rint -lm function broke

[Robert Thomas <thoma041 at info dot census dot gov>]

Hello Gnu (Skip narrative goto BUG label on left)

	This bug involves the rint function.  I have debugged it as far as
as I can and I'm not sure what is wrong without getting an assembler book
to debug the asm.

	We are using mysql to render web data and mysql was compiled on a
RH Linux 6.2 box using gcc (and egcs,gcc-2.95.2, it didn't matter).  This
error is also reproduceable on a Sun Solaris 2.8 box using gcc and sun's
libm.



BUG:


	rint under gcc rounds down if the major part is even and the
minor is exactly 5, rounds correctly if the major part is odd.  I really
don't think that this is the actual library rint problem, it seems to be a
gcc problem.  Here is some very reproduceable (raw) code to show this and 
this was rendered from the actual mysql code eliminating and
simplifying lines for readability:

test1.c
------
#include <math.h>

double Item_func_round(double value, int dec)
{
  unsigned int abs_dec=abs(dec);
  double tmp=pow(10.0,(double) abs_dec);
  return dec < 0 ? rint(value/tmp)*tmp : rint(value*tmp)/tmp;

}

main () {
  double num;
  int prec;
  double result;

  while (1) {
    scanf ("%lf %d",&num,&prec);
    result = Item_func_round (num,prec);
    printf ("%lf -> %lf\n",num,result);
  }
}

------cut-----

What this is designed to do is to take a number and round it depending on
the integer argument.  Negative values round to the left of the decimal,
zero is at the decimal and positive values to the right.

Brillig.ssd.census.gov is a very busy RH linux box (Dell 6300 4X).

[thoma041@brillig test] gcc -o test1 test1.c -lm
[thoma041@brillig test] ./test1
2.5 0
2.500000 -> 2.000000
3.5 0
3.500000 -> 4.000000

info.census.gov is a Sun 250 and the gcc will do the same thing there.  If
I compile the same code under /opt/SUNWspro/bin/cc (the new sun forte
compiler), it works as it should. a ldd shows:

test1:
        libm.so.1 =>     /usr/lib/libm.so.1
        libc.so.1 =>     /usr/lib/libc.so.1
        libdl.so.1 =>    /usr/lib/libdl.so.1
test1.sun:
        libc.so.1 =>     /usr/lib/libc.so.1
        libdl.so.1 =>    /usr/lib/libdl.so.1


The assembler between the 2 compilers is different enough I didn't want to
speculate on what is wrong for I may lead someone up the wrong path.

-Robert 

--------------------------------------------------------------------------
(Signature file)

Robert Thomas, CISSP
FOB-3, Room 1364
Washington, DC 20233
Phone 301-457-2157


No free man shall ever be debarred the use of arms.
-Thomas Jefferson, Proposed Virginia constitution, 1776

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