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Collapse inner loops with union-find during WTO construction - #9241
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Add `src/cfg/wto.h` with `WeakTopologicalOrdering` (`WTO`) and `WTOWorklist` built on top of `DomTree`. In a reducible CFG ordered in reverse postorder, every cycle is a natural loop headed by a block that dominates all blocks in the cycle, allowing a Bourdoncle Weak Topological Ordering to be constructed directly from the dominator tree and natural loops of the CFG. Include unit tests in `test/gtest/wto.cpp` and `TODO` comments noting follow-on optimizations.
Replace RPOQueue with WTOWorklist in ConstraintAnalysis and RedundantSetElimination so that loops stabilize before flow values propagate to downstream blocks. This avoids quadratic/cubic blowups on functions with sequential loops while also speeding up general workloads. Benchmark results across 16 WebAssembly modules (3 iterations): - --constraint-analysis: - esbuild.wasm: 381.60s -> 7.59s (-98.0%, 50.3x speedup) - 15 non-esbuild modules geomean: 1.564s -> 1.487s (-4.9%) - 15 non-esbuild modules total: 72.00s -> 57.24s (-20.5%) - All 16 modules geomean: 2.205s -> 1.646s (-25.4%) - All 16 modules total: 453.60s -> 64.83s (-85.7%) - --rse: - esbuild.wasm: >600s (timeout) -> 1.62s (>370x speedup) - 15 non-esbuild modules total: 26.40s -> 25.47s (-3.5%) - All 16 modules geomean: N/A -> 0.922s (total: 27.09s)
Read each basic block's reverse-postorder index from contents.index in DomTree instead of allocating and populating an unordered_map<BasicBlock*, Index>, and skip self-loop backedges immediately with predIndex >= index. Update OnceReduction and test/example/domtree.cpp to initialize contents.index, and remove the redundant index initialization loop in WeakTopologicalOrdering. Benchmark results across 16 WebAssembly modules (3 iterations, interleaved): - --constraint-analysis: - Geomean: 1.646s -> 1.576s (-4.3%) - Total time: 64.83s -> 62.82s (-3.1%) - --rse: - Geomean: 0.922s -> 0.859s (-6.8%) - Total time: 27.09s -> 25.40s (-6.3%)
During reverse-RPO natural loop discovery in WeakTopologicalOrdering, collapse each discovered loop body into its header using union-find with path compression, and skip over already-collapsed inner loops when walking immediate dominators in dominates(). This prevents outer loops from re-traversing inner loop bodies, bounding natural loop discovery to O(E alpha(N)) instead of O(N * depth) on deeply nested loops. Benchmark results across 16 WebAssembly modules (3 iterations, interleaved): - --constraint-analysis: - Geomean: 1.576s -> 1.564s (-0.7%) - Total time: 62.82s -> 62.40s (-0.7%) - --rse: - Geomean: 0.859s -> 0.853s (-0.7%) - Total time: 25.40s -> 25.06s (-1.3%)
# Conflicts: # src/cfg/wto.h
# Conflicts: # test/gtest/wto.cpp
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This was already reviewed in #9218, which I merged prematurely into the previous PR branch rather than main. |
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During reverse-RPO natural loop discovery in WeakTopologicalOrdering,
collapse each discovered loop body into its header using union-find with
path compression, and skip over already-collapsed inner loops when
walking immediate dominators in dominates(). This prevents outer loops
from re-traversing inner loop bodies, bounding natural loop discovery to
O(E alpha(N)) instead of O(N * depth) on deeply nested loops.
Benchmark results across 16 WebAssembly modules (3 iterations,
interleaved):