Currently, we assume that ScalarPair is always represented using
a two-element struct, both as an immediate value and when stored
in memory.
This currently works fairly well, but runs into problems with
https://github.com/rust-lang/rust/pull/116672, where a ScalarPair
involving an i128 type can no longer be represented as a two-element
struct in memory. For example, the tuple `(i32, i128)` needs to be
represented in-memory as `{ i32, [3 x i32], i128 }` to satisfy
alignment requirement. Using `{ i32, i128 }` instead will result in
the second element being stored at the wrong offset (prior to
LLVM 18).
Resolve this issue by no longer requiring that the immediate and
in-memory type for ScalarPair are the same. The in-memory type
will now look the same as for normal struct types (and will include
padding filler and similar), while the immediate type stays a
simple two-element struct type. This also means that booleans in
immediate ScalarPair are now represented as i1 rather than i8,
just like we do everywhere else.
The core change here is to llvm_type (which now treats ScalarPair
as a normal struct) and immediate_llvm_type (which returns the
two-element struct that llvm_type used to produce). The rest is
fixing things up to no longer assume these are the same. In
particular, this switches places that try to get pointers to the
ScalarPair elements to use byte-geps instead of struct-geps.
These tend to have special handling in a bunch of places anyway, so the variant helps remember that. And I think it's easier to grok than non-Scalar Aggregates sometimes being `Immediates` (like I got wrong and caused 109992). As a minor bonus, it means we don't need to generate poison LLVM values for them to pass around in `OperandValue::Immediate`s.
This takes a whole 3 lines in `compiler/` since it lowers to `CastKind::Transmute` in MIR *exactly* the same as the existing `intrinsics::transmute` does, it just doesn't have the fancy checking in `hir_typeck`.
Added to enable experimenting with the request in <https://github.com/rust-lang/rust/pull/106281#issuecomment-1496648190> and because the portable-simd folks might be interested for dependently-sized array-vector conversions.
It also simplifies a couple places in `core`.
LLVM can usually optimize these away, but especially for things like transmutes of newtypes it's silly to generate the `alloc`+`store`+`load` at all when it's actually a nop at LLVM level.
Updates `interpret`, `codegen_ssa`, and `codegen_cranelift` to consume the new cast instead of the intrinsic.
Includes `CastTransmute` for custom MIR building, to be able to test the extra UB.
Previously, it was only put on scalars with range validity invariants
like bool, was uninit was obviously invalid for those.
Since then, we have normatively declared all uninit primitives to be
undefined behavior and can therefore put `noundef` on them.
The remaining concern was the `mem::uninitialized` function, which cause
quite a lot of UB in the older parts of the ecosystem. This function now
doesn't return uninit values anymore, making users of it safe from this
change.
The only real sources of UB where people could encounter uninit
primitives are `MaybeUninit::uninit().assume_init()`, which has always
be clear in the docs about being UB and from heap allocations (like
reading from the spare capacity of a vec. This is hopefully rare enough
to not break anything.
