cvs commit: src/lib/msun/src s_expm1.c s_expm1f.c

[Bruce Evans]

bde         2008-02-07 09:42:19 UTC

  FreeBSD src repository

  Modified files:
    lib/msun/src         s_expm1.c s_expm1f.c 
  Log:
  Use a better method of scaling by 2**k.  Instead of adding to the
  exponent bits of the reduced result, construct 2**k (hopefully in
  parallel with the construction of the reduced result) and multiply by
  it.  This tends to be much faster if the construction of 2**k is
  actually in parallel, and might be faster even with no parallelism
  since adjustment of the exponent requires a read-modify-wrtite at an
  unfortunate time for pipelines.
  
  In some cases involving exp2* on amd64 (A64), this change saves about
  40 cycles or 30%.  I think it is inherently only about 12 cycles faster
  in these cases and the rest of the speedup is from partly-accidentally
  avoiding compiler pessimizations (the construction of 2**k is now
  manually scheduled for good results, and -O2 doesn't always mess this
  up).  In most cases on amd64 (A64) and i386 (A64) the speedup is about
  20 cycles.  The worst case that I found is expf on ia64 where this
  change is a pessimization of about 10 cycles or 5%.  The manual
  scheduling for plain exp[f] is harder and not as tuned.
  
  Details specific to expm1*:
  - the saving is closer to 12 cycles than to 40 for expm1* on i386 (A64).
    For some reason it is much larger for negative args.
  - also convert to __FBSDID().
  
  Revision  Changes    Path
  1.8       +8 -13     src/lib/msun/src/s_expm1.c
  1.8       +8 -13     src/lib/msun/src/s_expm1f.c