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digitalmars.D.bugs - [Issue 1681] New: cast(real) ulong.max == 0

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681

           Summary: cast(real) ulong.max == 0
           Product: D
           Version: 2.007
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: andrei metalanguage.com


The title says it all. Here's a test:

import std.stdio;
import std.conv;

void main()
{
    real r = ulong.max;
    ulong d = cast(ulong) r;
    writeln(d);
    assert(d == ulong.max); // should not fail
}


--
Nov 21 2007
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681








 Real is a floating point type, typically 80 bits. Ulong is a 64-bit 
 integer type. Real is not capable of holding a value as large as 
 ulong.max. That is why the assert fails.


Float must store integers with proper successors up to 2 ** 24 - 1. Double must
store integers with proper successors up to 2 ** 53 - 1. Real (i.e.
"double-extended") must store integers with proper successors up to at least 2
** 64 - 1. The program prints 0.


--
Nov 21 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681










 Real is a floating point type, typically 80 bits. Ulong is a 64-bit 
 integer type. Real is not capable of holding a value as large as 
 ulong.max. That is why the assert fails.


 
 Float must store integers with proper successors up to 2 ** 24 - 1. Double must
 store integers with proper successors up to 2 ** 53 - 1. Real (i.e.
 "double-extended") must store integers with proper successors up to at least 2
 ** 64 - 1. The program prints 0.
 


This is also interesting:

    writefln("ulong.max      %-30d", ulong.max);
    writefln("real ulong.max %-30f", cast(real)ulong.max);

-->

ulong.max      18446744073709551615          
real ulong.max 18446744073709551613.000000   


--
Nov 21 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681


baryluk mpi.int.pl changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |baryluk mpi.int.pl





writefln("ulong.max      %-30d", ulong.max);
writefln("real ulong.max %-30f", cast(real)ulong.max);

gdc (GCC) 4.1.3 20080114 (prerelease gdc 0.25 20071124, using dmd 1.022)
(Debian 0.25-4.1.2-19)

ulong.max      18446744073709551615          
real ulong.max 18446744073709551615.000000   

dmd 1.025:

ulong.max      18446744073709551615          
real ulong.max 18446744073709551615.000000  

This can depend on processor (some processors aren't strictly following IEEE
754, especially in extented precission). Also real isn't nacassary 80bit, real
can be double on some architectures and processors. As far as I known Intel
processors do real extended precission operations (in fact they use internally
128 bit precission).


Tested machines:
Debian GNU/Linux etch, Pentium III (Coppermine) 733MHz
Debian GNU/Linux unstable, Intel(R) Pentium(R) 4 CPU 2.80GHz
Debian GNU/Linux etch, AMD Duron 1200MHz (Unknown model)


Unfortunetly example from first post gives 0 in dmd (gdc works), so it is a
bug.


The largest integer real which gives nonzero result after cast is
18446744073709550591.000000   
18446744073709549568
but as you can see it gives wrong result. Then it is skiping every 2048 (11
bits), with some values correct:
18446744073709549568.000000
18446744073709549568
but most not.

Largest continues interval with correct answers:
0.0 ... 8744073709552616.000000  (about 2**53, so doubles are involved not
real!)

(similar in decimal expansion 18446744073709551615.000000, but probably
coincidence)


--
Jan 20 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681







 
 The largest integer real which gives nonzero result after cast is
 18446744073709550591.000000   
 18446744073709549568
 but as you can see it gives wrong result. Then it is skiping every 2048 (11
 bits), with some values correct:


53+11 = 64


--
Jan 20 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681


clugdbug yahoo.com.au changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|2.007                       |1.023





Also applies in D1.0


--
Feb 19 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681






Fixed dmd 1.041 and 2.026


--
Mar 11 2009
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1681


clugdbug yahoo.com.au changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED




--
Apr 08 2009