659 lines
24 KiB
Rust
659 lines
24 KiB
Rust
use std::cell::RefCell;
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use rustc::ty::{self, layout::Size};
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use rustc::hir::{Mutability, MutMutable, MutImmutable};
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use crate::{
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EvalResult, EvalErrorKind, MiriEvalContext, HelpersEvalContextExt, Evaluator, MutValueVisitor,
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MemoryKind, MiriMemoryKind, RangeMap, AllocId, Allocation, AllocationExtra, InboundsCheck,
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Pointer, MemPlace, Scalar, Immediate, ImmTy, PlaceTy, MPlaceTy,
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};
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pub type Timestamp = u64;
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/// Information about which kind of borrow was used to create the reference this is tagged
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/// with.
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#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
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pub enum Borrow {
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/// A unique (mutable) reference.
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Uniq(Timestamp),
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/// A shared reference. This is also used by raw pointers, which do not track details
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/// of how or when they were created, hence the timestamp is optional.
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/// Shr(Some(_)) does NOT mean that the destination of this reference is frozen;
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/// that depends on the type! Only those parts outside of an `UnsafeCell` are actually
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/// frozen.
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Shr(Option<Timestamp>),
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}
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impl Borrow {
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#[inline(always)]
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pub fn is_shared(self) -> bool {
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match self {
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Borrow::Shr(_) => true,
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_ => false,
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}
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}
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#[inline(always)]
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pub fn is_unique(self) -> bool {
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match self {
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Borrow::Uniq(_) => true,
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_ => false,
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}
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}
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}
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impl Default for Borrow {
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fn default() -> Self {
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Borrow::Shr(None)
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}
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}
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/// An item in the per-location borrow stack
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#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
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pub enum BorStackItem {
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/// Indicates the unique reference that may mutate.
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Uniq(Timestamp),
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/// Indicates that the location has been shared. Used for raw pointers, but
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/// also for shared references. The latter *additionally* get frozen
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/// when there is no `UnsafeCell`.
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Shr,
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/// A barrier, tracking the function it belongs to by its index on the call stack
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#[allow(dead_code)] // for future use
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FnBarrier(usize)
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}
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impl BorStackItem {
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#[inline(always)]
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pub fn is_fn_barrier(self) -> bool {
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match self {
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BorStackItem::FnBarrier(_) => true,
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_ => false,
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}
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}
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}
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/// Extra per-location state
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#[derive(Clone, Debug)]
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pub struct Stack {
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borrows: Vec<BorStackItem>, // used as a stack; never empty
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frozen_since: Option<Timestamp>, // virtual frozen "item" on top of the stack
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}
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impl Default for Stack {
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fn default() -> Self {
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Stack {
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borrows: vec![BorStackItem::Shr],
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frozen_since: None,
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}
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}
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}
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impl Stack {
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#[inline(always)]
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pub fn is_frozen(&self) -> bool {
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self.frozen_since.is_some()
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}
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}
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/// What kind of reference is being used?
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#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
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pub enum RefKind {
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/// &mut
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Unique,
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/// & without interior mutability
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Frozen,
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/// * (raw pointer) or & to `UnsafeCell`
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Raw,
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}
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/// Extra global machine state
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#[derive(Clone, Debug)]
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pub struct State {
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clock: Timestamp
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}
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impl State {
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pub fn new() -> State {
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State { clock: 0 }
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}
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fn increment_clock(&mut self) -> Timestamp {
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let val = self.clock;
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self.clock = val + 1;
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val
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}
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}
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/// Extra per-allocation state
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#[derive(Clone, Debug, Default)]
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pub struct Stacks {
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// Even reading memory can have effects on the stack, so we need a `RefCell` here.
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stacks: RefCell<RangeMap<Stack>>,
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}
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/// Core per-location operations: deref, access, create.
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/// We need to make at least the following things true:
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///
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/// U1: After creating a Uniq, it is at the top (+unfrozen).
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/// U2: If the top is Uniq (+unfrozen), accesses must be through that Uniq or pop it.
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/// U3: If an access (deref sufficient?) happens with a Uniq, it requires the Uniq to be in the stack.
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///
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/// F1: After creating a &, the parts outside `UnsafeCell` are frozen.
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/// F2: If a write access happens, it unfreezes.
