LI78225: IPA CAUSING INCORRECT POINTER ARITHMETIC

XL C/C++ for Linux Fix Pack 2 (February 2015 Update) for 13.1
XL C/C++ for Linux Fix Pack 1 (October 2014 Update) for 13.1

APAR status

• Closed as program error.

Error description

• When compiling with -qipa=level=0, certain pointer arithmetic is
  done incorrectly.  The result could be incorrect output or
  incorrect memory access.
  
  
     === EXAMPLE ===
  
  int func(mystruct *obj, char *bp, int copy)
  {
        char *pstart = bp;
        if (copy)
        {
              obj->val1 = (*(unsigned int *)bp); bp += 4;
              obj->val2 = (*(unsigned int *)bp); bp += 4;
        }
        else
        {
              bp += 8;
        }
        return (bp - pstart);
  }
  
  The bp pointer is only incremented by (4+4)=8, leaving
  (bp - pstart)=8, but for the code generated, the value of pstart
  used in the subtraction (in gr0) has been incremented by 4**,
  which ends up giving (bp+8)-(pstart+4)=4, when the return result
  should be 8:
  
       | 000000                       PDEF func
      0|                              PROC obj,bp,copy,gr3-gr5
      6| 000080 cmpwi    2C050000 1 C4   cr0=gr5,0
  **  8| 000084 addi     38040004 1 AI   gr0=gr4,4
      6| 000088 bc       41820020 1 BT   CL.2,cr0,0x4/eq
      8| 00008C lwz      80A40000 1 L4A  gr5=(*)uint.rns0.(gr4,0)
      9| 000090 lwz      80C40004 1 L4A  gr6=(*)uint.rns0.(gr4,4)
      9| 000094 addi     38840008 1 AI   gr4=gr4,8
      8| 000098 stw      90A30000 1 ST4A
                      (*)mystruct.mystruct.val1.rns1.(gr3,0)=gr5
      9| 00009C stw      90C30004 1 ST4A
                      (*)mystruct.mystruct.val2.rns2.(gr3,4)=gr6
     15| 0000A0 subf     7C602050 1 S    gr3=gr4,gr0
      0| 0000A4 bclr     4E800020 1 BA   lr
      9|                       CL.2:
     13| 0000A8 addi     38040008 1 AI   gr0=gr4,8
     15| 0000AC subf     7C640050 1 S    gr3=gr0,gr4
     16| 0000B0 bclr     4E800020 1 BA   lr
  

Local fix

• Do not use -qipa
  

Problem summary

• PROBLEM DESCRIPTION:
  An incorrect runtime value is calculated by an internal code
  optimization.
  
  USERS AFFECTED:
  Any users of -qipa, or any options that imply -qipa (for
  example, -qpdf1).
  

Problem conclusion

• The optimization was corrected, and IPA now generates correct
  code giving correct runtime values.
  

Temporary fix

Comments

APAR Information

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

  IV58760

• 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

• RD10 PSN IV58760

     UP06/09/13