...and then generates slow emulation code as the instruction isn't available. The only work-around for that is going to be to disable FMA on Clang/ARM. Affected Clang versions: at least Clang 3.8. (note though that 3.5 didn't seem to be affected).
https://bitbucket.org/eigen/eigen/commits/7403112d5871b40b63410e6ad937130e4a815362
Filed LLVM bug: https://llvm.org/bugs/show_bug.cgi?id=27216
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