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To: "PHP Internals" <internals@lists.php.net>, "Remi Collet"
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Date: Sun, 10 Feb 2013 20:27:48 +0100
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Subject: Re: [PHP-DEV] double val to long val conversion issue
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

On Sun, 10 Feb 2013 16:19:46 +0100, Remi Collet <remi@fedoraproject.org>=
  =

wrote:

> Le 10/02/2013 15:58, Gustavo Lopes a =C3=A9crit :
>
>> Can you test this program on ppc64:
>
> $ gcc -O3 -lm conv.c && ./a.out 9223372036854775808
> -9223372036854775808 -9223372036854775808
>
> $ gcc -O3 -lm conv.c && ./a.out 4e21
> -2943463994972700672 -1
>
> $ gcc -O3 -lm conv.c && ./a.out 4e19
> 3106511852580896768 -1
>
>
>> I get this:
>> $ gcc -O3 -lm conv.c && ./a.out 9223372036854775808
>> -9223372036854775808 -9223372036854775808
>> $ gcc -O3 -lm conv.c && ./a.out 4e21
>> -2943463994972700672 0
>> $ gcc -O3 -lm conv.c && ./a.out 4e19
>> 3106511852580896768 0

This was I was afraid. That bug was just the tip of the iceberg. I sugge=
st  =

we do change the the > operator to >=3D like you proposed, but also that=
 we  =

add the fmod call. The code I gave earlier had a bug btw, as fmod can gi=
ve  =

a negative number. I changed it to this:

long convert(double d)
{
     double dmod =3D fmod(d, pow(2., 64.));
     if (dmod < 0) {
         dmod +=3D pow(2., 64.);
     }
     return (long)(unsigned long)dmod;
}

I tested the doubles around -4e21 and it worked fine:

$ ./a.out -4000000000000001048576
2943463994971652096 -9223372036854775808
$ ./a.out -4000000000000000524288
2943463994972176384 -9223372036854775808
$ ./a.out -4000000000000000000000
2943463994972700672 -9223372036854775808
$ ./a.out -3999999999999999475712
2943463994973224960 -9223372036854775808
$ ./a.out -3999999999999998951424
2943463994973749248 -9223372036854775808

In[36]:=3D c /@ Table[-4.*^21 + i*Ulp[-4.*^21], {i, -2, 2}]

Out[36]=3D {2943463994971652096, 2943463994972176384, \
2943463994972700672, 2943463994973224960, 2943463994973749248}

Any reservations?

-- =

Gustavo Lopes