miri: rename intrinsic_fallback_checks_ub to intrinsic_fallback_is_spec
Checking UB is not the only concern, we also have to make sure we are not losing out on non-determinism.
r? ``@oli-obk`` (not urgent, take your time)
offset: allow zero-byte offset on arbitrary pointers
As per prior `@rust-lang/opsem` [discussion](https://github.com/rust-lang/opsem-team/issues/10) and [FCP](https://github.com/rust-lang/unsafe-code-guidelines/issues/472#issuecomment-1793409130):
- Zero-sized reads and writes are allowed on all sufficiently aligned pointers, including the null pointer
- Inbounds-offset-by-zero is allowed on all pointers, including the null pointer
- `offset_from` on two pointers derived from the same allocation is always allowed when they have the same address
This removes surprising UB (in particular, even C++ allows "nullptr + 0", which we currently disallow), and it brings us one step closer to an important theoretical property for our semantics ("provenance monotonicity": if operations are valid on bytes without provenance, then adding provenance can't make them invalid).
The minimum LLVM we require (v17) includes https://reviews.llvm.org/D154051, so we can finally implement this.
The `offset_from` change is needed to maintain the equivalence with `offset`: if `let ptr2 = ptr1.offset(N)` is well-defined, then `ptr2.offset_from(ptr1)` should be well-defined and return N. Now consider the case where N is 0 and `ptr1` dangles: we want to still allow offset_from here.
I think we should change offset_from further, but that's a separate discussion.
Fixes https://github.com/rust-lang/rust/issues/65108
[Tracking issue](https://github.com/rust-lang/rust/issues/117945) | [T-lang summary](https://github.com/rust-lang/rust/pull/117329#issuecomment-1951981106)
Cc `@nikic`
Directly implement native exception raise methods in miri
This implements the `_Unwind_RaiseException` function used on pretty much every unix system for starting unwinding. This allows removing the miri special case from libpanic_unwind for unix.
Windows still needs `miri_start_unwind` as SEH unwinding isn't supported by miri. Unlike DWARF unwinding, SEH preserves all stack frames until right after the do_catch function has executed. Because of this panic_unwind stack allocates the exception object. Miri can't currently model unwinding without destroying stack frames and as such will report a use-after-free of the exception object.
Windows still needs the old custom ABI as SEH unwinding isn't supported
by miri. Unlike DWARF unwinding it preserves all stack frames until
right after the do_catch function has executed. Because of this
panic_unwind stack allocates the exception object. Miri can't currently
model unwinding without destroying stack frames and as such will report
a use-after-free of the exception object.
Adjust Allocation Bytes used by Miri to custom MiriAllocBytes
Previously, the `MiriMachine` used `type Bytes = Box<[u8]>` for its allocations.
This PR swaps this out for a custom `MiriAllocBytes` type implemented in `alloc_bytes.rs`.
This is in anticipation of an extension to Miri's FFI, which will require its allocations to take care of alignment (the methods in `impl AllocBytes for Box<[u8]>` ignore this `_align: Align` argument).
Needs https://github.com/rust-lang/rust/pull/124492
Don't print unnecessary sysroot messages
Currently, when running `cargo miri setup`, we always print that a sysroot is being prepared, even if we just bail out of building after checking the hash. So that message is wrong; we didn't actually prepare a sysroot.
We also always print the preparing message for `cargo miri run`, even if no sysroot is prepared.
With this PR, `cargo miri run` prints no sysroot messages when an existing one is reused, and when a redundant `cargo miri setup` is requested, we print:
```
A sysroot for Miri is already available in `/home/ben/.cache/miri`.
```
