phc-discussions - Re: [PHC] optimizing memory access speed

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Message-ID: <CAOLP8p6Dme8A2OSRig+3roSipzF098ZOTSck5WpeGL7+Q-XhVQ@mail.gmail.com>
Date: Tue, 31 Dec 2013 16:49:10 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] optimizing memory access speed

That was it!  That's some serious out-of-order execution!  CPUs have gotten
a lot better at this since the 90's.  Thanks for the tip.


On Tue, Dec 31, 2013 at 4:12 PM, Samuel Neves <sneves@....uc.pt> wrote:

> On 31-12-2013 20:34, Bill Cox wrote:
> >
> >         for(j = 0; j < BLOCK_LENGTH; j++) {
> >             hash ^= sourceBlock[hash & BLOCK_MASK] * 0xdeadbeef + 12345;
> >             //hash ^= sourceBlock[j & BLOCK_MASK] * 0xdeadbeef + 12345;
> >             destBlock[j] = hash;
> >         }
> >
>
> It may have little to do with caching. Consider the instruction
> dependencies across the loop in either case: each sourceBlock memory
> load must wait for the hash value to be computed to know its next
> address, while in the commented line every memory load can be done
> independently of the value of hash.
>

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