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From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: What is the warning about builtin-function on gcc-4.6.0 ?
Date: Sat, 26 Mar 2011 19:48:49 +0100
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* Simon Wright:

>> My understanding is that an implementation must either raise
>> Constraint_Error, or the evaluation of an expression must result in a
>> valid value.
>
> ARM 13.9.2 doesn't include division by zero as a means of producing an
> invalid float.

Floating point arithmetic is not mentioned in in 13.9.1, either (which
seems an exhaustive list to me).

I'm concerned that any interpretation which makes 'Valid false here
would also allow to produce arbitrary values from any form of
arithmetic, floating point or not.  (If 'Valid is false, the object is
abnormal, so all bets are off.)

> I don't see anything in the above that says that 'Valid should produce
> False for Inf/NaN, but GNAT - on Intel & PowerPC hardware - certainly
> does.

And it's not an accident as the result of failing range checks or
something like that.  There is a deliberate check for Infs and Nans in
s-fatgen.adb.