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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