Fusion Pipeline

Gate optimizations before execution, gated by qubit count thresholds. Every pass returns Cow<Circuit>. Borrowed when no optimization applies, so circuits that do not benefit pay zero overhead.

flowchart TD
    IN[Input Circuit] --> P0["cancel_self_inverse_pairs (always)"]
    P0 --> P0r["fuse_rzz (always): CX·Rz·CX to Rzz"]
    P0r --> P0b["fuse_batch_rzz (>=16q): N×Rzz to BatchRzz"]
    P0b --> G{"qubits >= MIN_QUBITS_FOR_FUSION (10)?"}
    G -- no --> OUT[Output Circuit]
    G -- yes --> P1["fuse_single_qubit_gates (>=10q)"]
    P1 --> P1r["reorder_1q_gates (>=10q)"]
    P1r --> P1c["cancel_self_inverse_pairs (>=10q)"]
    P1c --> P1f["fuse_single_qubit_gates re-fuse (>=10q)"]
    P1f --> P2q["fuse_2q_gates (>=12q): CX/CZ + adjacent 1q to Fused2q"]
    P2q --> P2qb["fuse_same_pair_2q_blocks (>=12q)"]
    P2qb --> P2["fuse_multi_1q_gates (>=14q) to MultiFused"]
    P2 --> P2qr["reorder_disjoint_fused2q (>=12q)"]
    P2qr --> Pm2q["fuse_multi_2q_gates (>=12q) to Multi2q"]
    Pm2q --> Pcp["fuse_controlled_phases (>=16q) to BatchPhase"]
    Pcp --> Pdb["fuse_diagonal_batch (>=16q) to DiagonalBatch"]
    Pdb --> Ppp["batch_post_phase_1q (>=18q)"]
    Ppp --> OUT

Threshold constants