Function localiser_densmatr_getAmps

quest/src/core/localiser.cpp:540–575

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

    /// @todo improve the performance!
    /// this function simply serially invokes localiser_statevec_getAmps() upon 
    /// every indicated column, for simplicity, and since we believe this function
    /// will only ever be called upon tractably small sub-matrices. After all, the
    /// user is likely to serially process outAmps themselves. Our method incurs the
    /// below insignificant performance penalties:
    /// - we must allocate temporary memory that is the same size as outAmps,
    ///   but transposed, because we cannot make a pointer to a column of outAmps
    ///   in order to invoke localiser_statevec_getAmps() thereupon. Thereafter, we
    ///   serially populate outAmps with the transposed temp memory.
    /// - every invocation of localiser_statevec_getAmps() invokes synchronous
    ///   GPU-CPU copying (a total of #numCols), whereas a bespoke implementation
    ///   could perform each non-contiguous copy asynchronously then wait
    /// - every invocation invokes synchronous MPI broadcasting, whereas a bespoke
    ///   method could asynch all per-col broadcasts before a final wait
    /// A custom function to remedy these issues is complicated; it would involve
    /// e.g. exposing MPI_Request outside of comm_routines.cpp (unacceptable for
    /// compiler compatibility), or having comm_routines cache un-fulfilled asynch
    /// requests, etc.

    vector<vector<qcomp>> tempOut;
    util_tryAllocMatrix(tempOut, numCols, numRows, error_localiserFailedToAllocTempMemory); // transposed dim of outAmps

    for (qindex c=0; c<numCols; c++) {
        qindex flatInd = util_getGlobalFlatIndex(qureg, startRow, startCol + c);
        localiser_statevec_getAmps(tempOut[c].data(), qureg, flatInd, numRows);
    }

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


void localiser_fullstatediagmatr_getElems(qcomp* outElems, FullStateDiagMatr matr, qindex globalStartInd, qindex globalNumElems) {