[PATCH] D22630: Loop rotation

[Sanjoy Das via llvm-commits]

Hi,

I know I'm late to the party, but I didn't understand this example:

On Thu, Jul 21, 2016 at 1:43 PM, Sebastian Pop <sebpop at gmail.com> wrote:

> int fun(const std::vector<char>* A, const std::vector<char>* B, int n, int m)
> {
>   int res = 0;
>
>   for (int i = 0; i < m; ++i)
>     for (int j = 0; j < n; ++j)
>       res += A->at(n * i + j) + B->at(n * i + j);
>
>   return res;
> }

[snip]

> After loop rotate peels off one full iteration of the loop:
>
>    for (int i = 0; i < m; ++i) {
>     x = A->at(n * i);
>     if (exception)
>       throw;
>     y = B->at(n*i);
>     if(exception)
>       throw;
>     res += x + y;
>     for (int j = 0; j < n; ++j)
>       res += A->at(n * i + j) + B->at(n * i + j);
>    }

After rotating the inner loop, won't the program have to be

   for (int i = 0; i < m; ++i) {
    x = A->at(n * i);
    if (exception)
      throw;
    y = B->at(n*i);
    if(exception)
      throw;
    res += x + y;
    for (int j = 1; j < n; ++j)  // << changed lower count
      res += A->at(n * i + j) + B->at(n * i + j);
   }

Where is the redundancy here ^?

-- Sanjoy