[Bug c++/32073] New: Loop unrolling does not exploit VRP for loop bound

[scovich at gmail dot com]

Loops with a bounded, small number of iterations unroll too much. They should
be peeled away instead. For example, if I compile the following function with
``-O3 -funroll-loops'':

void short_loop(int* dest, int* src, int count) {
  // same happens for assert(count <= 4) and if(count > 4) exit(-1)
  if(count > 4)
    count = 4;

  for(int i=0; i < count; i++)
    dest[i] = src[i];
}

The assembly output (for i686-pc-cygwin) is an 8x duff's device, of which 75%
of the code will never execute (translated back to C++ here for readability):

void short_loop(int* dest, int* src, int count) {
  // same happens for assert(count <= 4) and if(count > 4) exit(-1)
  if(count > 4)
    count = 4;

  int mod = count % 8;
  switch(mod) {
  case 7:
    // loop body
    count--;
  case 6:
    // loop body
    count--;
  case 5:
    // loop body
    count--;
  case 4:
    // loop body
    count--;
  case 3:
    // loop body
    count--;
  case 2:
    // loop body
    count--;
  case 1:
    // loop body
    count--;
  default:
    for(int i=0; i < count; i+=8)
      // 8x unrolled loop body
  }
}

We need <25% of that code:

void short_loop(int* dest, int* src, int count) {
  // same happens for assert(count <= 4) and if(count > 4) exit(-1)
  if(count > 4)
    count = 4;

  switch(count) {
  case 4:
    // loop body
  case 3:
    // loop body
  case 2:
    // loop body
  case 1:
    // loop body
  default:
    break;
  }
}


-- 
           Summary: Loop unrolling does not exploit VRP for loop bound
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: scovich at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32073