[Bug libgcj/16122] New: gij - Incorrect result due to computations in extended precision on x86

[Bug libgcj/16122] New: gij - Incorrect result due to computations in extended precision on x86

[debian-gcc at lists dot debian dot org]

[forwarded from http://bugs.debian.org/255525]

3.3.4 release and 3.4.1 branch, bug submitter writes:

Concerning the following Java source: 
 
------------------------------------------------------------------ 
// $Id: test.java 3734 2004-06-21 15:36:52Z lefevre $ 
 
public class test { 
    public static void main(String[] args) throws Exception { 
        test t = new test(); 
        t.doTest(); 
    } 
 
    volatile double x, y, z, d; 
 
    public void doTest() { 
        x = 9007199254740994.0; /* 2^53 + 2 */ 
        y = 1.0 - 1/65536.0; 
        z = x + y; 
        d = z - x; 
        System.out.println("z = " + z); 
        System.out.println("d = " + d); 
    } 
} 
------------------------------------------------------------------ 
I've compiled it with "gcj -C test.java" (GCC 3.3.4). 
 
Both IBM's and Sun's JVM give the correct result: 
 
greux:~/wd/src/fp> /global/greux/lefevre/IBMJava2-131/jre/bin/java test 
z = 9.007199254740994E15 
d = 0.0 
 
greux:~/wd/src/fp> /usr/local/j2re1.4.1/bin/java test 
z = 9.007199254740994E15 
d = 0.0 
 
but not gij: 
 
greux:~/wd/src/fp> /usr/bin/gij test 
z = 9.007199254740996E15 
d = 2.0 
 
gij should switch the FPU of the x86 processor 
(Pentium III in my case) to rounding in double precision to avoid the 
effect of the "double rounding" (you may find some information about 
this effect here: <http://www.srware.com/linux_numerics.txt>).

-- 
           Summary: gij - Incorrect result due to computations in extended
                    precision on x86
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libgcj
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: debian-gcc at lists dot debian dot org
                CC: gcc-bugs at gcc dot gnu dot org,java-prs at gcc dot gnu
                    dot org
  GCC host triplet: i486-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16122

[Bug libgcj/16122] gij - Incorrect result due to computations in extended precision on x86

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-06-21 20:24 -------
Actually this is not a bug unless you use strictfp but IIRC strictfp does nothing to the problem anyways.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16122

[Bug libgcj/16122] gij - Incorrect result due to computations in extended precision on x86

[vincent at vinc17 dot org]

------- Additional Comments From vincent at vinc17 dot org  2004-06-22 06:04 -------
I don't know very much about Java, but I've talked with someone who has some
knowledge in Java and floating point, and according to him, Java FP arithmetic
operations must round exactly in double precision (this is also confirmed by the
documentation I could see). strictfp concerns the exponent range of the double
precision vs extended precision, but this is not the problem here.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16122

[Bug libgcj/16122] gij - Incorrect result due to computations in extended precision on x86

[vincent at vinc17 dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vincent at vinc17 dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16122

[Bug libgcj/16122] gij - Incorrect result due to computations in extended precision on x86

[mckinlay at redhat dot com]

------- Additional Comments From mckinlay at redhat dot com  2004-06-22 16:51 -------
Confirmed with HEAD. The problem doesn't occur with native compilation, it is a
gij issue only.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-06-22 16:51:42
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16122