[Cfp-interest 2733] Re: comparison macros and exceptions due to conversion to the semantic type

[Vincent Lefevre]

On 2023-03-27 21:01:25 -0700, Jim Thomas wrote:
> > and 7.12.17.6 "The isunordered macro" says
> > 
> >  It raises no floating-point exceptions if neither argument is a
> >  signaling NaN.
> > 
> > Now, if a conversion to the semantic type with the isunordered macro
> > yields an underflow or an overflow, is this exception expected to be
> > signaled? This seems unclear.
> 
> I think the intention is that if the argument is represented in
> wider format and is converted to semantic type, then any overflow or
> underflow resulting from the conversion would be signaled. Such a
> conversion may be considered to be prior to the comparison (similar
> to argument conversion for a function). If the argument is not
> converted to semantic type, then overflow and underflow would not
> occur.

This may not be obvious for the implementer, who could do the
following, in case the code would be based on signaling operations
(more likely to be available natively):

1. Save the exception state (flags).
2. Compute the isunordered result with signaling operations.
3. Restore the exception state and add "invalid" if needed.

Step 2 could involve a conversion to the semantic type, e.g. by
first storing the macro arguments to variables. And with such a
code, the potential underflow and overflow exceptions would be
cleared at step 3.

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