Re: [PATCH] Don't constant fold 1.0/0.0 at compile-time

[Fergus Henderson <fjh at cs dot mu dot OZ dot AU>]

On 04-Jul-2003, Segher Boessenkool <segher@koffie.nl> wrote:
> Roger Sayle wrote:
>  > Could one of the language lawyers post their opinions on "x = 1.0/0.0"?
> 
> If x has static storage duration, no exceptions are raised.
> See C99, F.7.4.

Right.  However, a warning should be issued.
See C99 section F.7.2:

 |        Recommended practice
 | 
 |        [#2] The implementation should produce a diagnostic  message
 |        for  each  translation-time  floating-point exception, other
 |        than ``inexact'';305) the implementation should then proceed
 |        with the translation of the program.

In C++, the program is ill-formed, so a diagnostic is required,
and the diagnostic should be an error, not a warning.
See C++98 section 5 - Expressions [expr]:

 | -5- If during the evaluation of an expression, the result is not
 |    mathematically defined or not in the range of representable values for
 |    its type, the behavior is undefined, unless such an expression is a
 |    constant expression (expr.const), in which case the program is
 |    ill-formed.

The above is all assuming you meant "static double x = 1.0/0.0;". 

For the case of "auto double x = 1.0/0.0;" or "static double x; x = 1.0/0.0;"
the answer would be different.  For C, such expressions should be evaluated
at run-time, raising a floating-point exception and potentially trapping etc.
For C++, it's not clear whether section 5[expr] paragraph 5 is supposed
to apply to any expression which has the form of a constant expression,
or only to expressions where the language requires a constant expression,
such as initializers for static constants.  With the former interpretation,
a program containing "auto double x = 1.0/0.0;" would be ill-formed, but
with the latter interpretation it would not.

This ambiguity in C++98 is the topic of defect report #261.  The proposed
response to that defect report is "The committee believes that the standard
is clear enough." -- which unfortunately does not answer the question!

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.