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Date: Thu, 22 Mar 2012 07:32:19 +0400
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: New patch for OpenCL SHA-512

On Mon, Mar 19, 2012 at 08:53:00AM -0300, Claudio Andr? wrote:
> For example of bad things in GPU code, data dependent decisions and
> paths (not avoidable, right?????????):

A way to avoid them is to speculatively follow both code paths, then
pick the right result with bitwise ops (bitselect() may help here).
Whether this change would result in a speedup or slowdown depends on
relative costs of the extra computation vs. branching.

BTW, speculative execution is commonly done by CPUs even when you do use
branch instructions, although CPUs tend to do it for one of the code
paths only (predicted-taken).  What I am proposing here is called "eager
execution" in the Wikipedia article below:

http://en.wikipedia.org/wiki/Speculative_execution

"Eager execution is a form of speculative execution where both sides of
the conditional branch are executed, however the results are committed
only if the predicate is true.  With unlimited resources, eager execution
(also known as oracle execution) would in theory provide the same
performance as perfect branch prediction.  With limited resources eager
execution should be employed carefully since the number of resources
needed grows exponentially with each level of branches executed
eagerly."

Alexander

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