GPU numerical compliance

Article: Predicting AMD and Nvidia GPU Performance
By: Sylvain Collange (full.name.delete@this.gmail.com), April 19, 2011 11:38 am
Room: Moderated Discussions
>Ian Ameline (ian.ameline@(NoSpam).gmail.com) on 4/18/11 wrote:
>---------------------------
>>*Fully* compliant? As in <= 1/2 ULP precision guarantees for add, subtract, divide, multiply *and* sqrt?

Yes. Unlike x86-SSE for instance, which is not compliant with the current IEEE-754 standard. Not until we get the FMA operator...

>>Denormal support?

Implemented in hardware at full speed. Unlike most CPUs, which trap on subnormals and handle them in microcode at a much lower speed.

>>+-0 and +- infinity correctly generated and handled?

These ones were already supported long before GPGPU APIs came out. At least since GeForce 6 / Radeon X1x00, and likely earlier.

Ping-Che Chen (pcchen@kimicat.com) on 4/19/11 wrote:
---------------------------
>Right now, OpenCL 1.1 requires basically the following:
>
>1. round to nearest even, other rounding modes are optional. Dynamic rounding mode selection is not supported.
>2. INF and (non signaling) NaN must be supported.
>3. Denormalized number is optional.
>4. No exception handling is supported right now.
>5. +, -, * must be infinitely precise. Division and sqrt are not required to be infinitely precise.

For double precision, it also requires FMA, subnormals, all 4 mandatory IEEE rounding modes, and correctly rounded division and sqrt.

Basically, current-generation GPUs are already far ahead of x86 and most other CPU architectures in terms of numerical compliance.
Power6/7 and Itanium are still arguably one notch ahead of GPUs, but that is about it.
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