016e9b5e33
rustc: Allow safe #[target_feature] on wasm This commit updates the compiler's handling of the `#[target_feature]` attribute when applied to functions on WebAssembly-based targets. The compiler in general requires that any functions with `#[target_feature]` are marked as `unsafe` as well, but this commit relaxes the restriction for WebAssembly targets where the attribute can be applied to safe functions as well. The reason this is done is that the motivation for this feature of the compiler is not applicable for WebAssembly targets. In general the `#[target_feature]` attribute is used to enhance target CPU features enabled beyond the basic level for the rest of the compilation. If done improperly this means that your program could execute an instruction that the CPU you happen to be running on does not understand. This is considered undefined behavior where it is unknown what will happen (e.g. it's not a deterministic `SIGILL`). For WebAssembly, however, the target is different. It is not possible for a running WebAssembly program to execute an instruction that the engine does not understand. If this were the case then the program would not have validated in the first place and would not run at all. Even if this were allowed in some hypothetical future where engines have some form of runtime feature detection (which they do not right now) any implementation of such a feature would generate a trap if a module attempts to execute an instruction the module does not understand. This deterministic trap behavior would still not fall into the category of undefined behavior because the trap is deterministic. For these reasons the `#[target_feature]` attribute is now allowed on safe functions, but only for WebAssembly targets. This notably enables the wasm-SIMD intrinsics proposed for stabilization in #74372 to be marked as safe generally instead of today where they're all `unsafe` due to the historical implementation of `#[target_feature]` in the compiler. |
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