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//! Regression test for <https://github.com/rust-lang/miri/issues/3341>:
//! If `Box` has a local allocator, then it can't be `noalias` as the allocator
//! may want to access allocator state based on the data pointer.
//@revisions: stack tree
//@[tree]compile-flags: -Zmiri-tree-borrows
#![feature(allocator_api)]
use std::alloc::{AllocError, Allocator, Layout};
use std::cell::{Cell, UnsafeCell};
use std::mem;
use std::ptr::{self, NonNull, addr_of};
use std::thread::{self, ThreadId};
const BIN_SIZE: usize = 8;
// A bin represents a collection of blocks of a specific layout.
#[repr(align(128))]
struct MyBin {
top: Cell<usize>,
thread_id: ThreadId,
memory: UnsafeCell<[usize; BIN_SIZE]>,
}
impl MyBin {
fn pop(&self) -> Option<NonNull<u8>> {
let top = self.top.get();
if top == BIN_SIZE {
return None;
}
// Cast the *entire* thing to a raw pointer to not restrict its provenance.
let bin = self as *const MyBin;
let base_ptr = UnsafeCell::raw_get(unsafe { addr_of!((*bin).memory) }).cast::<usize>();
let ptr = unsafe { NonNull::new_unchecked(base_ptr.add(top)) };
self.top.set(top + 1);
Some(ptr.cast())
}
// Pretends to not be a throwaway allocation method like this. A more realistic
// substitute is using intrusive linked lists, which requires access to the
// metadata of this bin as well.
unsafe fn push(&self, ptr: NonNull<u8>) {
// For now just check that this really is in this bin.
let start = self.memory.get().addr();
let end = start + BIN_SIZE * mem::size_of::<usize>();
let addr = ptr.addr().get();
assert!((start..end).contains(&addr));
}
}
// A collection of bins.
struct MyAllocator {
thread_id: ThreadId,
// Pretends to be some complex collection of bins, such as an array of linked lists.
bins: Box<[MyBin; 1]>,
}
impl MyAllocator {
fn new() -> Self {
let thread_id = thread::current().id();
MyAllocator {
thread_id,
bins: Box::new(
[MyBin { top: Cell::new(0), thread_id, memory: UnsafeCell::default() }; 1],
),
}
}
// Pretends to be expensive finding a suitable bin for the layout.
fn find_bin(&self, layout: Layout) -> Option<&MyBin> {
if layout == Layout::new::<usize>() { Some(&self.bins[0]) } else { None }
}
}
unsafe impl Allocator for MyAllocator {
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
// Expensive bin search.
let bin = self.find_bin(layout).ok_or(AllocError)?;
let ptr = bin.pop().ok_or(AllocError)?;
Ok(NonNull::slice_from_raw_parts(ptr, layout.size()))
}
unsafe fn deallocate(&self, ptr: NonNull<u8>, _layout: Layout) {
// Make sure accesses via `self` don't disturb anything.
let _val = self.bins[0].top.get();
// Since manually finding the corresponding bin of `ptr` is very expensive,
// doing pointer arithmetics is preferred.
// But this means we access `top` via `ptr` rather than `self`!
// That is fundamentally the source of the aliasing trouble in this example.
let their_bin = ptr.as_ptr().map_addr(|addr| addr & !127).cast::<MyBin>();
let thread_id = ptr::read(ptr::addr_of!((*their_bin).thread_id));
if self.thread_id == thread_id {
unsafe { (*their_bin).push(ptr) };
} else {
todo!("Deallocating from another thread");
}
// Make sure we can also still access this via `self` after the rest is done.
let _val = self.bins[0].top.get();
}
}
// Make sure to involve `Box` in allocating these,
// as that's where `noalias` may come from.
fn v1<T, A: Allocator>(t: T, a: A) -> Vec<T, A> {
(Box::new_in([t], a) as Box<[T], A>).into_vec()
}
fn v2<T, A: Allocator>(t: T, a: A) -> Vec<T, A> {
let v = v1(t, a);
// There was a bug in `into_boxed_slice` that caused aliasing issues,
// so round-trip through that as well.
v.into_boxed_slice().into_vec()
}
fn main() {
assert!(mem::size_of::<MyBin>() <= 128); // if it grows bigger, the trick to access the "header" no longer works
let my_alloc = MyAllocator::new();
let a = v1(1usize, &my_alloc);
let b = v2(2usize, &my_alloc);
assert_eq!(a[0] + 1, b[0]);
assert_eq!(addr_of!(a[0]).wrapping_add(1), addr_of!(b[0]));
drop((a, b));
}
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