make SaltedOutpointHasher noexcept
If the hash is not noexcept, unorderd_map has to assume that it can throw an exception. Thus when rehashing care needs to be taken. libstdc++ solves this by simply caching the hash value, which increases memory of each node by 8 bytes. Adding noexcept prevents this caching. In my experiments with -reindex-chainstate -stopatheight=594000, memory usage has decreased by 9.4% while runtime has increased by 1.6% due to additional hashing. Additionally, memusage::DynamicUsage() is now more accurate and does not underestimate.
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src/coins.h
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src/coins.h
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@ -95,8 +95,16 @@ public:
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* This *must* return size_t. With Boost 1.46 on 32-bit systems the
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* This *must* return size_t. With Boost 1.46 on 32-bit systems the
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* unordered_map will behave unpredictably if the custom hasher returns a
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* unordered_map will behave unpredictably if the custom hasher returns a
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* uint64_t, resulting in failures when syncing the chain (#4634).
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* uint64_t, resulting in failures when syncing the chain (#4634).
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*
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* Having the hash noexcept allows libstdc++'s unordered_map to recalculate
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* the hash during rehash, so it does not have to cache the value. This
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* reduces node's memory by sizeof(size_t). The required recalculation has
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* a slight performance penalty (around 1.6%), but this is compensated by
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* memory savings of about 9% which allow for a larger dbcache setting.
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*
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* @see https://gcc.gnu.org/onlinedocs/gcc-9.2.0/libstdc++/manual/manual/unordered_associative.html
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*/
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*/
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size_t operator()(const COutPoint& id) const {
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size_t operator()(const COutPoint& id) const noexcept {
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return SipHashUint256Extra(k0, k1, id.hash, id.n);
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return SipHashUint256Extra(k0, k1, id.hash, id.n);
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}
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}
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};
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};
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