[Remangler] Improve performance by caching hashes. #72874
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The deepHash() function gets called repeatedly as we descend the node tree, which results in O(n^2) behaviour because we're traversing entire node subtree from each node we try substitution in, in order to calculate the hash. Fix by adding a hash table for hashes, so that we can look up hashes we've already computed. This appears to yield a 26.8% saving in local tests. rdar://125739630
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Explanation: The remangler was traversing the node tree over and over again computing hashes, while iterating the node tree; this is both O(n²) behaviour and actually we're doing unnecessary work. This PR adds a cache for the node hashes, which lets us short-circuit all of the extra work we were doing. |
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@swift-ci Please test |
tbkka
approved these changes
Apr 8, 2024
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The deepHash() function gets called repeatedly as we descend the node tree, which results in O(n^2) behaviour because we're traversing entire node subtree from each node we try substitution in, in order to calculate the hash.
Fix by adding a hash table for hashes, so that we can look up hashes we've already computed.
This appears to yield a 26.8% saving in local tests.
rdar://125739630