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I added this back in 111999, but I no longer think it's a good idea - It had to get scaled back to only power-of-two things to not break a bunch of targets - LLVM seems to be getting better at memcpy removal anyway - Introducing vector instructions has seemed to sometimes (115515) make autovectorization worse So this removes it from the codegen crates entirely, and instead just tries to use <https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/traits/builder/trait.BuilderMethods.html#method.typed_place_copy> instead of direct `memcpy` so things will still use load/store for immediates.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.