LI77655: SUBOPTIMAL CODE SEQUENCES FOR MULTIPLICATION

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

• Compiler generates suboptimal code sequences for multiplication
  with characteristic immediate.
  
  Consider the following test case:
  
  $cat test.C
  /*********************************************************/
  extern "C" unsigned long long mulimm(unsigned long long in)
  {
    return in * 0xEFE000ULL;
  }
  /*********************************************************/
  
  Compile source test.C as:
  
  xlC -q64 -O2 -qlist test.C -qarch=pwr7 -c
  
  Review the content of test.lst file. Instead of sequence:
  
  ...
  ...
  | 000000                           PDEF     mulimm
      1|                                  PROC      in,gr3
      3| 000000 addis    3C0000EF   1     LIU       gr0=239
      3| 000004 ori      6000E000   1     OIL       gr0=gr0,0xE000
      3| 000008 mulld    7C6301D2   1     M         gr3=gr3,gr0
      4| 00000C bclr     4E800020   1     BA        lr
  
  The compiler now generates more effective sequence of
  instructions:
  
  ...
  ...
  | 000000                           PDEF     mulimm
      1|                                  PROC      in,gr3
      3| 000000 mulli    1C03077F   0     M8        gr0=gr3,1919
      3| 000004 rldicr   78036CA4   2     SLL8      gr3=gr0,13
      4| 000008 bclr     4E800020   1     BA        lr
  

Local fix

• Don't use this kind of immediate as one of the multiplier or
  shift it appropriately
  

Problem summary

• Compiler generates suboptimal code sequences for multiplication
  with characteristic immediate on Power7
  

Problem conclusion

• We now shift the imediate and issue MULLI instruction for
  multiplicat      and then shift back
  

Temporary fix

Comments

APAR Information

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

  IV37629

• 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 IV37629

     UP06/09/13