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Date: Tue, 26 May 2015 00:30:18 +0300
From: Solar Designer <>
Subject: Re: PHC: Parallel in OpenCL

On Mon, May 25, 2015 at 11:08:51PM +0200, Agnieszka Bielec wrote:
> Does interleaving make sense on gpu ?

Not at OpenCL source level.  The compiler and hardware take care of
interleaving for you, as long as LWS and GWS are high enough.  So please
don't bother doing it manually.

BTW, Intel's OpenCL when targeting CPU also manages to interleave things
well, at least in simple cases.  In fact, for a PHP mt_rand() seed
cracker competing with my php_mt_seed, it did that so well that until I
increased the interleave factor from 4x to 8x in php_mt_seed (in C), the
other cracker (in OpenCL) ran faster on a Core i5 CPU (lacking HT, so
needing that higher than 4x interleave factor to hide the SIMD integer
multiply latency).

So I currently see no need for interleaving at OpenCL source level.

Interleaving is something we use when writing in C (including with
intrinsics) or in assembly.


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