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January 10, 2011 -0 assigned to a FP variable | ||||
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A bug I've introduced once in my D code, are you able to spot it? void main() { double x = +0; double y = -0; } The bug: 'y' isn't the desired double -0.0 To avoid this bug DMD may keep the -0 and +0 integer literals represented in two distinct ways, so they have two different values when/if assigned to a floating point. (But here D will behave a little differently from C). An alternative is to just add a warning to DMD. A third possibility is to just ignore this probably uncommon bug :-) Bye, bearophile |
January 10, 2011 Re: -0 assigned to a FP variable | ||||
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Posted in reply to bearophile | On Sunday 09 January 2011 16:27:11 bearophile wrote:
> A bug I've introduced once in my D code, are you able to spot it?
>
> void main() {
> double x = +0;
> double y = -0;
> }
>
>
> The bug: 'y' isn't the desired double -0.0
>
> To avoid this bug DMD may keep the -0 and +0 integer literals represented
> in two distinct ways, so they have two different values when/if assigned
> to a floating point. (But here D will behave a little differently from C).
> An alternative is to just add a warning to DMD.
> A third possibility is to just ignore this probably uncommon bug :-)
>
> Bye,
> bearophile
I didn't even know that there _was_ such as thing as + or - 0 (or 0.0). I would have considered it a no-op, being identical to 0 or 0.0, and expected it to be compiled out completely. I haven't a clue what -0.0 would even mean. But I'm not exactly an expert on floating point values, so presumably, there's some weird floating point thing that - affects. Honestly, the more I deal with floating points, the more I wish that we were dealing with fixed points. Granted, there's probably a number of reasons why floating points are better than fixed points, but in general, I just don't see it.
- Jonathan M Davis
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January 10, 2011 Re: -0 assigned to a FP variable | ||||
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Posted in reply to bearophile | This is not really a bug.
I'd expect -0 to be an integer 0. Negative zero doesn't make sense for integers, and the behavior is consistent with C/C++.
A warning, like you proposed, could possibly help avoiding a mistake.
bearophile Wrote:
> A bug I've introduced once in my D code, are you able to spot it?
>
> void main() {
> double x = +0;
> double y = -0;
> }
>
>
> The bug: 'y' isn't the desired double -0.0
>
> To avoid this bug DMD may keep the -0 and +0 integer literals represented in two distinct ways, so they have two different values when/if assigned to a floating point. (But here D will behave a little differently from C).
> An alternative is to just add a warning to DMD.
> A third possibility is to just ignore this probably uncommon bug :-)
>
> Bye,
> bearophile
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January 10, 2011 Re: -0 assigned to a FP variable | ||||
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Posted in reply to Jonathan M Davis | On Sun, 09 Jan 2011 20:39:12 -0500, Jonathan M Davis <jmdavisProg@gmx.com> wrote:
> On Sunday 09 January 2011 16:27:11 bearophile wrote:
>> A bug I've introduced once in my D code, are you able to spot it?
>>
>> void main() {
>> double x = +0;
>> double y = -0;
>> }
>>
>>
>> The bug: 'y' isn't the desired double -0.0
>>
>> To avoid this bug DMD may keep the -0 and +0 integer literals represented
>> in two distinct ways, so they have two different values when/if assigned
>> to a floating point. (But here D will behave a little differently from C).
>> An alternative is to just add a warning to DMD.
>> A third possibility is to just ignore this probably uncommon bug :-)
>>
>> Bye,
>> bearophile
>
> I didn't even know that there _was_ such as thing as + or - 0 (or 0.0). I would
> have considered it a no-op, being identical to 0 or 0.0, and expected it to be
> compiled out completely. I haven't a clue what -0.0 would even mean. But I'm not
> exactly an expert on floating point values, so presumably, there's some weird
> floating point thing that - affects.
