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Reuse scratch space for Sabre best-swap choice #10783

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merged 2 commits into from
Sep 6, 2023
Merged

Reuse scratch space for Sabre best-swap choice #10783

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416e68a
Reuse scratch space for Sabre best-swap choice
jakelishman Sep 5, 2023
422964e
Add comment on scratch space
jakelishman Sep 6, 2023
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8 changes: 7 additions & 1 deletion crates/accelerate/src/sabre_swap/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -383,6 +383,10 @@ fn swap_map_trial(
// Main logic loop; the front layer only becomes empty when all nodes have been routed. At
// each iteration of this loop, we route either one or two gates.
let mut routable_nodes = Vec::<NodeIndex>::with_capacity(2);
// Reusable allocated storage space for choosing the best swap. This is owned outside of the
// `choose_best_swap` function so that we don't need to reallocate and then re-grow the
// collection on every entry.
let mut swap_scratch = Vec::<[VirtualQubit; 2]>::new();
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while !front_layer.is_empty() {
let mut current_swaps: Vec<[VirtualQubit; 2]> = Vec::new();
// Swap-mapping loop. This is the main part of the algorithm, which we repeat until we
Expand All @@ -397,6 +401,7 @@ fn swap_map_trial(
&qubits_decay,
heuristic,
&mut rng,
&mut swap_scratch,
);
front_layer.routable_after(&mut routable_nodes, &best_swap, &layout, coupling_graph);
current_swaps.push(best_swap);
Expand Down Expand Up @@ -688,9 +693,10 @@ fn choose_best_swap(
qubits_decay: &[f64],
heuristic: &Heuristic,
rng: &mut Pcg64Mcg,
best_swaps: &mut Vec<[VirtualQubit; 2]>,
) -> [VirtualQubit; 2] {
best_swaps.clear();
let mut min_score = f64::MAX;
let mut best_swaps: Vec<[VirtualQubit; 2]> = Vec::new();
// The decay heuristic is the only one that actually needs the absolute score.
let absolute_score = match heuristic {
Heuristic::Decay => {
Expand Down