From: invalid AT erehwon DOT invalid (Graaagh the Mighty)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Mysterious NaNs...
Organization: Low Charisma Anonymous
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Date: Tue, 19 Jun 2001 07:08:47 GMT
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

On Sat, 16 Jun 2001 23:25:58 +0300, "Eli Zaretskii"
<eliz AT is DOT elta DOT co DOT il> sat on a tribble, which squeaked:

>I'd advise against that: how can you trust a numeric computation which
>gives birth to NaNs out of thin air?  I suggest to find the reason for
>these overflows.  You could selectively enable the numeric exceptions
>by using `_control87', which might catch the overflowing code
>red-handed.

I have managed to improve around the overflows.

>> Regardless of all of this, the fundamental problem stays unaddressed:
>> a sequence of instructions is being misplaced by the optimizer.
>
>Not really.  The optimizer is allowed to do any transformations that
>don't change the outcome of a program.  Since operations on NaNs are
>arithmetic nonsense, and your tests look like something that should
>never happen (like a*a < 0), the optimizer might decide that some
>parts of your program are dead code, and optimize them out of
>existence.  GCC 2.95.x is known to be very aggressive in its
>optimizations.

Enough to actually recognize a test for something's square against
zero?

Incidentally, why is there no isnan() function? It seems like a
natural thing to have.

>If you insist on leaving NaNs instead of finding the reason for their
>being there, your best bet would be to use the library function
>`isnan' and maybe also `isinf' (link with -lm) for these tests.

Except that there is no such function. I could have sworn there was,
but when I went to check on what headers and syntax to use with "info
libc a isnan" I got a rude surprise. Maybe it was removed in a recent
version?

-- 
Bill Gates: "No computer will ever need more than 640K of RAM." -- 1980
"There's nobody getting rich writing software that I know of." -- 1980
"This antitrust thing will blow over." -- 1998
Combine neo, an underscore, and one thousand sixty-one to make my hotmail addy.