Function getMaxNumQubitsBeforeGlobalMemSizeofOverflow

quest/src/core/memory.cpp:129–170

}

int getMaxNumQubitsBeforeGlobalMemSizeofOverflow(bool isDensMatr, int numNodes, bool hasBuffer, bool isSuperOp) {

    // this function assumes we must be able to store the total 
    // CPU memory (in bytes) used by a single data structure, 
    // aggregate across all nodes, in a single size_t primitive. 
    // This is a defensively-designed constraint; we do not ever
    // actually need to know the full memory, but assuring that
    // we could principally store it in a size_t will futureproof
    // reporter functions against future overflows etc. Note it
    // does not meaningfully restrict the maximum simulatable size
    // except on ~8 EiB supercomputers. Looking at you, Jupiter!

    size_t maxSizeof = std::numeric_limits<size_t>::max();
    size_t maxGlobalNumAmps = maxSizeof / sizeof(qcomp); // floors
    size_t maxGlobalNumQubits = std::floor(std::log2(maxGlobalNumAmps)); // floors

    // distributing Quregs requires communication buffers, doubling memory, decreasing qubits by 1
    if (hasBuffer && numNodes > 1)
        maxGlobalNumQubits -= 1;

    // density matrices have square-more amps, halving the number of qubtis (AFTER buffer subtraction)
    if (isDensMatr)
        maxGlobalNumQubits /= 2; // floors

    // superoperators are square-bigger than their constituent dense matrices
    if (isSuperOp)
        maxGlobalNumQubits /= 2; // floors

    /// @todo
    /// above sometimes overestimates by one; suggesting N can fit
    /// in fact only N-1 can fit without causing overflows. It's
    /// a chore to correct this precision-agnostically, and we cannot
    /// just try get the total-memory and provoke the overflow because
    /// a call to mem_getLocalQuregMemoryRequired() would recurse! As
    /// this function is only needed for ridiculously overzealous
    /// validation, and does not risk any logical error, we simply
    /// subtract one to avoid the overflowing edge-case. The returned
    /// max-size remains completely unreachable by users of course!
    maxGlobalNumQubits -= 1;

    return maxGlobalNumQubits;
}

int mem_getMaxNumQuregQubitsBeforeGlobalMemSizeofOverflow(bool isDensityMatrix, int numNodes) {