Store protectors outside Item, pack Tag and Perm

Previously, Item was a struct of a NonZeroU64, an Option which was
usually unset or irrelevant, and a 4-variant enum. So collectively, the
size of an Item was 24 bytes, but only 8 bytes were used for the most
part.

So this takes advantage of the fact that it is probably impossible to
exhaust the total space of SbTags, and steals 3 bits from it to pack the
whole struct into a single u64. This bit-packing means that we reduce
peak memory usage when Miri goes memory-bound by ~3x. We also get CPU
performance improvements of varying size, because not only are we simply
accessing less memory, we can now compare a Vec<Item> using a memcmp
because it does not have any padding.

src/machine.rs

@@ -54,15 +54,18 @@ pub struct FrameData<'tcx> {
     /// for the start of this frame. When we finish executing this frame,
     /// we use this to register a completed event with `measureme`.
     pub timing: Option<measureme::DetachedTiming>,
+
+    pub protected_tags: Vec<SbTag>,
 }
 
 impl<'tcx> std::fmt::Debug for FrameData<'tcx> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         // Omitting `timing`, it does not support `Debug`.
-        let FrameData { call_id, catch_unwind, timing: _ } = self;
+        let FrameData { call_id, catch_unwind, timing: _, protected_tags } = self;
         f.debug_struct("FrameData")
             .field("call_id", call_id)
             .field("catch_unwind", catch_unwind)
+            .field("protected_tags", protected_tags)
             .finish()
     }
 }
@@ -788,6 +791,7 @@ fn memory_read(
                 range,
                 machine.stacked_borrows.as_ref().unwrap(),
                 machine.current_span(),
+                &machine.threads,
             )?;
         }
         if let Some(weak_memory) = &alloc_extra.weak_memory {
@@ -819,6 +823,7 @@ fn memory_written(
                 range,
                 machine.stacked_borrows.as_ref().unwrap(),
                 machine.current_span(),
+                &machine.threads,
             )?;
         }
         if let Some(weak_memory) = &alloc_extra.weak_memory {
@@ -852,6 +857,7 @@ fn memory_deallocated(
                 tag,
                 range,
                 machine.stacked_borrows.as_ref().unwrap(),
+                &machine.threads,
             )
         } else {
             Ok(())
@@ -892,7 +898,7 @@ fn init_frame_extra(
             stacked_borrows.borrow_mut().new_call()
         });
 
-        let extra = FrameData { call_id, catch_unwind: None, timing };
+        let extra = FrameData { call_id, catch_unwind: None, timing, protected_tags: Vec::new() };
         Ok(frame.with_extra(extra))
     }
 
@@ -936,7 +942,7 @@ fn after_stack_pop(
     ) -> InterpResult<'tcx, StackPopJump> {
         let timing = frame.extra.timing.take();
         if let Some(stacked_borrows) = &ecx.machine.stacked_borrows {
-            stacked_borrows.borrow_mut().end_call(frame.extra.call_id);
+            stacked_borrows.borrow_mut().end_call(&frame.extra);
         }
         let res = ecx.handle_stack_pop_unwind(frame.extra, unwinding);
         if let Some(profiler) = ecx.machine.profiler.as_ref() {

