cvs commit: src/lib/msun/src s_cosf.c

[Bruce Evans]

bde         2005-11-30 06:47:19 UTC

  FreeBSD src repository

  Modified files:
    lib/msun/src         s_cosf.c 
  Log:
  Fixed cosf(x) when x is a "negative" NaNs.  I broke this in rev.1.10.
  cosf(x) is supposed to return something like x when x is a NaN, and
  we actually fairly consistently return x-x which is normally very like
  x (on i386 and and it is x if x is a quiet NaN and x with the quiet bit
  set if x is a signaling NaN.  Rev.1.10 broke this by normalising x to
  fabsf(x).  It's not clear if fabsf(x) is should preserve x if x is a NaN,
  but it actually clears the sign bit, and other parts of the code depended
  on this.
  
  The bugs can be fixed by saving x before normalizing it, and using the
  saved x only for NaNs, and using uint32_t instead of int32_t for ix
  so that negative NaNs are not misclassified even if fabsf() doesn't
  clear their sign bit, but gcc pessimizes the saving very well, especially
  on Athlon XPs (it generates extra loads and stores, and mixes use of
  the SSE and i387, and this somehow messes up pipelines).  Normalizing
  x is not a very good optimization anyway, so stop doing it.  (It adds
  latency to the FPU pipelines, but in previous versions it helped except
  for |x| <= 3pi/4 by simplifying the integer pipelines.)  Use the same
  organization as in s_sinf.c and s_tanf.c with some branches reordered.
  These changes combined recover most of the performance of the unfixed
  version on A64 but still lose 10% on AXP with gcc-3.4 -O1 but not with
  gcc-3.3 -O1.
  
  Revision  Changes    Path
  1.15      +19 -11    src/lib/msun/src/s_cosf.c