topic Re: Bug in MKL/Lapack routine DLAMC2 in Intel® oneAPI Math Kernel Library

Bug in MKL/Lapack routine DLAMC2

mecej4 — Tue, 11 Apr 2023 00:41:18 GMT

The routine DLAMC2 is a routine in Lapack (now marked "deprecated", but still present and/or used in many codes that were developed with older versions of Lapack). This routine is present in MKL, and documented. Some of the returned values are wrong with the current version (20230320) of MKL on WIndows 11 64 bit.

Here is a very short reproducer:

program tdlamc2 implicit none integer beta, it, imin, imax, irnd double precision eps, rmin, rmax logical lrnd call dlamc2( beta, it, lrnd, eps, imin, rmin, imax, rmax ) if( lrnd ) then irnd = 1 eps = ( beta**( 1d0-it ) ) / 2 else irnd = 0 eps = beta**( 1d0-it ) end if print 77, beta,it,irnd,imin,imax 77 format('base,it,irnd, imin,imax: ',5I8) print 78,eps,rmin,rmax 78 format('eps,rmin,rmax: ',3ES20.12) end program

Compile and run:

ifort /Qmkl tdlamc2.f90 tdlamc2 base,it,irnd, imin,imax: 0 53 1 32759 1024 eps,rmin,rmax: Infinity 2.225073858507-308 1.797693134862+308

The results for BASE, IMIN and EPS are wrong. The correct results:

base,it,irnd, imin,imax: 2 53 1 -1021 1024 eps,rmin,rmax: 1.110223024625E-16 2.225073858507-308 1.797693134862+308

I tried to work around the problem by using the source code for DLAMC2 ( www.netlib.org/lapack/explore-html/d8/d32/dlamchf77_8f_source.html ), and built using the command

ifort /O2 /Qmkl tdlamc2.f90 dlamch.f

but the resulting EXE hangs when run. It runs and gives the correct results if the /Od (disable optimization) option is used, but that is not a good option for using in large programs that run long.

Re: Bug in MKL/Lapack routine DLAMC2

ShanmukhS_Intel — Wed, 28 Jun 2023 11:14:01 GMT

Hi Nagaraj,

Thanks for posting in Intel Communities.

Thanks for sharing the sample reproducer and other details. We could see the incorrect results being printed when compiling using "ifort /Qmkl tdlamc2.f90" and under other scenarios. Our engineering team is working on your issue internally. We will get back to you soon with an update.

Best Regards,

Shanmukh.SS

Re: Bug in MKL/Lapack routine DLAMC2

ShanmukhS_Intel — Mon, 07 Aug 2023 09:44:56 GMT

Hi Nagaraj,

Thank you for your patience. The issue raised by you has been fixed in version 2023.2. If the issue persists with the new release, please start a new discussion thread in the community forum and we’d investigate it further. Please check the release notes for other features and bug fixes

Below are the results captured at our end for ifort version 2021.10.0.

base,it,irnd, imin,imax: 2 53 1 -1021 1024

eps,rmin,rmax: 1.110223024625E-16 2.225073858507-308 1.797693134862+308

Best Regards,

Shanmukh.SS

Re:Bug in MKL/Lapack routine DLAMC2

ShanmukhS_Intel — Wed, 09 Aug 2023 04:25:26 GMT

Hi Nagaraj,

It’s great to know that the issue has been resolved, in case you run into any other issues please feel free to create a new thread.

Best Regards,

Shanmukh.SS