On some machines, expf() turns out to be very slow: much slower than its double
  precision counterpart exp().  Probably this is concerned with the version of
  glibc.
  
  Here are a couple of examples:
  
  Normal behaviour:
  
  configuration$ ldd --version | head -1 | awk '{print $NF}'
  2.15
  
  configuration$ ./exp-test
  exp() time: 0.00249362
  expf() time: 0.0017401
  
  Slow behaviour:
  
  configuration$ ldd --version | head -1 | awk '{print $NF}'
  2.11.1
  
  configuration$ ./exp-test
  exp() time: 0.0028439
  expf() time: 0.00713329
  
  If slow behaviour is detected, then KALDI_NO_EXPF macro will be used, and the
  Exp() wrapper in base/kaldi-math.h will use exp() even for single precision
  floats.  The behaviour of expf() is considered to be slow if it is slower than
  exp() by at least 10%.