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/// F3: If an access (well, a deref) happens with an & outside `UnsafeCell`, it requires the location to still be frozen.
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impl<'tcx> Stack {
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/// Deref `bor`: Check if the location is frozen and the tag in the stack.
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/// This dos *not* constitute an access! "Deref" refers to the `*` operator
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/// in Rust, and includs cases like `&*x` or `(*x).foo` where no or only part
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/// of the memory actually gets accessed. Also we cannot know if we are
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/// going to read or write.
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/// Returns the index of the item we matched, `None` if it was the frozen one.
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/// `kind` indicates which kind of reference is being dereferenced.
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fn deref(&self, bor: Borrow, kind: RefKind) -> Result<Option<usize>, String> {
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// Exclude unique ref and frozen tag.
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match (kind, bor) {
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(RefKind::Unique, Borrow::Shr(Some(_))) =>
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return Err(format!("Encountered mutable reference with frozen tag")),
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_ => {}
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}
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// Checks related to freezing
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match bor {
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Borrow::Shr(Some(bor_t)) if kind == RefKind::Frozen => {
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// We need the location to be frozen. This ensures F3.
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let frozen = self.frozen_since.map_or(false, |itm_t| itm_t <= bor_t);
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return if frozen { Ok(None) } else {
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Err(format!("Location is not frozen long enough"))
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}
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}
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Borrow::Shr(_) if self.frozen_since.is_some() => {
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return Ok(None) // Shared deref to frozen location, looking good
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}
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_ => {} // Not sufficient, go on looking.
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}
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// If we got here, we have to look for our item in the stack.
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for (idx, &itm) in self.borrows.iter().enumerate().rev() {
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match (itm, bor) {
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(BorStackItem::FnBarrier(_), _) => break,
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(BorStackItem::Uniq(itm_t), Borrow::Uniq(bor_t)) if itm_t == bor_t => {
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// Found matching unique item. This satisfies U3.
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return Ok(Some(idx))
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}
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(BorStackItem::Shr, Borrow::Shr(_)) => {
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// Found matching shared/raw item.
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return Ok(Some(idx))
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}
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// Go on looking.
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_ => {}
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}
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}
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// If we got here, we did not find our item. We have to error to satisfy U3.
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Err(format!(
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"Borrow being dereferenced ({:?}) does not exist on the stack, or is guarded by a barrier",
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bor
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))
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}
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/// Perform an actual memory access using `bor`. We do not know any types here
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/// or whether things should be frozen, but we *do* know if this is reading
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/// or writing.
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fn access(&mut self, bor: Borrow, is_write: bool) -> EvalResult<'tcx> {
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// Check if we can match the frozen "item".
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// Not possible on writes!
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if self.is_frozen() {
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if !is_write {
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// When we are frozen, we just accept all reads. No harm in this.
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// The deref already checked that `Uniq` items are in the stack, and that
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// the location is frozen if it should be.
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return Ok(());
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}
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trace!("access: Unfreezing");
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}
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// Unfreeze on writes. This ensures F2.
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self.frozen_since = None;
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// Pop the stack until we have something matching.
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while let Some(&itm) = self.borrows.last() {
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match (itm, bor) {
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(BorStackItem::FnBarrier(_), _) => break,
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(BorStackItem::Uniq(itm_t), Borrow::Uniq(bor_t)) if itm_t == bor_t => {
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// Found matching unique item.
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return Ok(())
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}
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(BorStackItem::Shr, _) if !is_write => {
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// When reading, everything can use a shared item!
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// We do not want to do this when writing: Writing to an `&mut`
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// should reaffirm its exclusivity (i.e., make sure it is
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// on top of the stack).
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return Ok(())
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}
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(BorStackItem::Shr, Borrow::Shr(_)) => {
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// Found matching shared item.
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return Ok(())
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}
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_ => {
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// Pop this. This ensures U2.
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let itm = self.borrows.pop().unwrap();
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trace!("access: Popping {:?}", itm);
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}
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}
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}
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// If we got here, we did not find our item.
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err!(MachineError(format!(
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"Borrow being accessed ({:?}) does not exist on the stack, or is guarded by a barrier",
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bor
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)))
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}
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/// Initiate `bor`; mostly this means pushing.
