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Add alignment parameter to `simd_masked_{load,store}`
This PR adds an alignment parameter in `simd_masked_load` and `simd_masked_store`, in the form of a const-generic enum `core::intrinsics::simd::SimdAlign`. This represents the alignment of the `ptr` argument in these intrinsics as follows
- `SimdAlign::Unaligned` - `ptr` is unaligned/1-byte aligned
- `SimdAlign::Element` - `ptr` is aligned to the element type of the SIMD vector (default behavior in the old signature)
- `SimdAlign::Vector` - `ptr` is aligned to the SIMD vector type
The main motive for this is stdarch - most vector loads are either fully aligned (to the vector size) or unaligned (byte-aligned), so the previous signature doesn't cut it.
Now, stdarch will mostly use `SimdAlign::Unaligned` and `SimdAlign::Vector`, whereas portable-simd will use `SimdAlign::Element`.
- [x] `cg_llvm`
- [x] `cg_clif`
- [x] `miri`/`const_eval`
## Alternatives
Using a const-generic/"const" `u32` parameter as alignment (and we error during codegen if this argument is not a power of two). This, although more flexible than this, has a few drawbacks
- If we use an const-generic argument, then portable-simd somehow needs to pass `align_of::<T>()` as the alignment, which isn't possible without GCE
- "const" function parameters are just an ugly hack, and a pain to deal with in non-LLVM backends
We can remedy the problem with the const-generic `u32` parameter by adding a special rule for the element alignment case (e.g. `0` can mean "use the alignment of the element type), but I feel like this is not as expressive as the enum approach, although I am open to suggestions
cc `@workingjubilee` `@RalfJung` `@BoxyUwU`
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.