Use of C99 extra long double math functions after r236148

[Stephen Montgomery-Smith]

On 07/18/2012 03:56 PM, Steve Kargl wrote:
> On Wed, Jul 18, 2012 at 10:07:41AM -0500, Stephen Montgomery-Smith wrote:
>>
>>> The most obvious immediate difficulty in translating the above into C is
>>> that y*y and z*z may overflow when the result shouldn't.
>>
>> This will be a lot easier than I originally expected.  When we are in
>> conditions when overflow might occur, we can simply make the approximations
>> sqrt(y*y-1) = y
>> csqrt(z*z+1) = signum(x)*z
>> because in floating point arithmetic, these will not be approximations,
>> but true exactly.  And I am thinking that the test I will use for when
>> to use these approximations will be (y==y+1) and (z==z+1) respectively.
>>   (I would use (z*z==z*z+1) but that test has the overflow problem.)
>
> I could be mistaken, but I believe that you need to raise the
> inexact flag with these approximations because in fact you
> are doing floating point math.
>


Thanks for this observation.  I am looking through the C99 standard, 
trying to understand the inexact flag.  But I am struggling to interpret it.

Am I to understand that the inexact flag should be set anytime a 
floating point operation produces an answer that is not guaranteed 
exact?  For example, should 1.0/3.0 and sqrt(2.0) raise the inexact flag?