LI77980: INEFFICIENT LOAD FROM CONSTANT AREA

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

• Inefficient load from constant area on strcpy, memcpy, strncpy
  when copying string constant.
  Suggest to replace the constant area loads with constant
  constant construction using immediate instructions.
  
  Sample:
  void strcpy1638E(char *out)
  {
     strcpy(out, "1638E");
  }
  
  Compiler generates:
       | 000000                           PDEF
  strcpy1638E(char *)
       | 000000                           AKA
  strcpy1638E__FPc
      9|                                  PROC      out,gr3
      0| 000040 ld       E8820008   1     L8
  gr4=.+CONSTANT_AREA(gr2,0)
    398| 000044 lwa      E804000A   1     L4A
  gr0=+CONSTANT_AREA(gr4,8)
    398| 000048 lhz      A084000C   1     L2Z
  gr4=+CONSTANT_AREA(gr4,12)
    398| 00004C stw      90030000   1     ST4A
  (char)(gr3,0)=gr0
    398| 000050 sth      B0830004   1     ST2Z
  (char)(gr3,4)=gr4
     12| 000054 bclr     4E800020   1     BA        lr
  
  This causes constant area growth or utilizing TOC. And loads
  are dependant on constant area load.
  

Local fix

• Use direct instructions to replace strcpy.
  void opt1638E(char *out)
  {
     const unsigned short c16 = '1' << 8 | '6';
     const unsigned short c38 = '3' << 8 | '8';
     const unsigned short cE  = 'E' << 8;
     ((unsigned short *)out)[0] = c16;
     ((unsigned short *)out)[1] = c38;
     ((unsigned short *)out)[2] = cE;
  }
  

Problem summary

• PROBLEM DESCRIPTION: Inefficient load from constant area on
  strcpy, memcpy, strncpy when copying string constant. This fix
  will replace the constant area loads with constant construction
  using immediate instructions.
  
  USERS AFFECTED: Users who use functions like strcpy, memcpy and
  strncpy etc. to copy a string literal into a memory area.
  

Problem conclusion

• This fix will optimize loads of the tail of string literals
  into immediate instructions to get performance improved.
  

Temporary fix

Comments

APAR Information

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

  IV56281

• 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