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
ordo..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.