[LLVMdev] determining the number of iteration of a loop

Eugene Toder eltoder at gmail.com
Wed Apr 21 14:48:38 PDT 2010

In your example the the number of iterations is known -- it is N. It
is not known at compile time, but it's known at run-time before you
enter the loop. So you can do transforms like if( N < threshold ) copy
of loop optimized for small iterations count; else copy of loop
optimized for large iterations count;

But you are right, in general, the number of iterations in unknown. I
think Khaled is interested in a large number of cases when it is
known, either as a constant or value.


On Wed, Apr 21, 2010 at 10:28 PM, Trevor Harmon
<Trevor.W.Harmon at nasa.gov> wrote:
> On Apr 21, 2010, at 7:31 AM, khaled hamidouche wrote:
>> I'm wandring to know how many times a block is executed inside a
>> loop  ?
>> knowing that I can't use  getSmallConstantTripCount() because the
>> number is unkown  "for (i=0;i<N;i++)  for example"
> In general, the number of iterations is undecidable. For example:
> int main(char **argv, int N) {
>     for (int i = 0; i < N; i++) {
>         std::cout << "arg: " << argv[i] << std::endl;
>     }
>     return 0;
> }
> If you are parsing code whose iterations can be determined, please
> post it.
> Trevor
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