LI80862: REORDERED ASSEMBLER INSTRUCTIONS BREAKING IF STATEMENT ARGUMENT EVALUATION ORDER

APAR status

• Closed as program error.

Error description

• When optimization is enabled, some generated floating point
  assembler instructions are scheduled in such a way as to mix up
  the order in which statements are evaluated in a compound IF
  statement.
  
  Depending on the external effect these IF conditional statements
  have (and the order in which they are called), the application
  behaviour may be incorrect.
  
  
     === EXAMPLE ===
  
  +16  if (!test(a) &&
  +17      !test(b) &&
  +18          a<b)
  +19    return TRUE;
  +20  else
  +21    return FALSE;
  
  The generated code looks like:
  
     16| 000204 xststdcdp  F0400DA8   1     TSTDCDP    cr0=vs1,64
     22| 000208 bc         40850008   0     TCFEX_B    cr1
     22| 00020C tw         7C8F7808   2
     18| 000210 xscmpodp   F3011158   1     CFLO       cr6=vs1,vs2
    ^^^^==> Line +18 is evaluated BEFORE line +17.  The code
            expects and requires test(b) to be run first.
  
     21| 000214 addi       38600000   1     LI         gr3=0
     17| 000218 xststdcdp  F0C015A8   1     TSTDCDP    cr1=vs2,64
     16| 00021C bc         41820010   0     BT  CL.10,cr0,0x4/eq
     17| 000220 bc         4186000C   1     BT  CL.10,cr1,0x4/eq
     18| 000224 bc         40980008   2     BF  CL.10,cr6,0x20/flt
  

Local fix

• Use a lower optimization level.
  

Problem summary

• PROBLEM DESCRIPTION:
  Speculative scheduling of conditionals with ordered compare can
  cause unexpected program behaviour.
  
  USERS AFFECTED:
  Users with ordered compares.
  

Problem conclusion

• Ordered compares are flagged for code reorganization
  optimization.
  

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:

Fix information

• Fixed component name

  XL C/C++ LINUX

• Fixed component ID

  5725C7310

Applicable component levels