[Bug tree-optimization/92839] New: Normalize memory address to same base in non-loop code

[fxue at os dot amperecomputing.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92839

            Bug ID: 92839
           Summary: Normalize memory address to same base in non-loop code
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fxue at os dot amperecomputing.com
  Target Milestone: ---

If possible, IVOPTs can transform memory accesses to make them use same base,
which can decrease register pressure in loop. But it does not handle memory
address that is not an iv, or in a pure non-loop code.

Consider the following example, compile with -O3, we can find that data[b + ?]
will use same base register. But if we remove the line "for (i = 0; i < n; i++,
b++)" to make it be non-loop code, data[b + ?] will use different bases, with
result that we have to consume 10 register as bases, which is a much higher
register pressure. And since live ranges of these base registers cross function
call, most of them will be spilled to memory.

int data[100];

int fn();

void foo(int b, int n)
{
  int i;

  for (i = 0; i < n; i++, b++)  // remove the statement to be non-loop 
    {
      data[b + 1] += 1;
      data[b + 3] += 3;
      data[b + 5] += 5;
      data[b + 7] += 7;
      data[b + 9] += 9;
      data[b + 11] += 11;
      data[b + 13] += 13;
      data[b + 15] += 15;
      data[b + 17] += 17;
      data[b + 19] += 19;

      fn ();

      data[b + 1] -= 1;
      data[b + 3] -= 3;
      data[b + 5] -= 5;
      data[b + 7] -= 7;
      data[b + 9] -= 9;
      data[b + 11] -= 11;
      data[b + 15] -= 13;
      data[b + 17] -= 15;
      data[b + 19] -= 19;
    }
}