LI76226: SEGMENTATION FAULT AT O3 WITHIN NESTED LOOP

June 2011 Update for XL C/C++ for Linux, V10.1
XL C/C++ for Linux Fix Pack 8 (January 2014 Update) for 10.1
January 2012 Update for XL C/C++ for Linux, V10.1
July 2012 Update for XL C/C++ for Linux, V10.1

APAR status

• Closed as program error.

Error description

• A segmentation fault is encountered at -O3 optimization whe
  runnning a test case that contains a nested loop.
  
  ==== Test case:
  test.c:
  #include <stdlib.h>
  #include <stdio.h>
  
  typedef struct {
    void *params;
  } Temp;
  
  void * get_params(Lattice *lat)
  {
    return lat->params;
  }
  
  void get_loop(Temp *lat, int x[], int nd, int index)
  {
    void *p = get_params(lat);
  
    for(int i=0; i<nd; i++) {
      x[i] = index;
    }
  }
  
  void CreateNestedBlock(int nd, Lattice *lat)
  {
    for(int b=0; b<2; b++) {
     int xf[nd];
      for(int j=0; j<1; j++) {
        get_loop(lat, xf, nd, j);
      }
    }
  }
  
  int main(int argc, char *argv[])
  {
    int ndim = 4;
    Temp lat;
    lat.params = NULL;
  
    printf("starting\n");
    CreateNestedBlock(ndim, &lat);
    printf("done\n");
  
    return 0;
  }
  
  
  ==== Observed Output:
  $ xlc -O3 test.c
  staring
  Segmentation fault
  $
  

Local fix

• Use a lower optimization level.
  

Problem summary

• PROBLEM DESCRIPTION:
  Segmentation fault at -O3 when returning from a function.
  
  USERS AFFECTED:
  Users of optimization levels -O3 or higher.
  

Problem conclusion

• Intermediate generated code was incorrectly moved outside of an
  internal optimized loop. Resulting in the corruption of the
  return address of the function. This fix involves ensuring that
  this particular block of code is not moved outside of the loop.
  

Temporary fix

Comments

APAR Information

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

  LI76071

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

Fix information

• Fixed component name

  XL C/C++ RHEL52

• Fixed component ID

  5724U8310

Applicable component levels

• RA10 PSY

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