Bug #574

Using autoconf produces zero forces with Altivec

Added by Justin Lemkul about 9 years ago. Updated over 7 years ago.

Target version:
Affected version - extra info:
Affected version:


Gromacs version 4.5.1 produces zero forces when using Altivec support. From md.log:

Configuring nonbonded kernels...
Configuring standard C nonbonded kernels...
Testing Altivec/VMX support... present.
Configuring PPC/Altivec nonbonded kernels...

The result is that energy minimization fails after a single step:

Steepest Descents converged to Fmax < 1000 in 1 steps
Potential Energy = -2.4873273e+04
Maximum force = 0.0000000e+00 on atom 0
Norm of force = nan

Any resulting simulation crashes immediately, indicating that the structure is insufficiently minimized.

Previous versions of Gromacs worked fine using Altivec support. The mdrun_mpi executable was installed using the following:

./configure --prefix=/home/rdiv1001/gromacs-4.5.1 CPPFLAGS="-I/home/rdiv1001/fftw-3.0.1-osx/include" LDFLAGS="-L/home/rdiv1001/fftw-3.0.1-osx/lib" --disable-threads --without-x --program-suffix=_4.5.1_mpi --enable-mpi CXXCPP="/nfs/compilers/mpich-1.2.5/bin/mpicxx -E"

make mdrun

make install-mdrun

In previous versions of Gromacs, I did not have to specify the CXXCPP environment variable; I don't know if this is related or not.

Related issues

Related to GROMACS - Bug #573: CMake doesn't detect Altivec supportClosed09/28/2010
Related to GROMACS - Bug #682: Gromacs 4.5.3 fails to compile on ppcClosed01/24/2011

Associated revisions

Revision 5ba7125c (diff)
Added by Erik Lindahl over 7 years ago

New CPU detection & AVX/SSE code, removed raw assembly files.

Removed all raw assembly files and deprecated altivec support.
Removed support for NASM and other assemblers, and replaced
previous SSE detection code with a new module using CPUID instead.
Added detection for SSE2, SSE4.1, AVX 128-bit with FMA, and AVX 256-bit.
Added Cmake detection of build platform based on CPUID, and output this
to the log file. The executables now compare the compile-time platform
and selected acceleration with the run-time platform and most suitable
acceleration and warns the user if they do not match. The compiler
detection code has also been reordered slightly to produce more readable
warnings when OpenMP is not available, and correctly disable pragma

Added intrinsics code and math functions for SSE2, SSE4.1, AVX128/256
both in single and double precision. All math functions and permutation
code have been tested & verified. Single precision math functions are
correct apart from the least significant bit, and double precision has
roughly twice the accuracy.

This has forced me to temporarily disable the SSE & Fortran acceleration.
SSE will be added back soon based on new intrinsics-only kernels currently
in testing, and we will test if Fortran still makes sense then.

Finally, the patch includes a modification to gmx_rmsdist where
a regression issue was introduced recently by using sqrtf() for
the norm function. This caused the intel compiler to produce slightly
different results at high optimization leves, which got evident here.

Closes #926 - Raw assembly code has been removed.
Refs #923 - Old kernels removed, new will be added shortly.
Fixes #914 - Cmake now does architecture-speficic optimization.
Fixes #912, #913
Fixes #857 - We detect rdtscp support with CPUID and use it if possible.
Fixes #750
Closes #537, #574 - Altivec is now deprecated.

Change-Id: Icfca5a940762f8d82ae67b59c65b2d2ac683256d


#1 Updated by Rossen Apostolov about 8 years ago

  • Description updated (diff)
  • Assignee deleted (Erik Lindahl)
  • Target version changed from 4.5.1 to 4.5.6

#2 Updated by Rossen Apostolov over 7 years ago

  • Assignee set to Erik Lindahl

Erik, shall we remove support for Altivec?

#3 Updated by Erik Lindahl over 7 years ago

  • Status changed from New to In Progress
  • Priority changed from Normal to Low
  • Target version deleted (4.5.6)


I'll try to have a quick look at this prior to 4.6, but since PowerPC hardware is getting pretty old now the Altivec kernels are likely to be deprecated quite soon.

#4 Updated by Justin Lemkul over 7 years ago

Erik Lindahl wrote:


I'll try to have a quick look at this prior to 4.6, but since PowerPC hardware is getting pretty old now the Altivec kernels are likely to be deprecated quite soon.

I agree. In fact, the supercomputer here that manifests this problem will be retired in July, so even if it's fixed I will have no way to test it. I wouldn't be offended at all if this were simply closed due to the antiquated nature of the hardware. It won't be a problem for us any longer, anyway.

#5 Updated by Rossen Apostolov over 7 years ago

Then lets drop Altivec support

#6 Updated by Roland Schulz over 7 years ago

Closed by 5ba7125c.

#7 Updated by Roland Schulz over 7 years ago

  • Status changed from In Progress to Closed

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