wasm-opt --flatten fails with exceptions or GC

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
wasm
Domain
compilers

Research direction

Start by reproducing the failure with demo.wasm and the provided wasm-opt command, then inspect Flatten.cpp around line 231. Compare behavior with exception-handling and GC-related inputs or flags. Done means wasm-opt completes without the unexpected expr type or UNREACHABLE failure on the supplied file.

Written by the indexing model from the issue text.

Description

When running wasm-opt on the following WASM file demo.wasm.zip, using -O4 or --flatten, the following exception is raised (tested in v124 and v125):

unexpected expr type
UNREACHABLE executed at /nix/var/nix/builds/nix-29457-2897100894/source/src/passes/Flatten.cpp:231

My complete command is

"wasm-opt" \
    "--enable-sign-ext" "--disable-threads" "--enable-mutable-globals" \
    "--enable-nontrapping-float-to-int" "--enable-simd" "--enable-bulk-memory" \
    "--enable-exception-handling" "--disable-tail-call" "--enable-reference-types" \
    "--enable-multivalue" "--disable-gc" "--disable-memory64" \
    "--disable-relaxed-simd" "--disable-extended-const" "--disable-strings" \
    "--disable-multimemory" "--strip-debug" "-O0" "--flatten" \
    "-o" "demo.opt.wasm" "demo.wasm"
Dominant language
WebAssembly
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Forks
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Avg merge
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Merged PRs (30d)
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