src/stacked_borrows.rs

@@ -92,27 +92,99 @@ pub enum Permission {
     Disabled,
 }
 
+impl Permission {
+    const UNIQUE: u64 = 0;
+    const SHARED_READ_WRITE: u64 = 1;
+    const SHARED_READ_ONLY: u64 = 2;
+    const DISABLED: u64 = 3;
+
+    fn to_bits(self) -> u64 {
+        match self {
+            Permission::Unique => Self::UNIQUE,
+            Permission::SharedReadWrite => Self::SHARED_READ_WRITE,
+            Permission::SharedReadOnly => Self::SHARED_READ_ONLY,
+            Permission::Disabled => Self::DISABLED,
+        }
+    }
+
+    fn from_bits(perm: u64) -> Self {
+        match perm {
+            Self::UNIQUE => Permission::Unique,
+            Self::SHARED_READ_WRITE => Permission::SharedReadWrite,
+            Self::SHARED_READ_ONLY => Permission::SharedReadOnly,
+            Self::DISABLED => Permission::Disabled,
+            _ => unreachable!(),
+        }
+    }
+}
+
+mod item {
+    use super::{Permission, SbTag};
+    use std::fmt;
+    use std::num::NonZeroU64;
+
     /// An item in the per-location borrow stack.
     #[derive(Copy, Clone, Hash, PartialEq, Eq)]
-pub struct Item {
+    pub struct Item(u64);
-    /// The permission this item grants.
+
-    perm: Permission,
+    // An Item contains 3 bitfields:
+    // * Bits 0-61 store an SbTag
+    // * Bits 61-63 store a Permission
+    // * Bit 64 stores a flag which indicates if we have a protector
+    const TAG_MASK: u64 = u64::MAX >> 3;
+    const PERM_MASK: u64 = 0x3 << 61;
+    const PROTECTED_MASK: u64 = 0x1 << 63;
+
+    const PERM_SHIFT: u64 = 61;
+    const PROTECTED_SHIFT: u64 = 63;
+
+    impl Item {
+        pub fn new(tag: SbTag, perm: Permission, protected: bool) -> Self {
+            assert!(tag.0.get() <= TAG_MASK);
+            let packed_tag = tag.0.get();
+            let packed_perm = perm.to_bits() << PERM_SHIFT;
+            let packed_protected = (protected as u64) << PROTECTED_SHIFT;
+
+            let new = Self(packed_tag | packed_perm | packed_protected);
+
+            debug_assert!(new.tag() == tag);
+            debug_assert!(new.perm() == perm);
+            debug_assert!(new.protected() == protected);
+
+            new
+        }
+
         /// The pointers the permission is granted to.
-    tag: SbTag,
+        pub fn tag(self) -> SbTag {
-    /// An optional protector, ensuring the item cannot get popped until `CallId` is over.
+            SbTag(NonZeroU64::new(self.0 & TAG_MASK).unwrap())
-    protector: Option<CallId>,
+        }
+
+        /// The permission this item grants.
+        pub fn perm(self) -> Permission {
+            Permission::from_bits((self.0 & PERM_MASK) >> PERM_SHIFT)
+        }
+
+        /// Whether or not there is a protector for this tag
+        pub fn protected(self) -> bool {
+            self.0 & PROTECTED_MASK > 0
+        }
+
+        /// Set the Permission stored in this Item to Permission::Disabled
+        pub fn set_disabled(&mut self) {
+            // Clear the current set permission
+            self.0 &= !PERM_MASK;
+            // Write Permission::Disabled to the Permission bits
+            self.0 |= Permission::Disabled.to_bits() << PERM_SHIFT;
+        }
     }
 
     impl fmt::Debug for Item {
         fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "[{:?} for {:?}", self.perm, self.tag)?;
+            write!(f, "[{:?} for {:?}]", self.perm(), self.tag())
-        if let Some(call) = self.protector {
-            write!(f, " (call {})", call)?;
-        }
-        write!(f, "]")?;
-        Ok(())
         }
     }
+}
+pub use item::Item;
 
 /// Extra per-allocation state.
 #[derive(Clone, Debug)]
@@ -136,8 +208,8 @@ pub struct GlobalStateInner {
     base_ptr_tags: FxHashMap<AllocId, SbTag>,
     /// Next unused call ID (for protectors).
     next_call_id: CallId,
-    /// Those call IDs corresponding to functions that are still running.
+    /// All tags currently protected
-    active_calls: FxHashSet<CallId>,
+    protected_tags: FxHashSet<SbTag>,
     /// The pointer ids to trace
     tracked_pointer_tags: HashSet<SbTag>,
     /// The call ids to trace
@@ -201,7 +273,7 @@ pub fn new(
             next_ptr_tag: SbTag(NonZeroU64::new(1).unwrap()),
             base_ptr_tags: FxHashMap::default(),
             next_call_id: NonZeroU64::new(1).unwrap(),
-            active_calls: FxHashSet::default(),
+            protected_tags: FxHashSet::default(),
             tracked_pointer_tags,
             tracked_call_ids,
             retag_fields,
@@ -221,17 +293,14 @@ pub fn new_call(&mut self) -> CallId {
         if self.tracked_call_ids.contains(&id) {
             register_diagnostic(NonHaltingDiagnostic::CreatedCallId(id));
         }
-        assert!(self.active_calls.insert(id));
         self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap();
         id
     }
 
