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
