Commits on May 28, 2021

1. 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 rust-lang#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.

   

   alexcrichton committed May 28, 2021
   Configuration menu

   • View commit details
   • Copy full SHA for 7fed92b
   • Browse repository at this point

   Copy the full SHA

   7fed92b View commit details

   Browse the repository at this point in the history