john-dev - Re: Ambiguous pointer increments

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Date: Tue, 28 Jul 2015 02:30:04 +0200
From: magnum <john.magnum@...hmail.com>
To: john-dev@...ts.openwall.com
Subject: Re: Ambiguous pointer increments

On 2015-07-28 01:11, jfoug@....net wrote:
>
> ---- magnum <john.magnum@...hmail.com> wrote:
>> The beignet OpenCL driver complained about oldoffice kernel, "multiple
>> unsequenced modifications to 'p'" for things like the below:
>>
>> -               for (i = 0; i < 32; i += 2)
>> -                       W[i >> 1] = (uint)*p++ | (*p++ << 16U);
>>
>> I already changed it but I'm curious - no other driver complained.
>>
>> +               for (i = 0; i < 32; i += 2) {
>> +                       W[i >> 1] = (uint)*p++;
>> +                       W[i >> 1] |= (*p++ << 16U);
>> +               }
>>
>> Originally I thought they (the use/increments of p) were guaranteed to
>> be left-to-right but after this I'm not sure at all. Anyone know for
>> sure? Alexander Cherepanov perhaps? In case it matters, OpenCL is C99.
>
> That is 100% undefined code, for sure. One of the only times you can do more than 1 increment in a statement is in the tertiary statement.

I was expecting that answer now. The code did work on most drivers and 
devices though. I was hoping this bugfix would make the format work on 
super's Tahitis, but it didn't :-(

I found some clues for that stupid issue now btw, 
https://github.com/magnumripper/JohnTheRipper/issues/1497 but I'm still 
trying to find a workaround that doesn't impact performance. Need some 
sleep now.

magnum