Re: [PATCH PR45098, 7/10] Nowrap limits iterations

[Tom de Vries <vries at codesourcery dot com>]

On 05/21/2011 02:24 PM, Zdenek Dvorak wrote:
>> 	* tree-ssa-loop-ivopts.c (may_eliminate_iv): Fix
>> 	estimated_loop_iterations comparison.
> 
> I don't think this part is correct, though:
> 
>> Index: gcc/tree-ssa-loop-ivopts.c
>> ===================================================================
>> --- gcc/tree-ssa-loop-ivopts.c (revision 173734)
>> +++ gcc/tree-ssa-loop-ivopts.c (working copy)
>> @@ -4391,8 +4391,13 @@ may_eliminate_iv (struct ivopts_data *da
>>              {
>>                if (!estimated_loop_iterations (loop, true, &max_niter))
>>                  return false;
>> -              /* The loop bound is already adjusted by adding 1.  */
>> -              if (double_int_ucmp (max_niter, period_value) > 0)
>> +              /* The max iterations applies also to the number of times the loop
>> +                 exit condition is executed.  The number of distinct values of
>> +                 the cand is period_value + 1.  So, test for
>> +                 'period_value + 1 >= max_iterations'.
>> +               */
>> +              period_value = double_int_add (period_value, double_int_one);
>> +              if (double_int_ucmp (max_niter, period_value) > 0)
>>                  return false;
>>              }
>>            else
> 

> max_niter is the upper bound on the number of iterations of the loop, i.e., the number
> of executions of its latch edge.

max_niter is set from estimated_loop_iterations, meaning from
loop->nb_iterations_upper_bound.

consider:
...
void f(int *a)
{
  int i;

  for (i = 0; i < 10; ++i)
    a[i] = 0;
}
...

at ivopts, it looks like this (compiled with -Os -fno-tree-vrp
-fno-tree-dominator-opts -fno-tree-loop-ivcanon, to get a source-like
representation)
...
f (int * a)
{
  int i;
  int * D.2009;
  unsigned int D.2008;
  unsigned int i.0;

<bb 2>:
  goto <bb 4>;

<bb 3>:
  i.0_3 = (unsigned int) i_1;
  D.2008_4 = i.0_3 * 4;
  D.2009_6 = a_5(D) + D.2008_4;
  *D.2009_6 = 0;
  i_7 = i_1 + 1;

<bb 4>:
  # i_1 = PHI <0(2), i_7(3)>
  if (i_1 <= 9)
    goto <bb 3>;
  else
    goto <bb 5>;

<bb 5>:
  return;

}
...


The header block of the loop is bb 4, the latch block is bb 3:
...
(gdb) p loop.header.index
$4 = 4
(gdb) p loop.latch.index
$5 = 3
...

The number of times the latch edge is executed, is 10.

But loop->nb_iterations_upper_bound, or max_niter is 11:
...
(gdb) p *loop
$1 = {num = 1, ninsns = 0, header = 0xf7dc2440, latch = 0xf7dc2400, lpt_decision
= {decision = LPT_NONE, times = 0}, av_ninsns = 0, num_nodes = 2, superloops =
0xf7db6ee8, inner = 0x0, next = 0x0,
  aux = 0x0, nb_iterations = 0xf7d3d540, nb_iterations_upper_bound = {low = 11,
high = 0}, nb_iterations_estimate = {low = 11, high = 0}, any_upper_bound = 1
'\001', any_estimate = 1 '\001',
  can_be_parallel = 0 '\000', estimate_state = EST_AVAILABLE, bounds =
0xf7d3da2c, exits = 0xf7dc3d70}
...

> Therefore, the control induction variable of the loop
> will (at the exit statement) achieve at most max_niter + 1 different values.

Based on what I observe, I'd say the control induction variable of the loop will
achieve at most max_niter different values.

> Conversely,
> the number of distinct values that the control iv can represent is period + 1 (naming of
> the "period" variable is a bit missleading).

agree.

Thanks,
- Tom