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feat(vcr-ra): flag-gated range re-allocation pass — first consequential allocator step (wiring 3a) - #288
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…ential allocator step (#209/#242, wiring step 3a) reallocate_function(instrs, pool): re-colour each maximal straight-line segment over the pool with VALUE RANGES as the allocation unit. Conservative pinning makes every segment independently sound with no cross-segment liveness: inputs (def==0) and per-register last-opened ranges (live-outs) keep their original registers; reserved-register ranges (R9-R12/SP) are identity-assigned outside the colouring. Renames only — never adds, removes, or reorders instructions, so labels and branch offsets are unaffected. Defense-in-depth: every interference edge is re-checked against the final assignment before a segment's rewrite is accepted; any decline passes the segment through untouched. Wired in arm_backend behind SYNTH_RANGE_REALLOC=1 (default OFF — every fixture stays bit-identical, verified by cmp). FLAG-ON evidence (the consequential gate): - differentials ALL RESULT-IDENTICAL: control_step 13/13 0x00210A55, flight_seam inlined+flat 0x07FDF307, div_const 338/338 - control_step + div_const: flag-on output is BYTE-identical to flag-off — the conservative pass re-derives the greedy assignment exactly (the strongest no-regression evidence) - flat_flight: 1/1 segment re-allocated, .text 1244 -> 1240 B (-4): the re-colouring enables more 16-bit Thumb encodings, result unchanged - stats per function: control_step 2/5 segments re-allocated, flight_seam 1/2, flat_flight 1/1 The flag stays default-off pending gale's on-target cycle confirmation (bytes != cycles, lesson #277). Step-4 spill insertion and the slack-aware assignment policy (const-CSE residency) build on this pass. Tests: pins honoured (inputs/live-outs/reserved identity), control flow verbatim, soundness on packed greedy code (output's own range interference conflict-free), structure preservation. 338/338 lib tests, clippy clean. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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This was referenced Jun 10, 2026
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What
The first consequential VCR-RA-001 step:
reallocate_functionre-colours each maximal straight-line segment with value ranges as the allocation unit, wired behindSYNTH_RANGE_REALLOC=1(default off — fixtures bit-identical, verified).Soundness by construction:
def==0) and per-register live-outs keep their original registers, so segments are independently sound with no cross-segment liveness; reserved registers (R9–R12/SP) identity-assigned outside the colouring.Flag-ON evidence (the gate that matters)
0x00210A55PASS0x07FDF307MATCH.text1244 → 1240 B (−4) — re-colouring enables more 16-bit Thumb forms, result unchangedFlag-OFF: all fixtures
cmp-bit-identical to v0.11.35. 338/338 lib tests, clippy clean.Why the flag stays off
Bytes ≠ cycles (lesson #277: the mla fusion removed instructions and still regressed on-target). The flip is gated on gale's on-target cycle confirmation. Step-4 spill insertion and the slack-aware assignment policy (the const-CSE residency unlock) build directly on this pass.
Falsification
Wrong if any flag-on module's result diverges from wasmtime — i.e. the pinning rules missed a cross-segment dependency or the interference convention let simultaneous values share a register. Watched by the four differentials (all green above) and the per-segment defense-in-depth check.
Traceability: VCR-RA-001 (epic #242,
release-v0.11.37).🤖 Generated with Claude Code