[gmx-developers] warning Emulating FMA instructions - this is probably not what you want!

[Szilárd Páll]

On Wed, Oct 17, 2012 at 4:46 PM, Berk Hess <hess at kth.se> wrote:

> Hi,
>
> Apparently icc can compile the code, so than it's not strange that you end
> up in this situation. But we should avoid this from happening.
> SSE4.1 would be the next supported level, but you really want the vex
> instructions and not plain SSE, as the latter is much slower.
>

AMD explicitly states that only up to -msse3 should be used on Bulldozer
and I have never tried to generate SSE4.1 instructions with icc and run it
on AMD. I would not be surprised if it didn't work.

--
Szilárd



> The best thing would be to use gcc, preferably 4.7.
>
> Cheers,
>
> Berk
>
> ----- Reply message -----
> From: "Szilárd Páll" <szilard.pall at cbr.su.se>
> To: "Discussion list for GROMACS development" <gmx-developers at gromacs.org>
> Subject: [gmx-developers] warning Emulating FMA instructions - this is
> probably not what you want!
> Date: Wed, Oct 17, 2012 15:25
>
>
> Hi,
>
> First of all, as far as I know, the new icc 13 can not generate
> AMD-compatible FMA4/XOP (v12 surely cant:
> http://developer.amd.com/Assets/CompilerOptQuickRef-62004200.pdf), so I
> find it strange that you've ended up with GMX_ACCELERATION=AVX_128_FMA
> using an Intel compiler -- unless you set the acceleration manually. To get
> FMA4 support you need to use a recent gcc version, the newer the better.
> The Verlet kernels don't benefit much from FMA4, so if you want to, you can
> use Intel Compiler, but then you need to set the acceleration to SSE2 (max
> SSE3 works, but we don't use these instructions).
>
> On Wed, Oct 17, 2012 at 11:14 AM, Jochen Hub <jhub at gwdg.de> wrote:
>
>> Hi developers,
>>
>> does anyone know how to interpret this icc warning:
>>
>> [  8%] /home/jhub/src/gromacs/**include/gmx_x86_avx_128_fma.h(**88):
>> warning #1224: #warning directive: Emulating FMA instructions - this is
>> probably not what you want!
>>
>>   #warning Emulating FMA instructions - this is probably not what you
>> want!
>>
>
> This warning is related to a bug in the build system, you'll need to add
> the -mfma4 flag to the compiler flags manually.
>
> --
> Szilard
>
>
>> I am compiling 46-release on a Interlagos 6378 with OpenMPI and icc
>> 13.0.0 20120731. My cmake line is:
>>
>> cmake $gmxsrc \
>>     -DFFTW_LIBRARY=$FFTW_LOCATION/**lib/libfftw3f.a \
>>     -DFFTW3F_INCLUDE_DIR=$FFTW_**LOCATION/include \
>>     -DFFTW3F_LIBRARIES=$FFTW_**LOCATION/lib/libfftw3f.a \
>>     -DCMAKE_INSTALL_PREFIX=$(pwd) \
>>     -DGMX_X11=OFF \
>>     -DCMAKE_CXX_COMPILER=$MPICXX \
>>     -DCMAKE_C_COMPILER=$MPICC \
>>     -DGMX_MPI=ON \
>>     -DGMX_PREFER_STATIC_LIBS=ON \
>>     -DGMX_GPU=OFF
>>
>> and cmake reported:
>>
>> -- Performing Test GNU_AVX_CFLAG
>> -- Performing Test GNU_AVX_CFLAG - Success
>> -- Enabling 128-bit AVX Gromacs acceleration (with fused-multiply add),
>> and it will help compiler optimization.
>>
>> Thanks a lot,
>> Jochen
>>
>>
>>
>>
>> --
>> ------------------------------**---------------------
>> Dr. Jochen Hub
>> Computational Molecular Biophysics Group
>> Institute for Microbiology and Genetics
>> Georg-August-University of Göttingen
>> Justus-von-Liebig-Weg 11, 37077 Göttingen, Germany.
>> Phone: +49-551-39-14189
>> http://cmb.bio.uni-goettingen.**de/ <http://cmb.bio.uni-goettingen.de/>
>> ------------------------------**---------------------
>> --
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>
>
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