LI75902: INCORRECT OUT WITH -O2 AND -QSTATICINLINE

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
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

• When an unused function is removed from the test case, the test
  case does not encounter a runtime logic error when compiled with
  -O2 and -qstaticinline.
  
  ===== Test case
  // Compile with:
  // xlC -O2 -qstaticinline xlcbug.C -o xlcbug
  
  #include <iostream>
  
  using namespace std;
  
  // This function is not used, but if I remove this, the bug goes
  away.
  static inline void foo(void)
  {
      cout << endl;
  }
  
  typedef unsigned long ulong;
  
  static inline ulong permute_inv(ulong inv, ulong perm)
  {
      if (perm >= 6) {
   ulong i = perm >= 18 ? 2 : (perm+4)/16;
   perm -= 6*(i+1);
   i = 3-i;
   ulong x = ((inv<<i) ^ inv) & 8;
   inv ^= x ^ (x>>i);
      }
      if (perm >= 2) {
   ulong i = 3 - perm/2;
   ulong x = ((inv<<i) ^ inv) & 4;
   inv ^= x ^ (x>>i);
      }
      if (perm & (inv ^ (inv>>1)) & 1)
   inv ^= 3;
      return inv;
  }
  
  struct FnPermT {
      unsigned inv : 5;
  };
  
  int bugfn = 16;
  
  static FnPermT minfn[1<<16];
  
  static void init_minfn(void)
  {
      ulong inv = permute_inv(1,2);
      minfn[bugfn].inv = permute_inv(1,2);
      if (minfn[16].inv != inv)
   printf("BUG: %u != %lu\n", minfn[bugfn].inv, inv);
  }
  
  int main(void)
  {
      init_minfn();
      return 0;
  }
  
  
  ==== Output
  #xlC -O2 -qstaticinline xlcbug.C -o a.opt
  # ./a.opt
  BUG: 0 != 4
  

Local fix

• Remove the 'static inline void foo()' from the test case.
  
  When the 'foo()' function is removed, the test case executes
  successfully.
  
  
  ==== Output
  # xlC -O2 -qstaticinline xlcbug.C -o b.opt
  # ./b.opt
  #
  

Problem summary

• Incorrect displacement calculation results in bad code
  generation, leading to incorrect output.
  

Problem conclusion

• Fix displacement calculation.
  

Temporary fix

Comments

APAR Information

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

  IZ83423

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

Fix information

• Fixed component name

  XL C/C++ RHEL52

• Fixed component ID

  5724U8310

Applicable component levels

• RA10 PSN IZ83423

     UP06/09/13