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Optimize Performance in PolarGrid Indexing #261

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Optimize Performance in PolarGrid Indexing #261

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9b7be06
Optimize performance in PolarGrid Indexing
julianlitz May 14, 2026
04a1959
Update polargrid.inl
julianlitz May 15, 2026
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24 changes: 19 additions & 5 deletions include/PolarGrid/polargrid.inl
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Original file line number Diff line number Diff line change
Expand Up @@ -82,10 +82,24 @@ KOKKOS_INLINE_FUNCTION int PolarGrid::wrapThetaIndex(const int unwrapped_theta_i
// This effectively computes unwrapped_theta_index % ntheta(), because it discards all higher bits.
//
// If ntheta is not a power of two, we use the standard modulo approach to handle wrapping.
int theta_index = is_ntheta_PowerOfTwo_ ? unwrapped_theta_index & (ntheta() - 1)
: (unwrapped_theta_index % ntheta() + ntheta()) % ntheta();
assert(0 <= theta_index && theta_index < ntheta());
return theta_index;
if (is_ntheta_PowerOfTwo_) {
const int theta_index = unwrapped_theta_index & (ntheta() - 1);
assert(0 <= theta_index && theta_index < ntheta());
return theta_index;
}
else {
// For non-power-of-two ntheta, we use a simple iterative approach to wrap the index.
// This is efficient for small deviations from the valid range, which is common in practice.
int theta_index = unwrapped_theta_index;
while (theta_index >= ntheta()) {
theta_index -= ntheta();
}
while (theta_index < 0) {
theta_index += ntheta();
}
assert(0 <= theta_index && theta_index < ntheta());
return theta_index;
Comment on lines +94 to +101
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@EmilyBourne EmilyBourne May 15, 2026

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Do you think that this option will still optimise performance on GPU? I agree that 1 -= is usually less costly than % but it introduces more branching 🤔

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@julianlitz julianlitz May 15, 2026

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I dont know

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@EmilyBourne EmilyBourne May 15, 2026

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Did you measure how much difference this PR makes? It would be interesting to compare the results with OpenMP and GPU

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@julianlitz julianlitz May 15, 2026

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Probably none as the bottleneck is in the double computations. Aaaand I dont have a ntheta != 2^k test case

}
}

KOKKOS_INLINE_FUNCTION int PolarGrid::index(const int r_index, const int unwrapped_theta_index) const
Expand All @@ -106,7 +120,7 @@ KOKKOS_INLINE_FUNCTION void PolarGrid::multiIndex(const int node_index, int& r_i
assert(0 <= node_index && node_index < numberOfNodes());
if (node_index < numberCircularSmootherNodes()) {
r_index = node_index / ntheta();
theta_index = wrapThetaIndex(node_index);
theta_index = node_index % ntheta();
}
else {
theta_index = (node_index - numberCircularSmootherNodes()) / lengthRadialSmoother();
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