------- Comment #11 from iano at apple dot com 2006-08-22 01:45 ------- About the manual wrongheadedness:
The major argument that I have heard from members of the GCC community (here and elsewhere) against isnan() in its various forms correctly detecting NaN when various hacky math flags are turned on, is that the hacky math flags are defined to preclude the presence of NaNs. The argument goes that the user actually asked for the possibility all NaNs to be ignored! This is circular reasoning. The fact of the matter is that GCC defines all the meanings of the flags. You can't claim the user wanted isnan(NaN) to return 0, because you provided him with no opportunity to say otherwise. I contend that given the choice, the user will want isnan(NaN) to correctly detect NaN's even if the rest of the application does not, because when one is walking on a tight rope, it is good to have a safety net in case something goes wrong. You can't deal with NaNs in special case code unless you have a way to find them. What you've given him is a choice between unavoidably wrong results, or "poor speed". If you can find a set of flags that would allow the user to do speed enhancing things like assume that make the assumption that x-x is always 0, while at the same time have __builtin_isnan(NaN) still work in the same compilation unit -- we want these things to inline for speed! -- then I will be happy to concede this point. Otherwise, I assert that the overly simple interpretation of these flags currently in practice does not serve the developer's needs. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28796
