LLVM and Cranelift disagree about how to return values that don't fit
in the registers designated for return values. LLVM will force the
entire return value to be passed by return area pointer, while
Cranelift will look at each IR level return value independently and
decide to pass it in a register or not, which would result in the
return value being passed partially in registers and partially through
a return area pointer.
While Cranelift may need to be fixed as the LLVM behavior is generally
more correct with respect to the surface language, forcing this
behavior in rustc itself makes it easier for other backends to conform
to the Rust ABI and for the C ABI rustc already handles this behavior
anyway.
In addition LLVM's decision to pass the return value in registers or
using a return area pointer depends on how exactly the return type is
lowered to an LLVM IR type. For example `Option<u128>` can be lowered
as `{ i128, i128 }` in which case the x86_64 backend would use a return
area pointer, or it could be passed as `{ i32, i128 }` in which case
the x86_64 backend would pass it in registers by taking advantage of an
LLVM ABI extension that allows using 3 registers for the x86_64 sysv
call conv rather than the officially specified 2 registers.
This adjustment is only necessary for the Rust ABI as for other ABI's
the calling convention implementations in rustc_target already ensure
any return value which doesn't fit in the available amount of return
registers is passed in the right way for the current target.
Except for `simd-intrinsic/`, which has a lot of files containing
multiple types like `u8x64` which really are better when hand-formatted.
There is a surprising amount of two-space indenting in this directory.
Non-trivial changes:
- `rustfmt::skip` needed in `debug-column.rs` to preserve meaning of the
test.
- `rustfmt::skip` used in a few places where hand-formatting read more
nicely: `enum/enum-match.rs`
- Line number adjustments needed for the expected output of
`debug-column.rs` and `coroutine-debug.rs`.
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.
