[LLVM][Transforms][Attributor] - Improvements in AAIntraFnReachability::isReachableImpl#2182
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Reduce overhead in the Attributor's reachability (intra-function) analysis. Improve AAIntraFnReachabilityFunction::isReachableImpl - Replace per-query SmallPtrSet<BasicBlock*> visited set with a persistent vector<unsigned> indexed by DominatorTree DFS numbers. Each basic block has a unique DFSNumIn in [0, N), making it a perfect dense array index. A monotonically increasing query ID avoids clearing the vector between queries: a block is "visited" iff VisitedMap[DFSNumIn] == CurrentQueryID. This eliminates per-query allocation, pointer hashing, and destruction for BFS traversal. Motivated by a representative workload where we were seeing ~318K BFS traverals when multiple during LTO with many GPU kernels (e.g., Fortran OpenMP offloading applications).
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Reduce overhead in the Attributor's reachability (intra-function) analysis.
Improve AAIntraFnReachabilityFunction::isReachableImpl - Replace per-query SmallPtrSet<BasicBlock*> visited set with a persistent vector indexed by DominatorTree DFS numbers. Each basic block has a unique DFSNumIn in [0, N), making it a perfect dense array index. A monotonically increasing query ID avoids clearing the vector between queries: a block is "visited" iff VisitedMap[DFSNumIn] == CurrentQueryID. This eliminates per-query allocation, pointer hashing, and destruction for BFS traversal.
Motivated by a representative workload where we were seeing ~381K BFS traverals during LTO with many GPU kernels (e.g., Fortran OpenMP offloading applications).
(Cherry-picked from llvm#189132)