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Add the `no-builtins` attribute to functions when `no_builtins` is applied at the crate level. **When `no_builtins` is applied at the crate level, we should add the `no-builtins` attribute to each function to ensure it takes effect in LTO.** This is also the reason why no_builtins does not take effect in LTO as mentioned in #35540. Now, `#![no_builtins]` should be similar to `-fno-builtin` in clang/gcc, see https://clang.godbolt.org/z/z4j6Wsod5. Next, we should make `#![no_builtins]` participate in LTO again. That makes sense, as LTO also takes into consideration function-level instruction optimizations, such as the MachineOutliner. More importantly, when a user writes a large `#![no_builtins]` crate, they would like this crate to participate in LTO as well. We should also add a function-level no_builtins attribute to allow users to have more control over it. This is similar to Clang's `__attribute__((no_builtin))` feature, see https://clang.godbolt.org/z/Wod6KK6eq. Before implementing this feature, maybe we should discuss whether to support more fine-grained control, such as `__attribute__((no_builtin("memcpy")))`. Related discussions: - #109821 - #35540 Next (a separate pull request?): - [ ] Revert #35637 - [ ] Add a function-level `no_builtin` attribute? |
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Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.