[maemo-developers] Performance of floating point instructions

[From: Siarhei Siamashka]

On Wednesday 10 March 2010, Laurent Desnogues wrote:
> Even if fast-math is known to break some rules, it only
> breaks C rules IIRC.  OTOH, NEON FP has no support
> for NaN and other nice things from IEEE754.

And just checked gcc man page to verify this stuff. 

-ffast-math
  Sets -fno-math-errno, -funsafe-math-optimizations,
  -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans
  and -fcx-limited-range.

-ffinite-math-only
  Allow optimizations for floating-point arithmetic that assume that arguments
  and results are not NaNs or +-Infs.

  This option is not turned on by any -O option since it can result in
  incorrect output for programs which depend on an exact implementation of
  IEEE or ISO rules/specifications for math functions. It may, however, yield
  faster code for programs that do not require the guarantees of these
  specifications.

So looks like -ffast-math already assumes no support for NaNs. Even if
there are other nice IEEE754 things preventing NEON from being used
with -ffast-math, an appropriate new option relaxing this requirement
makes sense to be invented.

-- 
Best regards,
Siarhei Siamashka