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

[Tom de Vries <vries@codesourcery.com>]

Hi Zdenek,

On 05/21/2011 07:59 PM, Tom de Vries wrote:
> 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.
> 

Any thoughts on my observations above?

Thanks,
- Tom