phc-discussions - Re: [PHC] omegacrypt and timing

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Message-ID: <20140921172424.7f9d6c85@lambda>
Date: Sun, 21 Sep 2014 17:24:24 +0000
From: Brandon Enright <bmenrigh@...ndonenright.net>
To: Bill Cox <waywardgeek@...hershed.org>
Cc: discussions@...sword-hashing.net, bmenrigh@...ndonenright.net
Subject: Re: [PHC] omegacrypt and timing

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On Sun, 21 Sep 2014 08:48:53 -0400
Bill Cox <waywardgeek@...hershed.org> wrote:

> I really hope there's something good here.  I would love to see a
> really good anti-SIMD moving forward, not that my opinion counts.
> 
> Can I rewrite your code this way?
> 
> depth = 0
> while true:
>     if coinFlip()
>         depth++
>     else
>         depth--
>     if depth <= 0:
>         return
>     switch depth:
>     case 1:
>         do_work_1()
>     case 2:
>         do_work_2()
>     ...
>     case N:
>         do_work_N()


Your code doesn't emulate where execution stays in the outer loop and
doesn't enter the inner loop.  I think you need to add a depth-- after
all of the cases.

Also, your case 1, case2, ... case n structure doesn't have a stack to
it so it can't emulate nested loops like:

while (coinflip()) {
   do_some_work();

   while (coinflip()) { ... }

   do_some_more_work();
}

You'd need a way of supporting cases nestled within other cases and
that needs a stack-like datastructure.

Brandon

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