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
whileordo..whileloop; hence if such a loop is fully unrolled, a note is emitted advising the user to partially unroll instead. - In
forloops, 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]++;
}