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

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Date: Tue, 31 Dec 2013 21:12:02 +0000
From: Samuel Neves <sneves@....uc.pt>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] optimizing memory access speed

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.