Function qureg_populateNonHeapFields

quest/src/api/qureg.cpp:38–84

Qureg qureg_populateNonHeapFields(int numQubits, int isDensMatr, int useDistrib, int useGpuAccel, int useMultithread) {

    QuESTEnv env = getQuESTEnv();

    // pre-prepare some struct fields (to avoid circular initialisation)
    int logNumNodes = (useDistrib)? 
        logBase2(env.numNodes) : 0;
    qindex logNumAmpsPerNode = (isDensMatr)? 
        (2*numQubits - logNumNodes) :
        (  numQubits - logNumNodes);

    // prepare output Qureg (avoiding C++20 designated initialiser)
    Qureg out;

    // bind deployment info
    out.isMultithreaded  = useMultithread;
    out.isGpuAccelerated = useGpuAccel;
    out.isDistributed    = useDistrib;

    // optionally bind distributed info, noting that in distributed environments,
    // the non-distributed quregs are duplicated on each node and each believe
    // they are the root node, with no other nodes existing; this is essential so
    // that these quregs can agnostically use distributed routines which consult
    // the rank, but it will interfere with naive root-only printing logic
    out.rank        = (useDistrib)? env.rank : 0;
    out.numNodes    = (useDistrib)? env.numNodes : 1;
    out.logNumNodes = (useDistrib)? logBase2(env.numNodes) : 0; // duplicated for clarity

    // bind dimensions
    out.isDensityMatrix = isDensMatr;
    out.numQubits  = numQubits;
    out.numAmps    = (isDensMatr)? powerOf2(2*numQubits) : powerOf2(numQubits);
    out.logNumAmps = (isDensMatr)?          2*numQubits  :          numQubits;

    // bind dimensions per node (even if not distributed)
    out.numAmpsPerNode = powerOf2(logNumAmpsPerNode);
    out.logNumAmpsPerNode = logNumAmpsPerNode;
    out.logNumColsPerNode = (isDensMatr)? numQubits - logNumNodes : 0; // used only by density matrices

    // caller will allocate heap memory as necessary
    out.cpuAmps       = nullptr;
    out.gpuAmps       = nullptr;
    out.cpuCommBuffer = nullptr;
    out.gpuCommBuffer = nullptr;

    return out;
}