IJ24657: IF CHECK MISSED WITH SIMD OPT CAUSING NEGATIVE VALUE TO VDFSQRT

APAR status

• Closed as program error.

Error description

• When using -O3 with SIMD instructions enabled, an IF statement
  in a looping structure is used to check a potential negative
  divisor to avoid calculating a negative square root.  At lower
  optimization levels this works, however SIMD optimization are
  packaging a full vector's worth of data from the loop into a
  single vector operation, without checking the IF conditional.
  This is leading to a runtime Floating Point Exception.
  
  
     === TEST CODE SNIPPET ===
  
  for(i=0; i<m; i++)
  {
      if (dy[i] < 0.)     // MISSING THIS CHECK
          dy[i] = 0.;
      else
          dy[i] = sqrt(2*hn / dy[i]) ;
  }
  

Local fix

• Lower optimization level, do not use SIMD
  

Problem summary

• USERS AFFECTED:
  Users who make use of the sqrt operation and compile with -O3
  and SIMD may be affected by this issue.
  
  PROBLEM DESCRIPTION:
  An internal compiler optimization causes an 'if' check to be
  missed leading to a floating point exception.
  

Problem conclusion

• The optimization has been modified to detect the sqrt operation
  and make necessary changes.
  

Temporary fix

Comments

APAR Information

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

  LI81412

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

Fix information

• Fixed component name

  XL C/C++ FOR AI

• Fixed component ID

  5725C7200

Applicable component levels