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Short-circuit boolean chains (`if b0 or b1 or ... or bN:`) previously threaded every live copyable value through the Input/Output signature of every CFG basic block, even blocks that neither read nor write it. For W booleans across W DataflowBlocks this is O(W^2) total signature width, which drives O(W^2) LLVM insertvalue/extractvalue packing and makes superlinear LLVM passes blow up. For a copyable scalar value whose single definition strictly dominates all its uses, drop it from the signatures of intervening blocks and instead let hugr wire a non-local Dom edge from the defining block directly to the use blocks. Linear/non-copyable values stay threaded (can't be duplicated). - cfg/analysis.py: add iterative dominators (`compute_dominators`), `reverse_postorder`, and natural-loop detection (`loop_blocks`). - cfg_compiler.py: `compute_dethread_info` + `DethreadInfo`; strict single-def-dominates-all-uses gate; RPO block ordering so a def compiles before its uses; supply/store de-threaded wires in `compile_bb`. `Measurement` is explicitly excluded: it is a future that guppy auto-copies at the surface level but Hugr models as linear, so the Helios QIS lowering rejects it on a non-local edge. Tracked as an edge case (TODO) rather than a general type restriction. Payoff on a bool or-chain (W=8/16/32/48): block-signature sum 28/120/496/1128 -> 0; insertvalue 21/105/465/1081 -> 0; extractvalue 35/135/527/1175 -> 0; alloca O(W^2) -> O(W). End-to-end opt-2 compile of a 16x if-chain over 48 measured bools: 20.0s -> 2.2s. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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## ss2165-revert-962-poc-current-stack #2091 +/- ##
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+ Coverage 93.21% 93.22% +0.01%
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Files 152 152
Lines 14676 14848 +172
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+ Hits 13680 13842 +162
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Stacked on #2090.
Copyable values live across CFG blocks but only used past a dominating block were threaded through every intervening block signature — O(W²) in the width of e.g. an
if a or b or ...:chain, which blows up LLVM (SLP/SCCP) to ~90min compiles in the worst cases.Emit a non-local Dom edge from the defining block instead, gated on a strict single-def-dominates-all-uses check (linear, multi-def and loop-carried values stay threaded). Block-signature width goes O(W²)→O(W); on a 16×
if/ 48-bool repro opt-2 compile drops 20s→2.2s.Measurementis carved out — it's future-like and the Helios QIS lowers it to a linear type, which can't ride a non-local edge. Tracked in Quantinuum/tket2#1883.