cvs commit: src/lib/msun/ld128 s_exp2l.c src/lib/msun/ld80 s_exp2l.c src/lib/msun/src e_exp.c e_expf.c s_exp2.c s_exp2f.c

Bruce Evans bde at
Wed Feb 6 19:17:06 PST 2008

bde         2008-02-07 03:17:05 UTC

  FreeBSD src repository

  Modified files:
    lib/msun/ld128       s_exp2l.c 
    lib/msun/ld80        s_exp2l.c 
    lib/msun/src         e_exp.c e_expf.c s_exp2.c s_exp2f.c 
  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.
  This change ld128/s_exp2l.c has not been tested.
  Revision  Changes    Path
  1.2       +15 -11    src/lib/msun/ld128/s_exp2l.c
  1.2       +14 -11    src/lib/msun/ld80/s_exp2l.c
  1.11      +8 -9      src/lib/msun/src/e_exp.c
  1.13      +8 -9      src/lib/msun/src/e_expf.c
  1.5       +9 -9      src/lib/msun/src/s_exp2.c
  1.5       +3 -6      src/lib/msun/src/s_exp2f.c

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