phc-discussions - Re: [PHC] data-dependent branching (Re: [PHC] A little nit which bothers me...)

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Message-ID: <9A043F3CF02CD34C8E74AC1594475C738A3452D4@uxcn10-tdc06.UoA.auckland.ac.nz>
Date: Thu, 3 Apr 2014 03:04:35 +0000
From: Peter Gutmann <pgut001@...auckland.ac.nz>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] data-dependent branching (Re: [PHC] A little nit which
 bothers me...)

Bill Cox <waywardgeek@...il.com> writes:

>Data dependent branching doesn't seem to buy as much ASIC resistance as some
>PHC submitters seem to think, IMO.  

Nor do data-dependent ops, which came up in a private discussion some time ago
(and were quickly shot down).  Imagine something like (in simplified form):

  switch( x & 3 )
    {
    case 0: x ^= y; break;
    case 1: x += y; break;
    case 2: x -= y; break;
    case 3: x <<<= y; break;
    case 4: puts( "WTF" );
    }

The idea is that this is difficult to pipeline since each op is data-
dependent, however if you use speculative execution and run each step in
parallel (easy on an FPGA/ASIC) and then discard all the ones that weren't
needed, you lose any advantage you've gained.

Peter.