[Cfp-interest 2693] Re: floating constants issue

[Vincent Lefevre]

On 2023-02-02 14:44:48 -0800, Jim Thomas wrote:
> On Feb 2, 2023, at 1:04 PM, Hans Boehm <boehm at acm.org> wrote:
> > 
> > I think many people are also surprised by the fact, pointed out by Vincent here, and already mentioned in a footnote, that -0.1 is effectively rounded in the opposite direction of what was specified, something you can't tell without carefully reading in the standard that negation is not part of the constant's matissa (though it clearly is for the exponent).
> 
> This is a long standing and well know problem. There are attempts to minimize the problem and warn about it. See (in N3054):
> 
> 6.4.4.2 #12 NOTE 1 (In a previous message I mistakenly referred to 6.4.4.3.)
> 7.24.1.5 #4 footnote 355
> 7.24.1.6 #4 (Since the strtodN functions are new, we were able to require negation before rounding.)
> F.5 #5 (For implementations that support Annex F "IEC 60559 floating-point arithmetic”, the strtod functions are required to negate before rounding.)

Warnings in the standard are better than nothing, but alone, they
are not really satisfactory because in practice, most developers
would not be aware of the issue, or may forget it. Warnings from
compilers would be much better, but this would be possible only
if the standard disallows (or at least discourages) such a form.

Another issue about constant rounding modes via pragmas is that
they depend on a context that may not be very visible. This is
even worse with macros. For instance, one may have in some header
file:

#define CST 0.1

where the developer wants 0.1 rounded to nearest.

Another example: GCC has

#define __FLT_MIN__ 1.17549435082228750796873653722224568e-38F

so that one gets

$ printf "#include <float.h>\nFLT_MIN\n" | gcc -E -xc -
[...]
1.17549435082228750796873653722224568e-38F

But the interpretation of this constant will not be correct
in all constant rounding modes.

-- 
Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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