Below is the reply from SUN.  It was indeed a bug which has been fixed already.

I have not yet applied the patches to my SUN compilers and tried again.

/Jean

Begin forwarded message:

From: Sun Microsystems <IncidentUpdateDaemon@sun.com>
Date: July 1, 2008 3:51:17 PM PDT

Subject: Re: (Incident Review ID: 1284413) Sun C 5.8 optimization bug for 64-bit Opteron

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Hi Jean Brouwers,

We appreciate your feedback.  Using your sample I was able to get the correct results using Sun 
Studio 12.  This may indicate that the issue was also fixed in a backported patch to Studio 11 (Sun 
C 5.8).  Check to be sure that the latest patches have been applied to your installation.  See:

<http://developers.sun.com/sunstudio/downloads/patches/ss11_patches.html>

Regards,
Brad Mayer

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--------------- Previous Messages ----------------


--------------------- Report ---------------------

      category : c
   subcategory : compiler
       release : other
          type : bug
      synopsis : Sun C 5.8 optimization bug for 64-bit Opteron
 customer name : Jean Brouwers
        sdn id : 
      language : en
      hardware : x86
            os : sol2.5.1
        bug id : 0
  date created : Sun Jun 29 11:49:10 MST 2008
date evaluated : Tue Jul 01 15:41:10 MST 2008
   description : 

FULL PRODUCT VERSION :
which cc
/opt/SUNWspro/bin/cc
cc -V
cc: Sun C 5.8 2005/10/13

ADDITIONAL OS VERSION INFORMATION :
uname -a
SunOS unknown 5.10 Generic_118855-14 i86pc i386 i86pc


EXTRA RELEVANT SYSTEM CONFIGURATION :
Solaris 10 on an Ultra20 Opteron machine.


A DESCRIPTION OF THE PROBLEM :
At higher optimization levels, the Sun C compiler does not handle certain function call patterns 
correctly.  Specifically, the values of complex_t  variables q and r in the code snippet below are 
different.

typedef struct {
	double real;
	double imag;
} complex_t;

complex_t c_comp(double real);
complex_t c_quot(complex_t a, complex_t b);
...
	complex_t_t x, y, q, r;

	x = c_comp(4.6051701859880918);
	y = c_comp(0.69314718055994529);

	q = c_quot(x, y);

	r = c_quot(x, c_comp(0.6931471805599452));
...

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
See the attached test case.  Instructions are included inside.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -

rm a.out ; cc -xtarget=native64 -xO0 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
6.643856 + j 0.000000

ACTUAL -

rm a.out ; cc -xtarget=native64 -xO3 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
Inf + j 0.000000


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------

/* Split this file in 3 separate file, c_main.c, c_quot.h and
   c_quot.c.  Compile with and without optimization using Sun
   C 5.8 compiler on Solaris 10 (Opteron) and run as follows.

rm a.out ; cc -xtarget=native64 -xO0 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
6.643856 + j 0.000000

rm a.out ; cc -xtarget=native64 -xO1 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
6.643856 + j 0.000000

rm a.out ; cc -xtarget=native64 -xO2 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
6.643856 + j 0.000000

rm a.out ; cc -xtarget=native64 -xO3 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
Inf + j 0.000000

rm a.out ; cc -xtarget=native64 -xO4 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
Inf + j 0.000000

rm a.out ; cc -xtarget=native64 -xO5 c_main.c c_quot.c -lm ; ./a.out
c_main.c:
c_quot.c:
6.643856 + j 0.000000
Inf + j 0.000000

*/


------------------ save as c_main.c -----------------
#include <stdio.h>
#include "c_quot.h"

int main(int argc, char* argv[])
{
	complex_t x, y, q, r;

	x = c_comp(4.6051701859880918);
	y = c_comp(0.69314718055994529);
	q = c_quot(x, y);
	/* expect: 6.643856 + j 0.000000 */
	printf("%f + j %f\n", q.real, q.imag);

	x = c_comp(4.6051701859880918);
	y = c_comp(0.69314718055994529);
	r = c_quot(x, c_comp(0.6931471805599452));
	/* expect: 6.643856 + j 0.000000, but ... */
	printf("%f + j %f\n", r.real, r.imag);
}


------------------ save as c_quot.h -----------------
typedef struct {
	double real;
	double imag;
} complex_t;

complex_t c_comp(double real);
complex_t c_quot(complex_t a, complex_t b);


------------------ save as c_quot.c -----------------
#include "c_quot.h"

complex_t
c_comp(double real)
{
	complex_t c;
	c.real = real;
	c.imag = 0.0; /* ignore imag */
	return c;
}

complex_t
c_quot(complex_t a, complex_t b)
{
	complex_t r;
	r.real = a.real / b.real;
	r.imag = 0.0; /* ignore imag */
	return r;
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Compiling with -xO... less than 3 avoids the problem.