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Re: [PATCH PR71503/PR71683]Fix ICE in tree-if-conv.c


On Thu, Jul 14, 2016 at 6:49 PM, Jeff Law <law@redhat.com> wrote:
> On 07/14/2016 10:12 AM, Bin Cheng wrote:
>>
>> Hi,
>> This is a simple patch fixing ICE in tree-if-conv.c.  Existing code does
>> not setup a variable (cond) when predicate of basic block is true and it
>> asserts on the variable.  Interesting thing is dead code is not cleaned up
>> before ifcvt, but that's another story.
>> Bootstrap and test on x86_64.  Is it OK?
>>
>> Thanks,
>> bin
>>
>> 2016-07-13  Bin Cheng  <bin.cheng@arm.com>
>>
>>         PR tree-optimization/71503
>>         PR tree-optimization/71683
>>         * tree-if-conv.c (gen_phi_arg_condition): Set cond when predicate
>>         is true.
>
> Maybe I'm missing something, but in the case where we COND is already set
> and we encounter a true predicate later, shouldn't that make the result true
> as well?
Yes, this is my understanding too, if the case does exist.  To my
understanding, conditions for phi arguments should be complimentary to
others.  If one argument has true predicate, then other must? be
false, then the case doesn't exist.

>
> I don't think the code will though -- we just throw away the true condition.
> ISTM that the right thing to do is
>
> if (is_true_predicate (c))
>   {
>     cond = c;
>     continue;
>   }
Anyway, I think we can make it more robust/efficient by using:
if (is_true_predicate (c))
 {
   cond = c;
   break;
  }
What do you think?  Actually we should discard other arguments if one
has true predicate.

>
> Can you see if you can trigger a case where we have an existing cond, then
> later find a true condition to verify the right thing happens?
Failed to do so.  It should be quite rare, if no impossible,

Thanks,
bin


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