🍒 [WebAssembly] Add segment RETAIN flag to support private retained data #8238
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llvm#81539) In WebAssembly, we have `WASM_SYMBOL_NO_STRIP` symbol flag to mark the referenced content as retained. However, the flag is not enough to express retained data that is not referenced by any symbol. This patch adds a new segment flag`WASM_SEG_FLAG_RETAIN` to support "private" linkage data that is retained by llvm.used. This kind of data that is not referenced but must be retained is usually used with encapsulation symbols (__start/__stop). Swift runtime uses this technique and depends on the fact "all metadata sections in live objects are retained", which was not guaranteed with `--gc-sections` before this patch. This is a revised version of https://reviews.llvm.org/D126950 (has been reverted) based on @MaskRay's comments
MaxDesiatov
approved these changes
Feb 20, 2024
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@swift-ci test |
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@swift-ci Please test macOS platform |
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Cherry-pick llvm@ba3c1f9 to enable
--gc-sectionsfor WebAssembly targets. With this fine-grained GC control, we can drop--no-gc-sectionspassed by driver and it removes unused import section entries and will get much smaller binary.