JIT: Factor reachability sets and base them on new DFS #96892
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- Introduce `BlockReachabilitySets` which encapsulates a data structure that can answer "A -> B" reachability queries in `O(1)`. It factors the old `fgComputeReachabilitySets` and `fgReachable`. - Base the new reachability sets on the new DFS instead of using the old DFS. Switch to postorder num indexing instead of bbNum based indexing. - Remove `BasicBlock::bbReach`; store the reachability sets within `BlockReachabilitySets` instead. This costs some extra memory in FullOpts if we end up computing it twice, but reduces memory in MinOpts. - We no longer propagate `BBF_GC_SAFE_POINT` during the reachability computation. The cycle-based GC safe point algorithm introduced in dotnet#95299 does not require this propagation. - Switch the dead block elimination in `fgComputeReachability` to use `fgDfsBlocksAndRemove`, which has more general handling of EH and does not require as much iteration to closure - Remove the compuation of "enter blocks". With recent work the reachability computation was the last remaining use. Minor diffs expected due to the more general dead block removal capacity of `fgDfsBlocksAndRemove`.
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch Issue Details
Minor diffs expected due to the more general dead block removal capacity of
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jakobbotsch
commented
Jan 12, 2024
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| } while (changed); | ||
| // fgDfsReversePostorder reassigns preorder numbers, so recompute the DFS. | ||
| m_dfsTree = fgComputeDfs(); |
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We gain a new DFS tree computation here, but it should be quite short lived; we'll be able to get rid of it once the old DFS is removed.
jakobbotsch
commented
Jan 12, 2024
| sets[i] = BitVecOps::MakeSingleton(&postOrderTraits, i); | ||
| } | ||
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| // Find the reachable blocks. Also, set BBF_GC_SAFE_POINT. |
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Oops, outdated comment. Will address in a follow-up.
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cc @dotnet/jit-contrib PTAL @BruceForstall Minor diffs. Some TP regressions that we will regain shortly once the old DFS is removed. |
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Jan 23, 2024
- Introduce `BlockReachabilitySets` which encapsulates a data structure that can answer "A -> B" reachability queries in `O(1)`. It factors the old `fgComputeReachabilitySets` and `fgReachable`. - Base the new reachability sets on the new DFS instead of using the old DFS. Switch to postorder num indexing instead of bbNum based indexing. - Remove `BasicBlock::bbReach`; store the reachability sets within `BlockReachabilitySets` instead. This costs some extra memory in FullOpts if we end up computing it twice, but reduces memory in MinOpts. - We no longer propagate `BBF_GC_SAFE_POINT` during the reachability computation. The cycle-based GC safe point algorithm introduced in dotnet#95299 does not require this propagation. - Switch the dead block elimination in `fgComputeReachability` to use `fgDfsBlocksAndRemove`, which has more general handling of EH and does not require as much iteration to closure - Remove the compuation of "enter blocks". With recent work the reachability computation was the last remaining use. Minor diffs expected due to the more general dead block removal capacity of `fgDfsBlocksAndRemove`.
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BlockReachabilitySetswhich encapsulates a data structure that can answer "A -> B" reachability queries inO(1). It factors the oldfgComputeReachabilitySetsandfgReachable.BasicBlock::bbReach; store the reachability sets withinBlockReachabilitySetsinstead. This costs some extra memory in FullOpts if we end up computing it twice, but reduces memory in MinOpts.BBF_GC_SAFE_POINTduring the reachability computation. The cycle-based GC safe point algorithm introduced in JIT: Generalize check for full interruptibility #95299 does not require this propagation.fgComputeReachabilityto usefgDfsBlocksAndRemove, which has more general handling of EH and does not require as much iteration to closureMinor diffs expected due to the more general dead block removal capacity of
fgDfsBlocksAndRemove.