Function localiser_densmatr_setAmps

quest/src/core/localiser.cpp:613–641

void localiser_densmatr_setAmps(qcomp** inAmps, Qureg qureg, qindex startRow, qindex startCol, qindex numRows, qindex numCols) {
    assert_localiserGivenDensMatr(qureg);

    /// @todo improve the performance!
    /// this function works by simply enumerating each column of inAmps
    /// (which requires explicit preparation, because inAmps is passed
    /// column-wise, rather than row-wise), passing each to the above
    /// statevec routine. It is ergo similar to the naive method used by
    /// localiser_densmatr_getAmps(), though is embarrassingly parallel.
    /// This func allocates temporary memory as large as numRows*numCols 
    /// (which could be an entire Qureg's worth), and serially computes 
    /// the transpose (which can be as bad as serial iteration of the
    /// whole qureg). This is grossly inefficient, and worse than merely
    /// parallel-overwriting CPU memory then copying to GPU.

    vector<vector<qcomp>> tempAmps;
    util_tryAllocMatrix(tempAmps, numCols, numRows, error_localiserFailedToAllocTempMemory); // transpose of inAmps

    // serially overwrite tempAmps = transpose(inAmps)
    for (qindex c=0; c<numCols; c++)
        for (qindex r=0; r<numRows; r++)
            tempAmps[c][r] = inAmps[r][c]; // writes contiguous, reads strided

    // call the statevector function upon each column
    for (qindex c=0; c<numCols; c++) {
        qindex flatInd = util_getGlobalFlatIndex(qureg, startRow, startCol + c);
        localiser_statevec_setAmps(tempAmps[c].data(), qureg, flatInd, numRows);
    }
}


void localiser_densmatr_setAmpsToPauliStrSum(Qureg qureg, PauliStrSum sum) {