fix(opt): defensive panic on unmapped vreg instead of silent R0 fallback#101
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fix(opt): defensive panic on unmapped vreg instead of silent R0 fallback#101avrabe wants to merge 1 commit into
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This was referenced May 11, 2026
Follow-up to issue #93 (silicon-blocking memset bug). The root cause was optimizer_bridge::wasm_to_ir silently dropping three wasm ops (I64ExtendI32U/S, I32WrapI64) — they produced vregs that never got mapped to ARM registers. The downstream lookup in get_arm_reg returned Reg::R0 as a silent fallback, so subsequent i64 shifts read R0 as rm_lo/rm_hi, destroying memset's destination pointer on real silicon. Replace the silent fallback with a panic that names the vreg. Future "wasm op silently dropped" bugs of this class will surface at the boundary instead of producing miscompiled firmware. A compiler crash is strictly better than a hung MCU. Verified: * cargo test --workspace — all pass (no path legitimately hits the fallback — the post-#97 codebase produces mappings for every wasm op it emits IR for) * cargo clippy --workspace --all-targets -- -D warnings — clean * cargo fmt --check — clean Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Follow-up to PR #97 (which fixed the silicon-blocking #93 memset bug).
The root cause of #93 was that
optimizer_bridge::wasm_to_irsilently dropped three wasm ops (I64ExtendI32U/S, I32WrapI64). The downstream lookup helperget_arm_regreturnedReg::R0as a silent fallback when a vreg had no assignment — so subsequent i64 shifts read R0 as theirrm_lo/rm_hi, destroying memset's destination pointer.This PR replaces the silent fallback with a diagnostic panic naming the offending vreg. Any future "wasm op silently dropped" bugs of this class will surface as a compiler crash with a useful error message, instead of producing miscompiled firmware that boots-and-loops on real silicon.
A compiler crash is strictly better than a hung MCU.
Test plan
cargo test --workspace— all pass (the post-fix(opt): i64 lowering miscompiles memset-style loop counter on Cortex-M (#93) #97 codebase produces mappings for every wasm op it emits IR for, so no test legitimately hits the fallback path)cargo clippy --workspace --all-targets -- -D warnings— cleancargo fmt --check— clean🤖 Generated with Claude Code