• d-bugmail puremagic.com Jan 11 2013
• d-bugmail puremagic.com Feb 26 2013

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=9297

           Summary: Formatting of floating point values in std.format
                    truncates reals to double
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: bugzilla digitalmars.com


--- Comment #0 from Walter Bright <bugzilla digitalmars.com> 2013-01-11
21:55:36 PST ---
The following from std.format shows the problem:

    n = snprintf(fbuf.ptr, sl, format.ptr, field_width,
            precision, cast(double)v);

It relies on the VC runtime library function snprintf to do the formatting, but
that function does not support 80 bit reals.

To fix we'll need to write our own 80 bit formatting routine instead of relying
on the host C compiler's.

-- 
Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email
------- You are receiving this mail because: -------

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=9297


hsteoh quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh quickfur.ath.cx


--- Comment #1 from hsteoh quickfur.ath.cx 2013-02-26 16:56:01 PST ---
Does gcc/g++ support 80-bit floats? I made a little test program in a Linux
environment, and it seems that in spite of the cast, some extra precision is
still getting through:

import std.stdio;

void main() {
        // Stuff with way too many digits than the types can handle, to see how
        // far they get.
        enum float phi_f = 1.6180339887_4989484820_4586834365f;
        enum double phi_d = 1.6180339887_4989484820_4586834365;
        enum real phi_r = 1.6180339887_4989484820_4586834365L;

        // The exact digits, taken from:
        //
        //
http://fabulousfibonacci.com/portal/index.php?option=com_content&view=article&id=7&Itemid=17
        //
        // (NOTE: the last digit is actually 6 if we round up the next digit
        // instead of truncating. But built-in types won't even come close to
        // that point).
        string phi_s = "1.618033988749894848204586834365";

        writeln(float.dig);
        writeln(double.dig);
        writeln(real.dig);

        writefln("%.25f", phi_f);
        writefln("%.25f", phi_d);
        writefln("%.25f", phi_r);
        writefln("%s", phi_s);
}


Output:

$ ./test
6
15
18
1.6180340051651000976562500
1.6180339887498949025257389
1.6180339887498948482072100
1.618033988749894848204586834365
$

Notice that the third line, which is the formatting of real, shows up with more
matching digits than double. Could this be something to do with the way the
Linux C/C++ ABI works (IIRC some floats gets passed in the FPU registers where
they retain their 80-bit precision)?

I don't have a way to test this on VC, though.

-- 
Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email
------- You are receiving this mail because: -------