LI77634: SUBOPTIMAL BRANCHLESS CODE FOR HI !=0

XL C/C++ for Linux Fix Pack 8 (October 2015 Update) for 12.1
XL C/C++ for Linux Fix Pack 4 (October 2013 Update) for 12.1
XL C/C++ for Linux Fix Pack 5 (December 2013 Update) for 12.1
XL C/C++ for Linux Fix Pack 6 (February 2014 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

• For the test case below, the current sequence
  of instructions are not optimal
  ==> addic+addze+neg+extsw+and
  
  It can be improved to the following:
  ==> addic+subfe+and
  
  
  ===== TESTCASE:
  $ cat test.cpp
  #include <builtins.h>
  
  #define Uint unsigned long long
  #define Uint64 unsigned long long
  #define Sint64 long long
  #define ossCountLeadingZeros64 __cntlz8
  
  
  
  Uint ossCountLeadingZeros128(Uint64 hi, Uint64 lo)
  {
      Uint64 clzHi = ossCountLeadingZeros64(hi);
      Uint64 clzLo = ossCountLeadingZeros64(lo);
      Uint64 mask = -(hi != 0);
      return clzHi + (clzLo & mask);
  }
  $
  
  
  ===== ACTUAL OUTPUT:
   CCR's set/used:   ---- ----
       | 000000                           PDEF
  ossCountLeadingZeros128(unsigned long long, unsigned long long)
       | 000000                           AKA
  ossCountLeadingZeros128__FULT1
     10|                                  PROC      hi,lo,gr3,gr4
     14| 000000 addic    3003FFFF   1     ADDC      gr0,ca=gr3,-1
     14| 000004 addi     38A00000   1     LI        gr5=0
     13| 000008 cntlzd   7C840074   1     CNTLZ8    gr4=gr4
     14| 00000C addze    7C050194   1     ADDE
  gr0,ca=gr5,0,ca
     14| 000010 neg      7CA000D0   1     COMP      gr5=gr0
     14| 000014 extsw    7CA007B4   1     EXTS4     gr0=gr5
     12| 000018 cntlzd   7C650074   1     CNTLZ8    gr5=gr3
     15| 00001C and      7C002038   1     N         gr0=gr0,gr4
     15| 000020 add      7C602A14   1     A         gr3=gr0,gr5
     16| 000024 bclr     4E800020   1     BA        lr
       |               Tag Table
  
  
  ===== EXPECTED OUTPUT:
  addic+subfe+and
  

Local fix

• N/A
  

Problem summary

• USERS AFFECTED:
  Users that have code sequences similar to the following:
  -(hi==0
  
  PROBLEM DESCRIPTION:
  The compiler generates inefficient code for a brnchless code
  -(hi==0).
  As a result, programs would issue two unnecessary instructions
  that would degrade run time performance.
  

Problem conclusion

• The compiler has been fixed such that two new opcodes were
  added and the unnecessary instructions were eliminated one by
  one.
  

Temporary fix

Comments

APAR Information

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

  IV43118

• 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 PSN IV43118

     UP06/09/13