Re: [Tinycc-devel] NAN and INFINITY break in different ways

[Vincent Lefevre]

On 2017-12-24 13:18:38 +0100, Michael Matz wrote:
> On Sat, 23 Dec 2017, avih wrote:
> > Alpine:
> > $ tcc test.c
> > testinfs.c:2: error: division by zero in constant
> > 
> > $ tcc test.c -E
> > ...
> > static const double infs[] = { (0.0f/0.0f), 1e5000f };
> > ...
> > 
> > So clearly tcc doesn't like musl's fallback definition (if not gcc) for NAN
> > in a constant, but tcc is fine with musl's INFINITY definition.
> 
> I think we could make an exception for 0.0/0.0 and always fold this into NaN
> (normally division by zero is indeed not allowable in constant expressions
> in c99).

AFAIK, the standard just implies that 0.0/0.0 is not specified as a
constant expression. But the standard also says: "An implementation
may accept other forms of constant expressions."

Moreover, if Annex F is supported, then FP division by zero is
well-defined. F7.4.2 gives examples with 0.0/0.0 in initializers,
such as:

  static float x = 0.0/0.0; // does not raise an exception

Nowadays, supporting Annex F is more or less a must.

-- 
Vincent Lefèvre <address@hidden> - Web: <https://www.vinc17.net/>
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