docs(erlang): design systematic wasm32 -O2 miscompilation coverage (kd-r8h7)#832
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Add a design handoff for kd-r8h7 under docs/plans/. Characterizes the LLVM wasm32 -O2 ESTACK/WSTACK miscompilation bug class behind the reactive -O1 downgrades in build-erlang.sh (erl_unicode, erl_db_util, erl_db_hash, erl_db; erl_bif_chksum via PR #824), and proposes a three-layer strategy: - Detection: a CI-gated on-Kandelo term/iodata smoke matrix (batched into one BEAM boot) so a new miscompilation fails fast instead of shipping silently. - Containment: a greppable workaround registry + a one-time ESTACK/WSTACK risk audit, keeping the reactive process systematic and bounded (not blanket -O1). - Root cause: a minimal reproducer + upstream LLVM/OTP report with explicit -O1 removal criteria. Design only; implements none of the coverage, smoke, audit, or upstream work. Follow-up to kd-qe2c (PR #824). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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What
Design handoff for
kd-r8h7(follow-up tokd-qe2c/ PR #824). Addsdocs/plans/2026-07-02-erlang-wasm32-o2-miscompilation-coverage-design.md.Design only — no coverage, smoke, audit, or upstream work is implemented here.
Why
build-erlang.shcompiles a growing, reactively discovered set of ERTS filesat
-O1to dodge LLVM wasm32-O2miscompilations (erl_unicode,erl_db_util,erl_db_hash,erl_db;erl_bif_chksumvia PR #824). Each wasfound only when a user hit it — the
erlang:md5/1-over-iolist case even brokeon-Kandelo compilation entirely. Other term/iodata-walking files may harbor the
same defect and ship silently until tripped.
The design
-O2miscompiles the ESTACK/WSTACK term-traversalidiom (a control struct whose pointer fields alias a shadow-stack local
array). Two facets — aggregate initialization (fixed globally in
global.h)and optimized traversal codegen (worked around per-file with
-O1). Theinit patch is necessary but not sufficient.
into one BEAM boot, so a new miscompilation fails fast; (B) a greppable
workaround registry + a one-time ESTACK/WSTACK risk audit to make the reactive
process systematic and bounded — explicitly not blanket
-O1; (C) a minimalreproducer + upstream LLVM/OTP report with concrete
-O1removal criteria.erlang.test.tsisskipIf(!hasErlang)anddoes not run in CI; the authoritative gate must run where the OTP runtime tree
(
erlang-otp.tar.zst) is present.Implementation sequence is broken into candidate child beads; steps 1–4 deliver
the core value at low risk.
🤖 Generated with Claude Code