IV09175: LONG TYPE LOOP INDEX CAUSES SLOWDOWN AT O3

January 2012 PTF for XL C for AIX, V11.1
January 2012 PTF for XL C/C++ for AIX, V11.1
April 2012 PTF for XL C for AIX, V11.1
April 2012 PTF for XL C/C++ for AIX, V11.1
July 2012 PTF for XL C/C++ for AIX, V11.1
October 2012 PTF for XL C/C++ for AIX, V11.1
October 2012 PTF for XL C for AIX, V11.1
February 2013 PTF for XL C for AIX, V11.1
February 2013 PTF for XL C/C++ for AIX, V11.1
XL C/C++ for AIX Fix Pack 14 (May 2013 PTF) for 11.1
XL C for AIX Fix Pack 14 (May 2013 PTF) for 11.1
XL C/C++ for AIX Fix Pack 15 (August 2013 PTF) for 11.1
XL C for AIX Fix Pack 15 (August 2013 PTF) for 11.1
XL C/C++ for AIX Fix Pack 16 (November 2013 PTF) for 11.1
XL C for AIX Fix Pack 17 (February 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 17 (February 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 18 (May 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 19 (August 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 20 (November 2014 PTF) for 11.1
XL C for AIX Fix Pack 21 (April 2015 PTF) for 11.1
XL C/C++ for AIX Fix Pack 21 (April 2015 PTF) for 11.1
XL C/C++ for AIX Fix Pack 22 (September 2015 PTF) for 11.1
XL C for AIX Fix Pack 23 (March 2016 PTF) for 11.1
XL C/C++ for AIX Fix Pack 23 (March 2016 PTF) for 11.1
XL C for AIX Fix Pack 24 (September 2016 PTF) for 11.1
XL C/C++ for AIX Fix Pack 24 (September 2016 PTF) for 11.1
July 2012 PTF for XL C for AIX, V11.1
XL C for AIX Fix Pack 16 (November 2013 PTF) for 11.1
XL C for AIX Fix Pack 18 (May 2014 PTF) for 11.1
XL C for AIX Fix Pack 19 (August 2014 PTF) for 11.1
XL C for AIX Fix Pack 20 (November 2014 PTF) for 11.1
XL C for AIX Fix Pack 22 (September 2015 PTF) for 11.1

APAR status

• Closed as program error.

Error description

• Consider the following testcase:
  
  $cat t.c
  ////////////////////////////////////////
  void foo(double *x)
  {
    TYPE i;
    for (i = 0; i < 100; i++)
      *x++ += 1.1;
  }
  
  double z[1000];
  
  int main()
  {
    int i;
    memset(z,0,sizeof(z));
    for (i = 0; i < 60*1000*1000; i++)
      foo(z);
    return 0;
  }
  ////////////////////////////////////////
  
  Compile with:
  
  xlc t.c -q64 -DTYPE=long -O
  xlc t.c -q64 -DTYPE=long -O3
  
  and
  
  xlc t.c -q64 -DTYPE=int -O
  xlc t.c -q64 -DTYPE=int -O3
  
  The executable compiled with -O3 and -DTYPE=long exhibits worse
  runtime performance than the one compiled with lower level of
  optimization (-O). This regression does not occur with
  -DTYPE=int.
  

Local fix

• N/A
  

Problem summary

• Performance degradation due to over-aggressive unrolling
  

Problem conclusion

• Fixed a potential performance problem due to complete unrolling
  of high iteration count loops.
  

Temporary fix

Comments

APAR Information

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

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

  LI76655

Fix information

• Fixed component name

  XL C FOR AIX

• Fixed component ID

  5724X1200

Applicable component levels