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Rollup merge of #122537 - RalfJung:interpret-allocation, r=oli-obk

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interpret/allocation: fix aliasing issue in interpreter and refactor getters a bit

That new raw getter will be needed to let Miri pass pointers to natively executed FFI code ("extern-so" mode).

While doing that I realized our get_bytes_mut are named less scary than get_bytes_unchecked so I rectified that. Also I realized `mem_copy_repeatedly` would break if we called it for multiple overlapping copies so I made sure this does not happen.

And I realized that we are actually [violating Stacked Borrows in the interpreter](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/I.20think.20Miri.20violates.20Stacked.20Borrows.20.F0.9F.99.88).^^ That was introduced in https://github.com/rust-lang/rust/pull/87777.

r? ```@oli-obk```
This commit is contained in:
Matthias Krüger 2024-03-22 11:36:59 +01:00 • committed by GitHub
commit 84e55be8da
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2 changed files with 45 additions and 14 deletions
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17
compiler/rustc_const_eval/src/interpret/memory.rs
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@ -1159,11 +1159,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
};
// Side-step AllocRef and directly access the underlying bytes more efficiently.
// (We are staying inside the bounds here so all is good.)
// (We are staying inside the bounds here and all bytes do get overwritten so all is good.)
let alloc_id = alloc_ref.alloc_id;
let bytes = alloc_ref
.alloc
.get_bytes_mut(&alloc_ref.tcx, alloc_ref.range)
.get_bytes_unchecked_for_overwrite(&alloc_ref.tcx, alloc_ref.range)
.map_err(move |e| e.to_interp_error(alloc_id))?;
// `zip` would stop when the first iterator ends; we want to definitely
// cover all of `bytes`.
@ -1184,6 +1184,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
self.mem_copy_repeatedly(src, dest, size, 1, nonoverlapping)
}
/// Performs `num_copies` many copies of `size` many bytes from `src` to `dest + i*size` (where
/// `i` is the index of the copy).
///
/// Either `nonoverlapping` must be true or `num_copies` must be 1; doing repeated copies that
/// may overlap is not supported.
pub fn mem_copy_repeatedly(
&mut self,
src: Pointer<Option<M::Provenance>>,
@ -1245,8 +1250,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
(dest_alloc_id, dest_prov),
dest_range,
)?;
// Yes we do overwrite all bytes in `dest_bytes`.
let dest_bytes = dest_alloc
.get_bytes_mut_ptr(&tcx, dest_range)
.get_bytes_unchecked_for_overwrite_ptr(&tcx, dest_range)
.map_err(|e| e.to_interp_error(dest_alloc_id))?
.as_mut_ptr();
@ -1280,6 +1286,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}
}
}
if num_copies > 1 {
assert!(nonoverlapping, "multi-copy only supported in non-overlapping mode");
}
let size_in_bytes = size.bytes_usize();
// For particularly large arrays (where this is perf-sensitive) it's common that
@ -1292,6 +1301,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
} else if src_alloc_id == dest_alloc_id {
let mut dest_ptr = dest_bytes;
for _ in 0..num_copies {
// Here we rely on `src` and `dest` being non-overlapping if there is more than
// one copy.
ptr::copy(src_bytes, dest_ptr, size_in_bytes);
dest_ptr = dest_ptr.add(size_in_bytes);
}