feat(vcr-ra): dead-frame elimination for promoted-local leaves, flag-off (#390, #242)

[avrabe]

VCR-RA-002 — lever 2 of the perf feature loop (uxth → 002 → #468 → #472)

compute_local_layout reserves a frame slot (sub sp,#N / add sp,#N) for every non-param wasm local it sees. Local promotion (v0.14.0) then homes the eligible i32 locals in registers, so when a function's locals all promote — and it neither spills, calls, nor touches i64 / stack-passed params — those frame bytes are never accessed. The sub/add sp pair is pure overhead (~2-3 cyc on a small leaf), AND because it writes SP it makes shrink_callee_saved_saves decline (that pass bails on any SP def/use).

elide_dead_frame removes the pair when the body provably never touches SP — saving the two instructions and restoring the SP-untouched precondition the shrink pass needs, so the two passes compose. It runs before shrink in the relocatable pipeline.

Safe-by-construction

Fires only when no instruction reads/writes SP except the matched frame sub/add and the prologue Push / epilogue Pop. For wasm locals that guard is exact deadness — locals are not addressable, so every other SP consumer (spills, #204 param-backing, the i64 pair-spill area, the #359 outgoing-arg region, incoming stack params) manifests as an [sp,#off] access the guard sees. Any such access — or any unmodeled op whose SP effect can't be confirmed absent — declines and leaves the bytes unchanged. Removal-only: no instruction added, rewritten, or reordered.

Gating

• Flag-off (opt-in SYNTH_DEAD_FRAME_ELIM=1); default path byte-identical → frozen byte gate green.
• Default-on flip held for on-silicon validation, like the realloc/shrink levers.

Validation

• 6 unit tests — removes / declines on sp-relative access / unbalanced add-sp / unmodeled sp-effect / no-frame noop / multiple epilogues.
• leaf_dead_frame_differential.py — leaf3 under unicorn, flag-off == flag-on == wasmtime over 10 vectors (signed + i32-wrap edges); 36 B → 28 B (-8 B). Both builds return cleanly via popped LR, confirming SP balance.

Honest scope

The push stays {r4-r8,lr} on leaf3 — a,b,c land in callee-saved r4,r5,r6 + scratch r7 = 4 saved regs, which shrink pads back to the even-count {r4-r8,lr}. Trimming the push needs the locals out of callee-saved (caller-saved leaf homing), tracked separately as #390.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.76471% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/synth-synthesis/src/liveness.rs	92.21%	13 Missing ⚠️
crates/synth-backend/src/arm_backend.rs	66.66%	1 Missing ⚠️

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