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/// This operation cannot fail; it is up to the caller to ensure that the precondition
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/// is met: We cannot push `Uniq` onto frozen stacks.
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/// `kind` indicates which kind of reference is being created.
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fn create(&mut self, bor: Borrow, kind: RefKind) {
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// First, push the item. We do this even if we will later freeze, because we
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// will allow mutation of shared data at the expense of unfreezing.
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if self.frozen_since.is_some() {
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// A frozen location, this should be impossible!
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bug!("We should never try pushing to a frozen stack");
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}
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// First, push.
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let itm = match bor {
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Borrow::Uniq(t) => BorStackItem::Uniq(t),
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Borrow::Shr(_) => BorStackItem::Shr,
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};
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if *self.borrows.last().unwrap() == itm {
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assert!(bor.is_shared());
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trace!("create: Sharing a shared location is a NOP");
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} else {
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// This ensures U1.
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trace!("create: Pushing {:?}", itm);
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self.borrows.push(itm);
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}
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// Then, maybe freeze. This is part 2 of ensuring F1.
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if kind == RefKind::Frozen {
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let bor_t = match bor {
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Borrow::Shr(Some(t)) => t,
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_ => bug!("Creating illegal borrow {:?} for frozen ref", bor),
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};
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trace!("create: Freezing");
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self.frozen_since = Some(bor_t);
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}
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}
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}
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/// Higher-level per-location operations: deref, access, reborrow.
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impl<'tcx> Stacks {
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/// Check that this stack is fine with being dereferenced
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fn deref(
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&self,
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ptr: Pointer<Borrow>,
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size: Size,
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kind: RefKind,
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) -> EvalResult<'tcx> {
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trace!("deref for tag {:?} as {:?}: {:?}, size {}",
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ptr.tag, kind, ptr, size.bytes());
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let mut stacks = self.stacks.borrow_mut();
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// We need `iter_mut` because `iter` would skip gaps!
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for stack in stacks.iter_mut(ptr.offset, size) {
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stack.deref(ptr.tag, kind).map_err(EvalErrorKind::MachineError)?;
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}
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Ok(())
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}
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/// `ptr` got used, reflect that in the stack.
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fn access(
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&self,
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ptr: Pointer<Borrow>,
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size: Size,
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is_write: bool,
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) -> EvalResult<'tcx> {
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trace!("{} access of tag {:?}: {:?}, size {}",
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if is_write { "read" } else { "write" },
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ptr.tag, ptr, size.bytes());
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// Even reads can have a side-effect, by invalidating other references.
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// This is fundamentally necessary since `&mut` asserts that there
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// are no accesses through other references, not even reads.
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let mut stacks = self.stacks.borrow_mut();
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for stack in stacks.iter_mut(ptr.offset, size) {
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stack.access(ptr.tag, is_write)?;
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}
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Ok(())
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}
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/// Reborrow the given pointer to the new tag for the given kind of reference.
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/// This works on `&self` because we might encounter references to constant memory.
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fn reborrow(
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&self,
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ptr: Pointer<Borrow>,
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size: Size,
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new_bor: Borrow,
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new_kind: RefKind,
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) -> EvalResult<'tcx> {
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assert_eq!(new_bor.is_unique(), new_kind == RefKind::Unique);
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trace!("reborrow for tag {:?} to {:?} as {:?}: {:?}, size {}",
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ptr.tag, new_bor, new_kind, ptr, size.bytes());
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let mut stacks = self.stacks.borrow_mut();
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for stack in stacks.iter_mut(ptr.offset, size) {
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// Access source `ptr`, create new ref.
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let ptr_idx = stack.deref(ptr.tag, new_kind).map_err(EvalErrorKind::MachineError)?;
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// If we can deref the new tag already, and if that tag lives higher on
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// the stack than the one we come from, just use that.
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// IOW, we check if `new_bor` *already* is "derived from" `ptr.tag`.
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// This also checks frozenness, if required.
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let bor_redundant = match (ptr_idx, stack.deref(new_bor, new_kind)) {
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// If the new borrow works with the frozen item, or else if it lives
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// above the old one in the stack, our job here is done.
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(_, Ok(None)) => true,
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(Some(ptr_idx), Ok(Some(new_idx))) if new_idx >= ptr_idx => true,
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// Otherwise we need to create a new borrow.
