LI77936: INEFFICIENT CODE FOR -(A!=0)

XL C/C++ for Linux Fix Pack 8 (October 2015 Update) for 12.1
XL C/C++ for Linux Fix Pack 7 (May 2014 Update) for 12.1

APAR status

• Closed as program error.

Error description

• The compiler generates inefficient code for the following:
  
  ===== COMPILE COMMAND:
  xlC -q64 -O2 -qlist test.cpp
  
  ===== TESTCASE:
  $ cat test.cpp
  #define Uint64 unsigned long long
  Uint64 notZeroMask(Uint64 in)
  {
    return -(in != 0);
  }
  $
  
  
  ===== ACTUAL OUTPUT:
  Looking at the listing file:
  Output:
       | 000000                           PDEF
  notZeroMask(unsigned long long)
       | 000000                           AKA
  notZeroMask__FUL
     51|                                  PROC      in,gr3
     53| 000160 addic    3003FFFF   1     ADDC      gr0,ca=gr3,-1
     53| 000164 addi     38600000   1     LI        gr3=0
     53| 000168 addze    7C030194   1     ADDE
  gr0,ca=gr3,0,ca
     53| 00016C neg      7C8000D0   1     COMP      gr4=gr0
     53| 000170 extsw    7C8307B4   1     EXTS4     gr3=gr4
     54| 000174 bclr     4E800020   1     BA        lr
  
  
  
  
  ===== EXPECTED OUTPUT:
  Optimal output consists of subfic + subfe such as the one
  created using the following code:
  Uint64 notZeroMask_opt(Uint64 in)
  {
     Uint64 mask;
     asm("subfic %0, %1, 0; subfe %0,%0,%0" : "=r"(mask) : "r"(in)
  : "xer");
     return mask;
  }
  

Local fix

• N/A
  

Problem summary

• USERS AFFECTED:
  Users who use the -(a!=0) syntax may be affected by this issue.
  
  PROBLEM DESCRIPTION:
  The compiler produces inefficient code for the -(a!=0)
  comparison.
  

Problem conclusion

• The compiler now generates better code for
  
  Uint64 notZeroMask(Uint64 in)
  {
   return in != 0 ? 0xFFFFFFFFFFFFFFFFULL : 0ULL;
  }
  
  which is semantically equivalent to
  
  Uint64 notZeroMask(Uint64 in)
  {
   return -(in != 0);
  }
  

Temporary fix

Comments

APAR Information

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

  IV55854

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

Fix information

• Fixed component name

  XL C/C++ FOR LI

• Fixed component ID

  5725C7300

Applicable component levels

• RC10 PSY

     UP