LI75801: INCORRECT RESULT FROM FP_INVALID_OP()

November 2010 Update for XL C/C++ for Linux, V11.1
XL C/C++ for Linux Fix Pack 9 (August 2013 Update) for 11.1
April 2011 Update for XL C/C++ for Linux, V11.1
July 2011 Update for XL C/C++ for Linux, V11.1
October 2011 Update for XL C/C++ for Linux, V11.1
June 2012 Update for XL C/C++ for Linux, V11.1
October 2012 Update for XL C/C++ for Linux, V11.1
XL C/C++ for Linux Fix Pack 8 (June 2013 Update) for 11.1

APAR status

• Closed as program error.

Error description

• After enabling floating point exceptions at runtime and
  compiling with -O3 and -qstrict, the following test case is
  giving incorrect output for fp_invalid_op().
  
  
       ====  TEST CASE  ====
  
  #include <stdio.h>
  #include <math.h>
  #include <fptrap.h>
  
  double x = -1;
  
  int main()
  {
    int i, j;
    double sum;
  
    (void) fp_enable(TRP_INVALID);
  
    sum = 0;
    if (x > 0)
      sum += log(x);
    printf("sum %g\n", sum);
  
    x += sum;
  
    for (i = 0; i < 1; i++)
      for (j = 0; j < 1; j++)
        ;
  
    printf("fp_invalid_op() => %d\n", fp_invalid_op());
    return 0;
  }
  
  
  $ xlc -O3 -qstrict test.c -lm
  sum 0
  fp_invalid_op() => 1
  $ xlc test.c -lm
  sum 0
  fp_invalid_op() => 0
  $
  

Local fix

• Compile with either lower optimization, -qnohot, or -qflttrap.
  

Problem summary

• PROBLEM DESCRIPTION:
  Floating point instruction speculation triggers floating point
  traps when traps are enabled at runtime via fp_enable().  These
  traps are detected by the fp_invalid_op() function.
  
  USERS AFFECTED:
  Users who utilize fp_enable to enable floating point traps at
  runtime.
  

Problem conclusion

• Floating point speculation that may cause traps can now be
  disabled by compiling with the options:  -qfloat=fenv
  -qstrict=guards
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

  IZ83163

• APAR is sysrouted TO one or more of the following:

Fix information

• Fixed component name

  XL C/C++ FOR LI

• Fixed component ID

  5724X1400

Applicable component levels

• RB10 PSN IZ83163

     UP06/09/13