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Subject: Re: [boost] [compute] Review period starts today December 15, 2014, ends on December 24, 2014
From: Thomas M (firespot71_at_[hidden])
Date: 2014-12-22 03:01:43

On 21/12/2014 23:24, Asbjørn wrote:
> On 21.12.2014 20:39, Kyle Lutz wrote:
>> Strongly disagree, the floating-point operations on the device are
>> well defined and their output should be identical to the host results
>> (barring optimizations like "-cl-fast-relaxed-math").
> While I agree, I've found Intel's OpenCL CPU device to return results
> which make me think it uses some relaxed math regardless. With NVIDIA
> and AMD I can get (essentially) the same results as reference CPU
> calculations, but with Intel I sometimes get quite large discrepancies.
> Of course, it's possible I'm just doing it wrong...

Intel's OpenCL CPU implementation (it was SDK 2013) is exactly the one
from which I am used to that results normally deviate, quite
considerably indeed.
I don't have it installed on this machine, but Kyle could you run some
kernel code on doubles yourself? My code used basic arithmetic operands,
summed values up (up to tens-of-thousands) and there were several exp /
log / sqrt along the way. I hadn't set any special compiler flag, surely
not -cl-fast-relaxed-math.
My suspicion was that they use their own math library which provides
highly optimized calculation variants. FWIW I am not even sure if a
plain C++ program compiled with the Intel C++ compiler and linking in
their math library will produce the same results as e.g. MSVC.

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