Re: Problem with PowerPC Altivec engine

> I'm getting occasional bad outputs on my Mac iBook when minting 29 bit
> coins with a long resource name.  This is for my rpow.net project which
> uses a fixed resource name of 71672bc400000017-000003c2-0001.rpow.net.
>
> Here is a sample GOOD output:
>
> ./hashcash -m -b 29 -r 71672bc400000017-000003c2-0001.rpow.net
> hashcash token:  
> 1:29:050131:71672bc400000017-000003c2-0001.rpow.net::TP+l7rpzbtJoqygB: 
> 000000000000000000000HJZop
>
> Here is a sample BAD output:
>
> ./hashcash -m -b 29 -r 71672bc400000017-000003c2-0001.rpow.net
> hashcash token:  
> 1:29:050131:71672bc400000017-000zFsHIgwYn9MnogZF: 
> 0000000000000000000000000000000000000000007/FQn
>
> You will notice that it's not well formed, there are only 4 colons  
> rather
> than 6.
>
> It fails about one time in 5 or so.

I'm guessing that it fails more often when higher bit counts are used -  
the bug appears to be in the "wraparound" code used when more than 2^31  
iterations are needed to find a collision.  The problem is that when  
wrapping, the "tail" position is not reset, but it is updated in the  
body of the loop.

> This test was done compiling with no optimization, because I thought
> the Mac's optimizer might be having problems.  The Mac gcc compiler
> actually gets an internal compiler error when compiling with -O3 on
> fastmint_altivec_standard_2.c, so I compile that one manually without
> optimization.

I've recently reported this compiler bug to Apple, so we should see  
that particular problem improve soonish.

--------------------------------------------------------------
from:     Jonathan "Chromatix" Morton
mail:     [email protected]
website:  http://www.chromatix.uklinux.net/
tagline:  The key to knowledge is not to rely on people to teach you it.