LIM/Alias Analysis performance issue

[Lu, John]

Hi,

I've encountered a performance issue in a port of GCC I'm working on, 
where the behavior of LIM is affected by the ordering of fields in a 
structure.  I've been able to re-create it with a GCC 4.3.1
Linux X86 compiler.  With GCC 4.3.1 Linux X86, when I compile:

    struct foo {
      int *p;
      int  t;
    } T;

    void o() {
      unsigned int  i;
  
      for (i = 0; i < 256; i++) {
        T.p[i]=0;
      }
    }

with the command:

    gcc -S -O2 -fdump-tree-all good.c

the file good.c.095t.lim shows T.p being moved outside the loop:

    <bb 2>:
      pretmp.10_8 = T.p;
    
    <bb 3>:
      # i_14 = PHI <i_7(4), 0(2)>
      D.1556_4 = (long unsigned int) i_14;
      D.1557_5 = D.1556_4 * 4;
      D.1558_6 = pretmp.10_8 + D.1557_5;
      *D.1558_6 = 0;
      i_7 = i_14 + 1;
      if (i_7 <= 255)
        goto <bb 4>;
      else
        goto <bb 5>;
    
    <bb 4>:
      goto <bb 3>;

If the fields of the structure are reversed:

    struct foo {
      int  t;
      int *p;
    } T;

T.p is kept inside the loop:

    <bb 3>:
      # i_21 = PHI <i_7(4), 0(2)>
      D.1555_3 = T.p;
      D.1556_4 = (long unsigned int) i_21;
      D.1557_5 = D.1556_4 * 4;
      D.1558_6 = D.1555_3 + D.1557_5;
      *D.1558_6 = 0;
      i_7 = i_21 + 1;
      if (i_7 <= 255)
        goto <bb 4>;
      else
        goto <bb 5>;

    <bb 4>:
      goto <bb 3>;

On my port, this causes a large performance degradation, and I suspect
the
cause is ultimately in the alias analysis pass.  I was wondering if
there 
is a way to configure GCC to avoid this issue.

Thanks,
John Lu