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_ => false,
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};
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if bor_redundant {
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assert!(new_bor.is_shared(), "A unique reborrow can never be redundant");
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trace!("reborrow is redundant");
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continue;
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}
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// We need to do some actual work.
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stack.access(ptr.tag, new_kind == RefKind::Unique)?;
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stack.create(new_bor, new_kind);
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}
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Ok(())
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}
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}
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/// Hooks and glue
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impl AllocationExtra<Borrow> for Stacks {
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#[inline(always)]
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fn memory_read<'tcx>(
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alloc: &Allocation<Borrow, Stacks>,
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ptr: Pointer<Borrow>,
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size: Size,
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) -> EvalResult<'tcx> {
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alloc.extra.access(ptr, size, /*is_write*/false)
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}
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#[inline(always)]
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fn memory_written<'tcx>(
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alloc: &mut Allocation<Borrow, Stacks>,
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ptr: Pointer<Borrow>,
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size: Size,
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) -> EvalResult<'tcx> {
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alloc.extra.access(ptr, size, /*is_write*/true)
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}
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#[inline(always)]
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fn memory_deallocated<'tcx>(
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alloc: &mut Allocation<Borrow, Stacks>,
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ptr: Pointer<Borrow>,
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size: Size,
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) -> EvalResult<'tcx> {
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// This is like mutating
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alloc.extra.access(ptr, size, /*is_write*/true)
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// FIXME: Error out of there are any barriers?
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}
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}
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impl<'tcx> Stacks {
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/// Pushes the first item to the stacks.
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pub fn first_item(
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&mut self,
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itm: BorStackItem,
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size: Size
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) {
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assert!(!itm.is_fn_barrier());
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for stack in self.stacks.get_mut().iter_mut(Size::ZERO, size) {
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assert!(stack.borrows.len() == 1);
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assert_eq!(stack.borrows.pop().unwrap(), BorStackItem::Shr);
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stack.borrows.push(itm);
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}
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}
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}
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pub trait EvalContextExt<'tcx> {
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fn ptr_dereference(
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&self,
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place: MPlaceTy<'tcx, Borrow>,
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size: Size,
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mutability: Option<Mutability>,
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) -> EvalResult<'tcx>;
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fn tag_new_allocation(
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&mut self,
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id: AllocId,
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kind: MemoryKind<MiriMemoryKind>,
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) -> Borrow;
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/// Reborrow the given place, returning the newly tagged ptr to it.
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fn reborrow(
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&mut self,
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place: MPlaceTy<'tcx, Borrow>,
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size: Size,
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new_bor: Borrow
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) -> EvalResult<'tcx, Pointer<Borrow>>;
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/// Retag an indidual pointer, returning the retagged version.
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fn retag_reference(
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&mut self,
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ptr: ImmTy<'tcx, Borrow>,
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mutbl: Mutability,
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) -> EvalResult<'tcx, Immediate<Borrow>>;
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fn retag(
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&mut self,
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fn_entry: bool,
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place: PlaceTy<'tcx, Borrow>
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) -> EvalResult<'tcx>;
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fn escape_to_raw(
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&mut self,
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place: MPlaceTy<'tcx, Borrow>,
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size: Size,
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) -> EvalResult<'tcx>;
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}
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impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for MiriEvalContext<'a, 'mir, 'tcx> {
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fn tag_new_allocation(
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&mut self,
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id: AllocId,
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kind: MemoryKind<MiriMemoryKind>,
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) -> Borrow {
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let time = match kind {
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MemoryKind::Stack => {
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// New unique borrow. This `Uniq` is not accessible by the program,
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// so it will only ever be used when using the local directly (i.e.,
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// not through a pointer). IOW, whenever we directly use a local this will pop
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// everything else off the stack, invalidating all previous pointers
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// and, in particular, *all* raw pointers. This subsumes the explicit
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// `reset` which the blog post [1] says to perform when accessing a local.
