altera-unroll-loops

Finds inner loops that have not been unrolled, as well as fully unrolled loops with unknown loop bounds or a large number of iterations.

Unrolling inner loops could improve the performance of OpenCL kernels. However, if they have unknown loop bounds or a large number of iterations, they cannot be fully unrolled, and should be partially unrolled.

Notes:

• This check is unable to determine the number of iterations in a while or do..while loop; hence if such a loop is fully unrolled, a note is emitted advising the user to partially unroll instead.

• In for loops, our check only works with simple arithmetic increments ( +, -, *, /). For all other increments, partial unrolling is advised.

• Depending on the exit condition, the calculations for determining if the number of iterations is large may be off by 1. This should not be an issue since the cut-off is generally arbitrary.

Based on the Altera SDK for OpenCL: Best Practices Guide.

for (int i = 0; i < 10; i++) {  // ok: outer loops should not be unrolled
   int j = 0;
   do {  // warning: this inner do..while loop should be unrolled
      j++;
   } while (j < 15);

   int k = 0;
   #pragma unroll
   while (k < 20) {  // ok: this inner loop is already unrolled
      k++;
   }
}

int A[1000];
#pragma unroll
// warning: this loop is large and should be partially unrolled
for (int a : A) {
   printf("%d", a);
}

#pragma unroll 5
// ok: this loop is large, but is partially unrolled
for (int a : A) {
   printf("%d", a);
}

#pragma unroll
// warning: this loop is large and should be partially unrolled
for (int i = 0; i < 1000; ++i) {
   printf("%d", i);
}

#pragma unroll 5
// ok: this loop is large, but is partially unrolled
for (int i = 0; i < 1000; ++i) {
   printf("%d", i);
}

#pragma unroll
// warning: << operator not supported, recommend partial unrolling
for (int i = 0; i < 1000; i<<1) {
   printf("%d", i);
}

std::vector<int> someVector (100, 0);
int i = 0;
#pragma unroll
// note: loop may be large, recommend partial unrolling
while (i < someVector.size()) {
   someVector[i]++;
}

#pragma unroll
// note: loop may be large, recommend partial unrolling
while (true) {
   printf("In loop");
}

#pragma unroll 5
// ok: loop may be large, but is partially unrolled
while (i < someVector.size()) {
   someVector[i]++;
}

Options

MaxLoopIterations

Defines the maximum number of loop iterations that a fully unrolled loop can have. By default, it is set to 100.

In practice, this refers to the integer value of the upper bound within the loop statement’s condition expression.