topic Re: subnormal rounding issue on i7-7820HK in Mobile and Desktop Processors

subnormal rounding issue on i7-7820HK

DGore2 — Sat, 29 Jun 2019 11:40:58 GMT

Hi,

I am writing a simple double precision floating point library for a micro controller... and I tested its results vs what my computer calculates.

My computer has a i7-7820HK CPU @ 2.90GHz, 2901 Mhz.

Where I found issues is when I am multiplying two numbers that result in a subnormal/denormal number... sometimes, according to my calculations, my computer (your CPU?) is giving me a result that is rounded incorrectly.

Here are a couple examples:

My lib rounded to higher magnitude, CPU didn't

-1.0913716444947332E-161 * 5.5880945523004242E-148

Lib -6.0986879411361762E-309

CPU -6.09868794113617E-309

Lib 0x800462ab6512d055

CPU 0x800462ab6512d054

CPU rounded to higher magnitude, my lib didn't

6.4439637432002375E-265 * -1.8323993047591027E-44

Lib -1.1807914682932978E-308

CPU -1.1807914682932983E-308

Lib 0x80087da541ae0aa1

CPU 0x80087da541ae0aa2

On inspection of the rounded off bits, it looks like my lib is correct. I'm using: round to nearest, if half, round to even.

Please confirm/test on your system? Fortunately, if this is a hardware bug, it should normally be below the noise floor.

You should be able to verify which rounding is correct using 128 bit floating point math.

Cheers,

Dean

Re: subnormal rounding issue on i7-7820HK

Alberto_R_Intel — Tue, 02 Jul 2019 04:00:45 GMT

DGore2, Thank you for posting in the Intel® Communities Support.

In this case, we can always run the Intel® Processor Diagnostic Tool, it does an overall testing on the unit and if it passes the test then it means it is working fine:

https://downloadcenter.intel.com/download/19792/Intel-Processor-Diagnostic-Tool

Is there any demo or link that you can share in order for us to download the application that you are using to try to replicate this scenario?

Once you get the chance, please let us know the results.

Just to let you know, we will do further research on this matter, as soon as I get any updates I will post all the details on this thread.

Regards,

Alberto R.

Intel Customer Support Technician

Under Contract to Intel Corporation

Re: subnormal rounding issue on i7-7820HK

Alberto_R_Intel — Wed, 03 Jul 2019 02:29:19 GMT

DGore2, I just received an update on this matter.

At this point the next thing to do will be to make sure that the latest BIOS version is installed in your system, please get in contact directly with the manufacturer of the computer to confirm which is the latest BIOS version available for your platform to install it.

Regards,

Alberto R.

Intel Customer Support Technician

Under Contract to Intel Corporation

Re: subnormal rounding issue on i7-7820HK

Alberto_R_Intel — Tue, 09 Jul 2019 05:01:56 GMT

DGore2, I just wanted to check if the information posted previously was useful for you and if you need further assistance on this matter?

Regards,

Alberto R.

Intel Customer Support Technician

Under Contract to Intel Corporation

Re: subnormal rounding issue on i7-7820HK

Alberto_R_Intel — Fri, 12 Jul 2019 02:38:25 GMT

DGore2, I just received an update on this matter.

I just wanted to let you know that if the problem persists after doing the BIOS update, we recommend to visit, register and submit your inquiry in our Intel® Developer Zone site, where you will receive further peer to peer assistance for this scenario:

https://software.intel.com/en-us/forum

Regards,

Alberto R.

Intel Customer Support Technician

Under Contract to Intel Corporation