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//
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// [1] https://www.ralfj.de/blog/2018/08/07/stacked-borrows.html
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self.machine.stacked_borrows.increment_clock()
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}
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_ => {
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// Nothing to do for everything else
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return Borrow::default()
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}
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};
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// Make this the active borrow for this allocation
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let alloc = self.memory_mut().get_mut(id).expect("This is a new allocation, it must still exist");
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let size = Size::from_bytes(alloc.bytes.len() as u64);
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alloc.extra.first_item(BorStackItem::Uniq(time), size);
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Borrow::Uniq(time)
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}
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/// Called for value-to-place conversion. `mutability` is `None` for raw pointers.
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///
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/// Note that this does NOT mean that all this memory will actually get accessed/referenced!
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/// We could be in the middle of `&(*var).1`.
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fn ptr_dereference(
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&self,
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place: MPlaceTy<'tcx, Borrow>,
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|
size: Size,
|
|
mutability: Option<Mutability>,
|
|
) -> EvalResult<'tcx> {
|
|
trace!("ptr_dereference: Accessing {} reference for {:?} (pointee {})",
|
|
if let Some(mutability) = mutability { format!("{:?}", mutability) } else { format!("raw") },
|
|
place.ptr, place.layout.ty);
|
|
let ptr = place.ptr.to_ptr()?;
|
|
if mutability.is_none() {
|
|
// No further checks on raw derefs -- only the access itself will be checked.
|
|
return Ok(());
|
|
}
|
|
|
|
// Get the allocation
|
|
self.memory().check_bounds(ptr, size, InboundsCheck::Live)?;
|
|
let alloc = self.memory().get(ptr.alloc_id).expect("We checked that the ptr is fine!");
|
|
// If we got here, we do some checking, *but* we leave the tag unchanged.
|
|
if let Borrow::Shr(Some(_)) = ptr.tag {
|
|
assert_eq!(mutability, Some(MutImmutable));
|
|
// We need a frozen-sensitive check
|
|
self.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
|
|
let kind = if frozen { RefKind::Frozen } else { RefKind::Raw };
|
|
alloc.extra.deref(cur_ptr, size, kind)
|
|
})?;
|
|
} else {
|
|
// Just treat this as one big chunk
|
|
let kind = if mutability == Some(MutMutable) { RefKind::Unique } else { RefKind::Raw };
|
|
alloc.extra.deref(ptr, size, kind)?;
|
|
}
|
|
|
|
// All is good
|
|
Ok(())
|
|
}
|
|
|
|
/// The given place may henceforth be accessed through raw pointers.
|
|
#[inline(always)]
|
|
fn escape_to_raw(
|
|
&mut self,
|
|
place: MPlaceTy<'tcx, Borrow>,
|
|
size: Size,
|
|
) -> EvalResult<'tcx> {
|
|
self.reborrow(place, size, Borrow::default())?;
|
|
Ok(())
|
|
}
|
|
|
|
fn reborrow(
|
|
&mut self,
|
|
place: MPlaceTy<'tcx, Borrow>,
|
|
size: Size,
|
|
new_bor: Borrow
|
|
) -> EvalResult<'tcx, Pointer<Borrow>> {
|
|
let ptr = place.ptr.to_ptr()?;
|
|
let new_ptr = Pointer::new_with_tag(ptr.alloc_id, ptr.offset, new_bor);
|
|
trace!("reborrow: Creating new reference for {:?} (pointee {}): {:?}",
|
|
ptr, place.layout.ty, new_bor);
|
|
|
|
// Get the allocation. It might not be mutable, so we cannot use `get_mut`.
|
|
self.memory().check_bounds(ptr, size, InboundsCheck::Live)?;
|
|
let alloc = self.memory().get(ptr.alloc_id).expect("We checked that the ptr is fine!");
|
|
// Update the stacks.
|
|
if let Borrow::Shr(Some(_)) = new_bor {
|
|
// Reference that cares about freezing. We need a frozen-sensitive reborrow.
|
|
self.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
|
|
let kind = if frozen { RefKind::Frozen } else { RefKind::Raw };
|
|
alloc.extra.reborrow(cur_ptr, size, new_bor, kind)
|
|
})?;
|
|
} else {
|
|
// Just treat this as one big chunk.
|
|
let kind = if new_bor.is_unique() { RefKind::Unique } else { RefKind::Raw };
|
|
alloc.extra.reborrow(ptr, size, new_bor, kind)?;
|
|
}
|
|
Ok(new_ptr)
|
|
}
|
|
|
|
fn retag_reference(
|
|
&mut self,
|
|
val: ImmTy<'tcx, Borrow>,
|
|
mutbl: Mutability,
|
|
) -> EvalResult<'tcx, Immediate<Borrow>> {
|
|
// We want a place for where the ptr *points to*, so we get one.
