[Bug c/34473] New: casting negative __int128_t to float/double rounds to nearest multiple of 2048

[Bug c/34473] New: casting negative __int128_t to float/double rounds to nearest multiple of 2048

[james at albanarts dot com]

casting a negative _int128_t value to a float or double rounds the resulting
floating point number to the nearest multiple of 2048.

the following C program demonstrates which I compiled with the compile string
"gcc":

int main()
{
        __int128_t x = -1025;
        double f = x;
        return f < -1500.0;
}

I would expect the exit code of the program to be 0. f should get set to
-1025.0, which is not < -1500.0.

However when I compile and run this the exit code is 1.

Further examination shows that f gets the following values:
    0.0 for x in the range -1024 to 0
-2048.0 for x in the range -3060 to -1025
    ...etc...

When x is positive f gets set to the expected (the same) value.

"gcc -v" output:

Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-cpu=generic
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)


-- 
           Summary: casting negative __int128_t to float/double rounds to
                    nearest multiple of 2048
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: james at albanarts dot com
 GCC build triplet: x86_64-redhat-linux
  GCC host triplet: x86_64-redhat-linux
GCC target triplet: x86_64-redhat-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34473

[Bug target/34473] casting negative __int128_t to float/double rounds to nearest multiple of 2048

[james at albanarts dot com]

------- Comment #1 from james at albanarts dot com  2007-12-14 23:27 -------
Created an attachment (id=14758)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=14758&action=view)
Very simple failure test case

exit code is 0 for pass, 1 for fail
compiled with compile line "gcc"


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34473

[Bug target/34473] casting negative __int128_t to float/double rounds to nearest multiple of 2048

[james at albanarts dot com]

------- Comment #2 from james at albanarts dot com  2007-12-14 23:30 -------
Oh and i forgot to mention: casting negative __int128_t to "long double"
instead of "float" or "double" compiles correctly.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34473

[Bug target/34473] casting negative __int128_t to float/double rounds to nearest multiple of 2048

[ubizjak at gmail dot com]

------- Comment #3 from ubizjak at gmail dot com  2007-12-16 16:02 -------
>From libgcc2.c, __floatdisf():

  /* Protect against double-rounding error.
     Represent any low-order bits, that might be truncated by a bit that
     won't be lost.  The bit can go in anywhere below the rounding position
     of the SFmode.  A fixed mask and bit position handles all usual
     configurations.  It doesn't handle the case of 128-bit DImode, however. 
*/

Please note that "di" in fact represents TImode for 64bit x86_64. I guess that
__floatdidf() also can't handle 128bit DImode values properly.

This works OK for 4.3.0, beacuse 4.3 branch implements 128bit TFmode floating
point mode and _floatdi{s,d}f function is changed to use this mode.


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
      Known to work|                            |4.3.0
         Resolution|                            |WONTFIX


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34473

[Bug target/34473] casting negative __int128_t to float/double rounds to nearest multiple of 2048

[joseph at codesourcery dot com]

------- Comment #4 from joseph at codesourcery dot com  2007-12-16 16:44 -------
Subject: Re:  casting negative __int128_t to float/double
 rounds to nearest multiple of 2048

On Sun, 16 Dec 2007, ubizjak at gmail dot com wrote:

> This works OK for 4.3.0, beacuse 4.3 branch implements 128bit TFmode floating
> point mode and _floatdi{s,d}f function is changed to use this mode.

I think the fix was actually

2005-12-15  Joseph S. Myers  <joseph@codesourcery.com>

        PR other/25028
        * libgcc2.h (SF_SIZE, DF_SIZE, XF_SIZE, TF_SIZE): Define.
        * libgcc2.c (__floatdixf, __floatundixf, __floatditf,
        __floatunditf): Use #error if type sizes don't match requirements
        of implementation.
        (__floatdisf, __floatdidf): Unify.  Possibly use XFmode or TFmode
        as wider floating-point type.  Use #error if type sizes don't
        match requirements of implementation.  Avoid overflow in computing
        Wtype_MAXp1_F * Wtype_MAXp1_F.  When special casing conversion,
        shift one more bit.  Cast 1 to DWtype or UDWtype for shifting.
        (__floatundisf, __floatundidf): Likewise.
        * config/ia64/hpux.h (XF_SIZE, TF_SIZE): Define.
        * config/ia64/ia64.c (ia64_init_libfuncs): Use
        _U_Qfcnvfxt_quad_to_quad and _U_Qfcnvxf_quad_to_quad for
        TFmode-TImode conversions.
        * doc/tm.texi (SF_SIZE, DF_SIZE, XF_SIZE, TF_SIZE): Document.

(which is in 4.2) and that this is a duplicate of that bug.  If software 
TFmode is being used internally in conversions for narrower modes, that's 
a bug caused by x86_64 not defining WIDEST_HARDWARE_FP_SIZE.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34473

[Bug target/34473] casting negative __int128_t to float/double rounds to nearest multiple of 2048

[ubizjak at gmail dot com]

------- Comment #5 from ubizjak at gmail dot com  2007-12-16 23:27 -------
Reopened ...


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|WONTFIX                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34473

[Bug target/34473] casting negative __int128_t to float/double rounds to nearest multiple of 2048

[ubizjak at gmail dot com]

------- Comment #6 from ubizjak at gmail dot com  2007-12-16 23:27 -------
... to close as duplicate of PR 25028.

*** This bug has been marked as a duplicate of 25028 ***


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34473