[ET Trac] [Einstein Toolkit] #1563: Provide always-working isnan etc.

[Einstein Toolkit]

#1563: Provide always-working isnan etc.
-------------------------+--------------------------------------------------
 Reporter:  eschnett     |       Owner:                     
     Type:  enhancement  |      Status:  new                
 Priority:  major        |   Milestone:                     
Component:  Cactus       |     Version:  development version
 Keywords:               |  
-------------------------+--------------------------------------------------
 Certain math optimization options (e.g. -ffast-math) tell the compiler
 that IEEE floating point numbers such as inf and nan do not need to be
 handled correctly (in the sense specified by the IEEE standard). This
 greatly improves floating-point speed and is commonly used in numerical
 HPC applications.

 For example, fmax() specifies:
 {{{
 If exactly one argument is a NaN, fmax() returns the other argument. If
 both arguments are NaNs, fmax() returns a NaN.
 }}}
 Implementing this correctly requires checking whether each argument is a
 nan. To improve speed, one can omit this check, which means that fmax()
 may return NaN, even if one of its argument is not a NaN. This is fine in
 most cases, and people appreciate the added speed.

 However, since compilers then don't need to handle inf and nan correctly,
 they have begun to optimise isnan(x) to simply returning false all the
 time. This improves speed (since the check does not actually need to
 occur) and reduces code size (since the nan-handling if branches can be
 omitted). Of course, this makes it then impossible to actually check for
 nan by calling isnan.

 Currently, e.g. g++ performs this optimisation, whereas gcc does not.
 Things vary with other compilers. In the future, with link-time
 optimisations, I expect other compilers to follow g++.

 The enclosed patch provides functions CCTK_IEEE_isnan etc. that always
 check for nan, independent of the chosen optimisation flags.

-- 
Ticket URL: <https://trac.einsteintoolkit.org/ticket/1563>
Einstein Toolkit <http://einsteintoolkit.org>
The Einstein Toolkit