|
|
let place = self.ref_to_mplace(val)?;
|
|
let size = self.size_and_align_of_mplace(place)?
|
|
.map(|(size, _)| size)
|
|
.unwrap_or_else(|| place.layout.size);
|
|
if size == Size::ZERO {
|
|
// Nothing to do for ZSTs.
|
|
return Ok(*val);
|
|
}
|
|
|
|
// Compute new borrow.
|
|
let time = self.machine.stacked_borrows.increment_clock();
|
|
let new_bor = match mutbl {
|
|
MutMutable => Borrow::Uniq(time),
|
|
MutImmutable => Borrow::Shr(Some(time)),
|
|
};
|
|
|
|
// Reborrow.
|
|
let new_ptr = self.reborrow(place, size, new_bor)?;
|
|
|
|
// Return new ptr
|
|
let new_place = MemPlace { ptr: Scalar::Ptr(new_ptr), ..*place };
|
|
Ok(new_place.to_ref())
|
|
}
|
|
|
|
fn retag(
|
|
&mut self,
|
|
_fn_entry: bool,
|
|
place: PlaceTy<'tcx, Borrow>
|
|
) -> EvalResult<'tcx> {
|
|
// TODO: Honor `fn_entry`.
|
|
|
|
// We need a visitor to visit all references. However, that requires
|
|
// a `MemPlace`, so we have a fast path for reference types that
|
|
// avoids allocating.
|
|
// Cannot use `builtin_deref` because that reports *immutable* for `Box`,
|
|
// making it useless.
|
|
if let Some(mutbl) = match place.layout.ty.sty {
|
|
ty::Ref(_, _, mutbl) => Some(mutbl),
|
|
ty::Adt(..) if place.layout.ty.is_box() => Some(MutMutable),
|
|
_ => None, // handled with the general case below
|
|
} {
|
|
// fast path
|
|
let val = self.read_immediate(self.place_to_op(place)?)?;
|
|
let val = self.retag_reference(val, mutbl)?;
|
|
self.write_immediate(val, place)?;
|
|
return Ok(());
|
|
}
|
|
let place = self.force_allocation(place)?;
|
|
|
|
let mut visitor = RetagVisitor { ecx: self };
|
|
visitor.visit_value(place)?;
|
|
|
|
// The actual visitor
|
|
struct RetagVisitor<'ecx, 'a, 'mir, 'tcx> {
|
|
ecx: &'ecx mut MiriEvalContext<'a, 'mir, 'tcx>,
|
|
}
|
|
impl<'ecx, 'a, 'mir, 'tcx>
|
|
MutValueVisitor<'a, 'mir, 'tcx, Evaluator<'tcx>>
|
|
for
|
|
RetagVisitor<'ecx, 'a, 'mir, 'tcx>
|
|
{
|
|
type V = MPlaceTy<'tcx, Borrow>;
|
|
|
|
#[inline(always)]
|
|
fn ecx(&mut self) -> &mut MiriEvalContext<'a, 'mir, 'tcx> {
|
|
&mut self.ecx
|
|
}
|
|
|
|
// Primitives of reference type, that is the one thing we are interested in.
|
|
fn visit_primitive(&mut self, place: MPlaceTy<'tcx, Borrow>) -> EvalResult<'tcx>
|
|
{
|
|
// Cannot use `builtin_deref` because that reports *immutable* for `Box`,
|
|
// making it useless.
|
|
let mutbl = match place.layout.ty.sty {
|
|
ty::Ref(_, _, mutbl) => mutbl,
|
|
ty::Adt(..) if place.layout.ty.is_box() => MutMutable,
|
|
_ => return Ok(()), // nothing to do
|
|
};
|
|
let val = self.ecx.read_immediate(place.into())?;
|
|
let val = self.ecx.retag_reference(val, mutbl)?;
|
|
self.ecx.write_immediate(val, place.into())?;
|
|
Ok(())
|
|
}
|
|
}
|
|
|
|
Ok(())
|
|
}
|
|
}
|