LI75124: XLC ISSUE ON SHORT CONSTANT FOLDING

October 2009 Update for XL C/C++ for Linux, V10.1
June 2011 Update for XL C/C++ for Linux, V10.1
XL C/C++ for Linux Fix Pack 8 (January 2014 Update) for 10.1
August 2010 Update for XL C/C++ for Linux, V10.1
January 2011 Update for XL C/C++ for Linux, V10.1
January 2012 Update for XL C/C++ for Linux, V10.1
July 2012 Update for XL C/C++ for Linux, V10.1

APAR status

• Closed as program error.

Error description

• The following test case demonstrates a C++ specific issue with
  xlC that relates to "short" constant folding.
  
  ===== COMPILE COMMAND:
  xlC test.cpp
  
  ===== TESTCASE:
  ===Start of test.cpp=====
  #include <stdio.h>
  int main(int argc, char *argv[])
  {
      //fails
      const short x = ((short)-5 + (short)4) < (short)4;
      //fails
      //const short x = ((short)-5 + (short)4) < (short)4;
      //fails
      //const short x = (short)(((short)-5 +(short)4) < (short)4);
      //fails
      //const short x = (-5 + (short)4) < 4;
      //fails
      //const short x = (short)4 - 5 < 4;
      //fails
      //const short x = (short)-5 + 4 < 4;
  
      //passes
      //const short x = (short)(-5 + 4) < 4;
  
  
      printf("x = %d\n",(int)x);
      printf("value = %d\n",((short)-5 + (short)4));
      return 0;
  }
  
  ====End of test.cpp=====
  
  ===== ACTUAL OUTPUT:
  $ ./a.out
  x = 0
  value = -1
  
  
  ===== EXPECTED OUTPUT:
  $ ./a.out
  x = 1
  value = -1
  

Local fix

• Use the following type caste:
  const short x = (short)(-5 + 4) < 4;
  

Problem summary

• Users of arithmetic operations on operands requiring
  integral promotion in the context of a constant expression maybe
  affected by this issue.
  
  Incorrect values from constant folding will appear
  in the form of a signed operand being evaluated as
  an unsigned type.
  

Problem conclusion

• Code for integral promotions in evaluating
  the value of constant expressions compared a value in
  bits to a value in bytes and otherwise did not
  allow unsigned types to be promoted to signed types.
  This caused all types requiring promotion to be
  promoted to unsigned int.  Correction is to multiply the
  value in bytes by the number of bits per byte and
  adjust the check to allow unsigned types to be promoted
  to signed types when appropriate.
  

Temporary fix

Comments

APAR Information

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

  IZ44945

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

Fix information

• Fixed component name

  XL C/C++ SLES10

• Fixed component ID

  5724U8300

Applicable component levels

• RA10 PSN IZ44945

     UP06/09/13