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Re: [VRP][PATCH] Improve value range for loop index



On 24/04/14 23:05, Richard Biener wrote:
> On Wed, Apr 9, 2014 at 10:07 PM, Kugan
> <kugan.vivekanandarajah@linaro.org> wrote:
>> Value range propagation simplifies convergence in vrp_visit_phi_node by
>> setting minimum to TYPE_MIN when the computed minimum is smaller than
>> the previous minimum. This can however result in pessimistic value
>> ranges in some cases.
>>
>> for example,
>>
>>         unsigned int i;
>>         for (i = 0; i < 8; i++)
>>         {
>>           ....
>>         }
>>
>>         # ivtmp_19 = PHI <ivtmp_17(5), 8(2)>
>>         ...
>>         <bb 5>:
>>         ivtmp_17 = ivtmp_19 - 1;
>>         if (ivtmp_17 != 0)
>>         ....
>>         goto <bb 5>;
>>
>> min value of ivtmp_19  is simplified to 0 (in tree-vrp.c:8465) where as
>> it should have been 1. This prevents correct value ranges being
>> calculated for ivtmp_17 in the example.
>>
>> We should be able to see the step (the difference from previous minimum
>> to computed minimum) and if there is scope for more iterations (computed
>> minimum is greater than step), and then we should be able set minimum to
>> do one more iteration and converge to the right minimum value.
>>
>> Attached patch fixes this. Is this OK for stage-1?
> 
> In principle the code in adjust_range_with_scev is supposed to
> fix this up by using number-of-iteration analysis.  I can see this is not
> working for the testcase but I'm curious exactly why.

Thanks for pointing me to adjust_range_with_scev.  I will look into it.


> Your patch basically makes us converge to the correct value by
> iterating (but faster than by just iterating).  That's an interesting
> idea but the way you do it looks very special.  If we really want to
> go down this route (instead of fixing up adjust_range_with_scev for IVs)
> then I'd like to see a more general solution - like by making the code
> skip to TYPE_MIN/MAX_VALUE +-1.  I'm also not sure the case
> handling the supposed bouncing needs to bump to MIN/MAX at all,
> it could simply retain the old values.

TYPE_MIN/MAX_VALUE +-1 might not always work as there might be some
cases where the stride (or the steps in convergence) is not 1 but more
than 1 (?). In those cases, if we set it to TYPE_MIN/MAX_VALUE +-1, they
will not converge from there. therefore should that be,
TYPE_MIN/MAX_VALUE +- stride?


Thanks,
Kugan


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