See e.g. http://lists.llvm.org/pipermail/llvm-dev/2013-December/068806.html For us this is a disaster, since this means that we can't trust EIGEN_HAS_SINGLE_INSTRUCTION_MADD. So we end up mis-shooting the number of registers, and spilling! The work-around for this is going to be to reimplement the pmadd in inline assembly.
Notes: * this affects all known versions of Clang/ARM, but not Aarch64. * this depends on -mcpu= settings. Affected settings include -mcpu=cortex-a8 and -mcpu=cortex-a7. Since the former is the default on -march=armv7-a, that is a very common case.
https://bitbucket.org/eigen/eigen/commits/d86a203e9913f08aec96ccc8fec8ae8d2a23ee20
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