LI77327: INEFFICIENT 64-BIT CONSTANT LOAD OF 32-BIT VALUE

April 2013 Update for XL C/C++ for Linux, V12.1
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

• xlC loads from constant area while it is more efficient to use
  immediate instructions.
  
  Pattern:
  First 32-bits are zero
  Next 16-bit start with 1-bit
  Next 16-bit value does not start with 1-bit
  
  Take this sample:
  extern "C" unsigned long long res()
  {
          return 0x82160001ULL;
  }
  
  xlC listing outputs:
       | 000000                           PDEF     res
      2|                                  PROC
      4| 000000 ld       E8620008   1     L8
  gr3=.+CONSTANT_AREA(gr2,0)
      4| 000004 ld       E8630000   1     L8
  gr3=+CONSTANT_AREA(gr3,0)
      5| 000008 bclr     4E800020   1     BA        lr
       |               Tag Table
       | 00000C        00000000 00092000 00000000 0000000C
       |               Instruction count            3
       |               Straight-line exec time      3
       |               Constant Area
       | 000000        00000000 82160001
  
  If using immediate instructions for above example, it would be
  more efficient:
  
  li rx,1
  oris rx,rx,0x8216
  

Local fix

• N/A
  

Problem summary

• PROBLEM DESCRIPTION: The compiler is not as efficient as it
  could be when loading certain constants.
  
  USERS AFFECTED: Users who use a constant that starts with 32
  zero-bits, has a 0 in the 49th bit, and a 1 for the 33rd bit
  when represented with 64-bits.
  

Problem conclusion

• Added code to identify these constants and build them instead
  of loading them from the constant area.
  

Temporary fix

Comments

APAR Information

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

  IV30067

• 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 IV30067

     UP06/09/13