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506848f563
gcov computes the line execution count as the sum of (a) counts from predecessors on other lines and (b) the sum of loop execution counts of blocks on the same line (think of loops on one line). For (b), we use Donald B. Johnson's cycle enumeration algorithm and perform cycle cancelling for each cycle. This number of candidate cycles were exponential and D93036 made it polynomial by skipping zero count cycles. The time complexity is high (O(V*E^2) (it could be O(E^2) but the linear `Blocks` check made it higher) and the implementation is complex. We could just identify loops and sum all back edges. However, this requires a dominator tree construction which is more complex. The time complexity can be decreased to almost linear, though. This patch just performs cycle cancelling iteratively. Add two members `traversable` and `incoming` to GCOVArc. There are 3 states: * `!traversable`: blocks not on this line or explored blocks * `traversable && incoming == nullptr`: unexplored blocks * `traversable && incoming != nullptr`: blocks which are being explored (on the stack) If an arc points to a block being explored, a cycle has been found. Let E be the number of arcs. Every time a cycle is found, at least one arc is saturated (`edgeCount` reduced to 0), so there are at most E cycles. Finding one cycle takes O(E) time, so the overall time complexity is O(E^2). Note that we always augment through a back edge and never need to augment its reverse edge so reverse edges in traditional flow networks are not needed. Reviewed By: xinhaoyuan Differential Revision: https://reviews.llvm.org/D93073 |
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| .. | ||
| Coverage | ||
| GCOV.h | ||
| InstrProf.h | ||
| InstrProfData.inc | ||
| InstrProfReader.h | ||
| InstrProfWriter.h | ||
| ProfileCommon.h | ||
| SampleProf.h | ||
| SampleProfReader.h | ||
| SampleProfWriter.h | ||