[NSWI131] 03-counter-spinner: Weird branch missprediction counter behavior

[Pavel Marek]

Hello,

I am working on 03-counter-spinner assignment and I am confused from the 
way how my CPU predicts/miss predicts branches (PAPI_BR_MSP, and 
PAPI_BR_PRC counters).

Here is the particular example: I have quite a huge array of booleans 
(100MB), initialize it to a random values, and then in `execute` just 
increase the index and conditionally jump based on the content of the 
array. Here is the code:

```c++
class branch_misspredictions : public workload {
private:
     static constexpr size_t arr_size = 100*1024*1024; // 100MB
     volatile bool arr[arr_size];
     size_t idx;

public:
     void before() {
         idx = 0;
         srand(42);
         for (size_t i = 0; i < arr_size; i++) {
             arr[i] = rand() % 2 == 0 ? true : false;
         }
     }

     int execute() {
         idx = (idx < arr_size - 1) ? idx + 1 : 0;
         if (arr[idx]) {
             return 1;
         } else {
             return 2;
         }
     }
};
```

The score (ratio events/instructions) for correct predictions 
(PAPI_BR_PRC) is: 0.09 and for miss predictions (PAPI_BR_MSP) is: 0.00007

My intuition tells me that the scores should be other way around ie. 
there should be a lot of miss predictions and few correct predictions.

Can you provide some insight of what I am doing wrong here? It seems 
really weird to me, but maybe I missed something trivial..

Type of my CPU: Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz


Thanks for any comments.
Pavel Marek