffffffc0001, which is 2^51 - 2^18 + 1.

After researching it, I think that the "high bits of a multiply" is
in fact a decent way to do such a hash.  Interestingly, for a randomly
chosen odd multiplier A, the high k bits of the w-bit product A*x is a
universal hash function in the cryptographic sense.  See section 2.3 of
http://arxiv.org/abs/1504.06804


One thing I note is that the advice in the comments to choose a prime
number is misquoting Knuth!  Knuth says (vol. 3 section 6.4) the number
should be *relatively* prime to the word size, which for binary computers
simply means odd.

When we have a hardware multiplier, keeping the Hamming weight low is
a waste of time.  When we don't, clever organization can do
better than the very naive addition/subtraction chain in the
current hash_64().

To multiply by the 32-bit constant 1640531527 = 0x61c88647 (which is
the negative of the golden ratio, so has identical distribution
properties) can be done in 6 shifts + adds, with a critical path
length of 7 operations (3 shifts + 4 adds).

#define GOLDEN_RATIO_32 0x61c88647	/* phi^2 = 1-phi */
/* Returns x * GOLDEN_RATIO_32 without a hardware multiplier */
unsigned hash_32(unsigned x)
{
        unsigned y, z;
				/* Path length */
        y = (x << 19) + x;	/* 1 shift + 1 add */
        z = (x << 9) + y;	/* 1 shift + 2 add */
        x = (x << 23) + z;	/* 1 shift + 3 add */
        z = (z << 8) + y;	/* 2 shift + 3 add */
	x = (x << 6) - x;	/* 2 shift + 4 add */
        return (z << 3) + x;	/* 3 shift + 4 add */
}

Finding a similarly efficient chain for the 64-bit golden ratio
0x9E3779B97F4A7C15 = 11400714819323198485
or
0x61C8864680B583EB = 7046029254386353131

is a bit of a challenge, but algorithms are known.