[wasm-split] Remove dead globals#8505
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Currently dead module items are not split and just end up remaining in the primary module. Usually a user runs DCE before or after the splitting, and the goal of wasm-split is not DCE, so from the optimization perspective it shouldn't be a problem. But after WebAssembly#8441, this can be a problem because a dead global's initializer can refer to another global that is moved to a secondary module: ```wast ;; Primary (global.get $dead i32 (global.get $a)) ;; Secondary (global $a i32 (...)) ``` This PR just removes those dead globals. We leave it and do some post processing to make it work but that's more complicated, or we can move it to the same secondary module but this requires scanning of the reverse mapping. Removing it seems the simplest. We can remove those dead items for other module items (memories, tables, and tags) but they are not necessary for wasm-split to run, and they can be handled later in DCE. Fixes WebAssembly#8442 (comment).
aheejin
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Mar 21, 2026
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| (drop | ||
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After this PR, glob1 and glob2 are dead and the test doesn't do the intended testing anymore, so we use them here in the primary function
tlively
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Mar 21, 2026
Co-authored-by: Thomas Lively <tlively123@gmail.com>
aheejin
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We currently just leave unused module items in the primary module, except for globals (#8505). This removes other unused module items too, because it is simpler for upcoming PRs. We currently pin all segments in the primary module, but future PRs can change that, in which case segments' usage can be dependent on tables, in which case unused tables can become tricky as in the case of globals. This PR just removes all unused module items for simplicity. This doesn't mean this runs a comprehensive fixed-point analysis like RemoveUnusedModuleElements pass; it just drops elements that are never referenced anywhere in any modules when exporting them. Update some tests so that tables are "used" in not droped.
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Currently dead module items are not split and just end up remaining in the primary module. Usually a user runs DCE before or after the splitting, and the goal of wasm-split is not DCE, so from the optimization perspective it shouldn't be a problem.
But after #8441, this can be a problem because a dead global's initializer can refer to another global that is moved to a secondary module:
This PR just removes those dead globals. We leave it and do some post processing to make it work but that's more complicated, or we can move it to the same secondary module but this requires scanning of the reverse mapping. Removing it seems the simplest. We can remove dead items for other module items (memories, tables, and tags) but it is not necessary for wasm-split to run, and they can be handled later in DCE.
Fixes #8442 (comment).