build: add -flto for wasm32 to fix setjmp/longjmp with Emscripten#218
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Makes sense to me! Thanks for fixing this |
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When building Lua for wasm32 with Emscripten, WebAssembly does not natively support traditional
setjmp/longjmpsemantics. Emscripten implements support by applying compiler and linker transformations that rewrite and instrumentsetjmp/longjmpusage. But when Lua is compiled by Zig as a separate static library outside Emscripten's normal compilation pipeline. Without-flto, Zig compiles Lua's source files into object code that Emscripten cannot fully analyze or rewrite during the final link step which prevents the requiredsetjmp/longjmptransformations from being applied correctly.Adding
-fltofor the wasm32 build causes Zig to preserve LLVM IR in the generated artifacts. When emcc performs the final link, it can see inside the Lua code and apply itssetjmp/longjmpinstrumentation passes across the entire program.Note: This issue was discovered while building Neovim for WebAssembly as part of the active Neovim wasm project: neovim/neovim#39987