[Bug c/30813] New: Numerical error--#define value differs from declared variable value

[Bug c/30813] New: Numerical error--#define value differs from declared variable value

[kevin dot glass at pnl dot gov]

I've used a #define to set a value, I've set a declared variable to the same
text value and I've set a second declared variable to the #define value. The
declared values are the same, the #define value differs.

If this were a simple round off error, all three values should be the same. As
they are not the same, the #define has a different value. I think the #define
might have access to all of the guard bits in the FPU whereas the declared
values are truncated.

Also, please note, this problem does not happen with gcc 2.9.2 and I'm running
on Red Hat Linux

==============================================================================
cat prec.c
==============================================================================
#include <stdio.h>

#define VALUE 1e-4

int
main()
{
   double v = 1.0;
   double a = 1e-4;
   double b = VALUE;

   printf("a = %e\n", a);
   printf("def/var - 1 = %e, var/def - 1 = %e\n", (VALUE*VALUE)/(a*a) - v,
      (a*a)/(VALUE*VALUE) - v);
   printf("def/var2 - 1 = %e, var2/def - 1 = %e\n", (VALUE*VALUE)/(b*b) - v,
      (b*b)/(VALUE*VALUE) - v);
   printf("var/var - 1 = %e, def/def - 1 = %e\n", (a*a)/(a*a) - v,
      (VALUE*VALUE)/(VALUE*VALUE) - v);
   printf("var2/var1 - 1 = %e, var2/var2 - 1 = %e\n", (b*b)/(a*a) - v,
      (b*b)/(b*b) - v);
}
========================================================================================
$ gcc prec.c
$ ./a.out
a = 1.000000e-04
def/var - 1 = -7.486416e-17, var/def - 1 = 7.491837e-17
def/var2 - 1 = -7.486416e-17, var2/def - 1 = 7.491837e-17
var/var - 1 = 0.000000e+00, def/def - 1 = 0.000000e+00
var2/var1 - 1 = 0.000000e+00, var2/var2 - 1 = 0.000000e+00
$


-- 
           Summary: Numerical error--#define value differs from declared
                    variable value
           Product: gcc
           Version: 3.4.5
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kevin dot glass at pnl dot gov


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30813

[Bug target/30813] Numerical error--#define value differs from declared variable value

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2007-02-15 21:23 -------
This works correctly on powerpc-linux-gnu.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30813

[Bug target/30813] Numerical error--#define value differs from declared variable value

[kevin dot glass at pnl dot gov]

------- Comment #2 from kevin dot glass at pnl dot gov  2007-02-15 21:26 -------
Subject: RE:  Numerical error--#define value differs from declared variable
value


I ran on a 64-bit Itanium running Red Hat linux, I'm not sure which
version. 

-----Original Message-----
From: pinskia at gcc dot gnu dot org [mailto:gcc-bugzilla@gcc.gnu.org] 
Sent: Thursday, February 15, 2007 1:24 PM
To: Glass, Kevin A
Subject: [Bug target/30813] Numerical error--#define value differs from
declared variable value



------- Comment #1 from pinskia at gcc dot gnu dot org  2007-02-15 21:23
------- This works correctly on powerpc-linux-gnu.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30813

[Bug target/30813] Numerical error--#define value differs from declared variable value

[kevin dot glass at pnl dot gov]

------- Comment #3 from kevin dot glass at pnl dot gov  2007-02-16 15:29 -------
I ran this on a Pentium III and a Pentium IV using gcc 3.4.5 and gcc 4.01(?).
These produced incorrect results. I also ran them on an itanium using an older
gcc (2.9.2) in these cases, it produced correct results.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30813

[Bug target/30813] Numerical error--#define value differs from declared variable value

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30813

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
ok, this is really PR 323.
With -std=c90 or -std=c99, it provides the correct thing for x86 (but note it
is not 0 really).

Also you could use -mfpmath=sse these days.

*** This bug has been marked as a duplicate of bug 323 ***