Re: RFA: don't try to compare IEEE NaN's

[Eli Zaretskii <eliz at is dot elta dot co dot il>]

On 6 Jun 2001, Jim Blandy wrote:

> > I think it is better to initialize the integral members of the union
> > with an explicit bit pattern, just not a pattern which gets
> > interpreted as a NaN or an Inf.  With initialization such as above,
> > you risk losing due to subtleties of compile-time conversion of a
> > literal constant to a floating-point value.  This is a GDB test suite,
> > so we are not interested in testing the compiler.
> 
> I'm not sure what you mean.  Once the test has assigned a value to
> testval.float_testval, we only use that variable.  The compile-time
> conversion happens exactly once, and then we always use the result of
> that conversion.

Let's begin by asking why do you at all set the members of the union to 
specific values?  Why not initialize them to zero, or even leave them at 
some random garbage they pick up from the stack?

My assumption was that whoever wrote the test wanted to see that GDB 
doesn't lose bits due to all kinds of conversions that are going under 
the hood.  If that is true, you want to make sure the value you work with 
has the same bit pattern you wanted it to have.  If not, you don't really 
know what you are testing here; for example, imagine an (absurdly 
unrealistic) case that the compiler turns your literal constant into an 
all-zero bit pattern, or into a NaN.  Then you are back to square one.

The only way to make sure you get the bit patterns you wanted is to 
initialize the integral members of the union with those bit patterns.  
You just want them to be different from a NaN or an Inf, because they 
cause trouble in comparisons.

Am I making any sense?