[MachineOutliner] Efficient Implementation of MachineOutliner::findCandidates()

[xuanzhang816]

We reduce the complexity of the main loop of findCandidates() method from O(n^2) to O(n log n).

We sort RS.StartIndices in SuffixTree and change replace find_if function with a simple check.

For each SuffixTree::RepeatedSubstring RS, the time complexity to find a set of candidates that do not overlap with each other is O(n^2) where n is the number of occurrence of this repeated substring (i.e., the size of RS.StartIndices). This is due to the use of the find_if method which has complexity O(n). The quadratic runtime becomes a problem when n gets larger. For clang, with [3], the maximum n goes from 12k to 100k, and the time to complete the main loop in MachineOutliner::findCandidates() goes from 17 seconds to 120 seconds.

To improve the runtime, we implement a more efficient algorithm with complexity O(n log n), using the fact that once RS.StartIndices is sorted, find_if can be achieved with an O(1) check. The O(n log n) complexity comes from the sorting. For clang, with [1], the time to complete the loop is reduced to only 28 seconds.

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