-0.0 is an artifact of the floating point sign bit: i.e. there is a + and - for each value, so naturally there's also + and - 0. The difference isn't generally something you care about, as they are practically identical except in bit pattern (i.e. assert( 0.0 == -0.0 )).
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January 10, 2011 Re: -0 assigned to a FP variable | ||||
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Posted in reply to bearophile | bearophile wrote:
> A bug I've introduced once in my D code, are you able to spot it?
>
> void main() { double x = +0; double y = -0; }
>
>
> The bug: 'y' isn't the desired double -0.0
>
> To avoid this bug DMD may keep the -0 and +0 integer literals represented in
> two distinct ways, so they have two different values when/if assigned to a
> floating point. (But here D will behave a little differently from C). An
> alternative is to just add a warning to DMD. A third possibility is to just
> ignore this probably uncommon bug :-)
This is not a bug in the language or compiler. While there is such a thing as -0.0, there is no such thing as -0 as being distinct from 0. If you want to flag mathematical no-ops as errors, what's next? a+0 is an error, too?
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January 10, 2011 Re: -0 assigned to a FP variable | ||||
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Posted in reply to bearophile | On 10/01/11 11:27, bearophile wrote:
> A bug I've introduced once in my D code, are you able to spot it?
>
> void main() {
> double x = +0;
> double y = -0;
> }
>
>
> The bug: 'y' isn't the desired double -0.0
>
> To avoid this bug DMD may keep the -0 and +0 integer literals represented in two distinct ways, so they have two different values when/if assigned to a floating point. (But here D will behave a little differently from C).
> An alternative is to just add a warning to DMD.
> A third possibility is to just ignore this probably uncommon bug :-)
>
> Bye,
> bearophile
Hi bearophile,
This time it really looks like you are trying to pull the wings off of a butterfly.
But in this case me thinks the butterfly wins.
Best regards and Happy New Year,
Justin Johansson
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January 10, 2011 Re: -0 assigned to a FP variable | ||||
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Posted in reply to bearophile | On 10/01/11 11:27, bearophile wrote:
> A bug I've introduced once in my D code, are you able to spot it?
>
> void main() {
> double x = +0;
> double y = -0;
> }
>
>
> The bug: 'y' isn't the desired double -0.0
>
> To avoid this bug DMD may keep the -0 and +0 integer literals represented in two distinct ways, so they have two different values when/if assigned to a floating point. (But here D will behave a little differently from C).
> An alternative is to just add a warning to DMD.
> A third possibility is to just ignore this probably uncommon bug :-)
>
> Bye,
> bearophile
> A bug I've introduced once in my D code, are you able to spot it?
>
> void main() {
> double x = +0;
> double y = -0;
> }
The butterfly answers back to you.
There is no bug. Did you mean
<code>
void main() {
double x = +0.0;
double y = -0.0;
}
</code> ?
Are you able to spot it?
Is it still a bug?
Cheers
Justin
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January 10, 2011 Re: -0 assigned to a FP variable | ||||
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Posted in reply to bearophile | Le 10/01/2011 01:27, bearophile a écrit :
> A bug I've introduced once in my D code, are you able to spot it?
>
> void main() {
> double x = +0;
> double y = -0;
> }
>
>
> The bug: 'y' isn't the desired double -0.0
>
> To avoid this bug DMD may keep the -0 and +0 integer literals represented in two distinct ways, so they have two different values when/if assigned to a floating point. (But here D will behave a little differently from C).
> An alternative is to just add a warning to DMD.
> A third possibility is to just ignore this probably uncommon bug :-)
>
> Bye,
> bearophile
Hi,
Do you suggest the compiler should guess you want -0.0 and convert 0 to double before applying the unary minus operator ?
I suppose such a fix would be likely to create many more bugs than the one it is suppose to solve.
I don't have a clue on what kind of user code needs to differentiate 0.0 and -0.0 fp litterals, but I humbly suppose that it must be kind of rare.
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