john-dev - Re: bcrypt-parallella on 64-core (was: Katja's weekly report #13)

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Message-ID: <20131030120510.GA27010@openwall.com>
Date: Wed, 30 Oct 2013 16:05:11 +0400
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: bcrypt-parallella on 64-core (was: Katja's weekly report #13)

Yaniv, Katja -

On Wed, Oct 30, 2013 at 07:51:38AM -0400, Yaniv Sapir wrote:
> The problem is not with certain registers, but with certain cores. Cores on
> the 2nd row should not *load *or *fetch *from external memory. DMA reads
> are OK.

Oh.  I was referring to:

http://www.openwall.com/lists/john-dev/2013/09/18/3

"(A very long shot:) In the stuck cores, do you have external loads into
registers R53, R0, R2, R38, respectively?"

so it sounded like the problem had something to do with certain
registers (as well as certain cores).

Further in that thread, Katja confirmed that "there exists external load
intto r38 in compiler generated code":

http://www.openwall.com/lists/john-dev/2013/09/19/6

and asked "Is it possible to force e-gcc not to use r38 and r53?":

http://www.openwall.com/lists/john-dev/2013/10/01/8

Maybe this can in fact be done by placing some local variables into
those problematic registers (using the syntax I just posted about),
and either only using those variables for other than external loads or
keeping them unused.  Like I said, I am unsure if this syntax prevents
gcc from making other use of the same registers, though.

Yaniv, it'd be helpful if you post a full set of relevant Epiphany-IV
errata (specifically for E64G401?) in one message.

For example, I am confused whether the problem with certain registers
occurs only on certain cores (that is, when both the core and the
register number are problematic) or whether it's independent (that is,
a problem occurs when either register or core number is problematic).

Thanks,

Alexander

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