High-level approach looks good! It would be nice to use `EnterJITStackEntry` more directly in `generateEnterJIt` too, [especially here](https://searchfox.org/mozilla-central/rev/9ae77e4ce3378bd683ac9a86b729ea6b6bd22cb8/js/src/jit/x64/Trampoline-x64.cpp#66-94). There you can probably subtract `sizeof(EnterJITStackEntry)` from the stack pointer, and then fill in each field using `storePtr` and `offsetof(EnterJITStackEntry, foo)`.
Bug 1635987 Comment 20 Edit History
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High-level approach looks good! It would be nice to use `EnterJITStackEntry` more directly in `generateEnterJIT` too, [especially here](https://searchfox.org/mozilla-central/rev/9ae77e4ce3378bd683ac9a86b729ea6b6bd22cb8/js/src/jit/x64/Trampoline-x64.cpp#66-94). There you can probably subtract `sizeof(EnterJITStackEntry)` from the stack pointer, and then fill in each field using `storePtr` and `offsetof(EnterJITStackEntry, foo)`.