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Deferred initialization of EmptyMerklePath structures till first use … #4131

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Deferred initialization of EmptyMerklePath structures till first use … #4131

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502507e
Deferred initialization of EmptyMerklePath structures till first use …
Sep 9, 2019
cf89467
Marked the empty_roots array mutable, simplified locking to use LOCK …
Sep 11, 2019
99e4f2f
Update src/zcash/IncrementalMerkleTree.hpp
zebambam Oct 8, 2019
1f82216
Update src/zcash/IncrementalMerkleTree.hpp
zebambam Oct 8, 2019
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22 changes: 17 additions & 5 deletions src/zcash/IncrementalMerkleTree.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@

#include "uint256.h"
#include "serialize.h"
#include "sync.h"

#include "Zcash.h"
#include "zcash/util.h"
Expand Down Expand Up @@ -57,25 +58,36 @@ class MerklePath {
template<size_t Depth, typename Hash>
class EmptyMerkleRoots {
public:
EmptyMerkleRoots() {
empty_roots.at(0) = Hash::uncommitted();
for (size_t d = 1; d <= Depth; d++) {
empty_roots.at(d) = Hash::combine(empty_roots.at(d-1), empty_roots.at(d-1), d-1);
}
void initialize() {
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if (!initialized) {
LOCK(cs_EmptyMerkleRoots_private_variables_write);
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if (!initialized) {
empty_roots.at(0) = Hash::uncommitted();
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for (size_t d = 1; d <= Depth; d++) {
empty_roots.at(d) = Hash::combine(empty_roots.at(d-1), empty_roots.at(d-1), d-1);
}
initialized = true;
}
}
}
EmptyMerkleRoots() { initialized = false; }
Hash empty_root(size_t depth) {
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initialize();
return empty_roots.at(depth);
}
template <size_t D, typename H>
friend bool operator==(const EmptyMerkleRoots<D, H>& a,
const EmptyMerkleRoots<D, H>& b);
private:
CCriticalSection cs_EmptyMerkleRoots_private_variables_write;
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bool initialized;
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I think this would be better as a std::atomic<bool>. We could then replace the locking with initialized.compare_exchange_strong(...).

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template<size_t Depth, typename Hash>
class EmptyMerkleRoots {
public:
    EmptyMerkleRoots() : initialized(false) { }
    void initialize() const {
        bool unitialized = false;
        if (initialized.compare_exchange_strong(unitialized, true)) {
            empty_roots.at(0) = Hash::uncommitted();
            for (size_t d = 1; d <= Depth; d++) {
                empty_roots.at(d) = Hash::combine(empty_roots.at(d-1), empty_roots.at(d-1), d-1);
            }
        }
    }
    Hash empty_root(size_t depth) const {
        initialize();
        return empty_roots.at(depth);
    }
    template <size_t D, typename H>
    friend bool operator==(const EmptyMerkleRoots<D, H>& a,
                           const EmptyMerkleRoots<D, H>& b);
private:
    mutable std::atomic<bool> initialized;
    mutable std::array<Hash, Depth+1> empty_roots;
};

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This is a neat way to do it I guess. I'm not sure that it's so much better than what's proposed that we should hold up the PR though. Maybe someone else disagrees.

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Agreed. The two ways should both be fine.

std::array<Hash, Depth+1> empty_roots;
};

template<size_t Depth, typename Hash>
bool operator==(const EmptyMerkleRoots<Depth, Hash>& a,
const EmptyMerkleRoots<Depth, Hash>& b) {
a.initialize(); b.initialize();
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return a.empty_roots == b.empty_roots;
}

Expand Down