-    pub fn end_call(&mut self, id: CallId) {
+    pub fn end_call(&mut self, frame: &machine::FrameData<'_>) {
-        assert!(self.active_calls.remove(&id));
+        for tag in &frame.protected_tags {
+            self.protected_tags.remove(tag);
         }
-
-    fn is_active(&self, id: CallId) -> bool {
-        self.active_calls.contains(&id)
     }
 
     pub fn base_ptr_tag(&mut self, id: AllocId) -> SbTag {
@@ -287,7 +356,7 @@ impl<'tcx> Stack {
     /// Find the first write-incompatible item above the given one --
     /// i.e, find the height to which the stack will be truncated when writing to `granting`.
     fn find_first_write_incompatible(&self, granting: usize) -> usize {
-        let perm = self.get(granting).unwrap().perm;
+        let perm = self.get(granting).unwrap().perm();
         match perm {
             Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
             Permission::Disabled => bug!("Cannot use Disabled for anything"),
@@ -299,7 +368,7 @@ fn find_first_write_incompatible(&self, granting: usize) -> usize {
                 // The SharedReadWrite *just* above us are compatible, to skip those.
                 let mut idx = granting + 1;
                 while let Some(item) = self.get(idx) {
-                    if item.perm == Permission::SharedReadWrite {
+                    if item.perm() == Permission::SharedReadWrite {
                         // Go on.
                         idx += 1;
                     } else {
@@ -325,34 +394,46 @@ fn item_popped(
         provoking_access: Option<(SbTagExtra, AllocRange, Size, AccessKind)>, // just for debug printing and error messages
         global: &GlobalStateInner,
         alloc_history: &mut AllocHistory,
+        threads: &ThreadManager<'_, 'tcx>,
     ) -> InterpResult<'tcx> {
-        if global.tracked_pointer_tags.contains(&item.tag) {
+        if global.tracked_pointer_tags.contains(&item.tag()) {
             register_diagnostic(NonHaltingDiagnostic::PoppedPointerTag(
                 *item,
                 provoking_access.map(|(tag, _alloc_range, _size, access)| (tag, access)),
             ));
         }
 
-        if let Some(call) = item.protector {
+        if !item.protected() {
-            if global.is_active(call) {
+            return Ok(());
+        }
+
+        if global.protected_tags.contains(&item.tag()) {
+            let call_id = threads
+                .all_stacks()
+                .flatten()
+                .find(|t| t.extra.protected_tags.contains(&item.tag()))
+                .map(|frame| frame.extra.call_id)
+                .unwrap(); // FIXME: Surely we should find something, but a panic seems wrong here?
             if let Some((tag, _alloc_range, _offset, _access)) = provoking_access {
                 Err(err_sb_ub(
                     format!(
-                            "not granting access to tag {:?} because incompatible item is protected: {:?}",
+                        "not granting access to tag {:?} because incompatible item is protected: {:?} (call {:?})",
-                            tag, item
+                        tag, item, call_id
                     ),
                     None,
-                        tag.and_then(|tag| alloc_history.get_logs_relevant_to(tag, Some(item.tag))),
+                    tag.and_then(|tag| alloc_history.get_logs_relevant_to(tag, Some(item.tag()))),
                 ))?
             } else {
                 Err(err_sb_ub(
-                        format!("deallocating while item is protected: {:?}", item),
+                    format!(
+                        "deallocating while item is protected: {:?} (call {:?})",
+                        item, call_id
+                    ),
                     None,
                     None,
                 ))?
             }
         }
-        }
         Ok(())
     }
 
@@ -369,6 +450,7 @@ fn access(
         current_span: &mut CurrentSpan<'_, '_, 'tcx>,
         alloc_history: &mut AllocHistory,
         exposed_tags: &FxHashSet<SbTag>,
+        threads: &ThreadManager<'_, 'tcx>,
     ) -> InterpResult<'tcx> {
         // Two main steps: Find granting item, remove incompatible items above.
 
@@ -399,8 +481,9 @@ fn access(
                     Some((tag, alloc_range, offset, access)),
                     global,
                     alloc_history,
+                    threads,
                 )?;
-                alloc_history.log_invalidation(item.tag, alloc_range, current_span);
+                alloc_history.log_invalidation(item.tag(), alloc_range, current_span);
                 Ok(())
             })?;
         } else {
@@ -425,8 +508,9 @@ fn access(
                     Some((tag, alloc_range, offset, access)),
                     global,
                     alloc_history,
+                    threads,
                 )?;
-                alloc_history.log_invalidation(item.tag, alloc_range, current_span);
+                alloc_history.log_invalidation(item.tag(), alloc_range, current_span);
                 Ok(())
             })?;
         }
@@ -439,9 +523,9 @@ fn access(
             for i in 0..self.len() {
                 let item = self.get(i).unwrap();
                 // Skip disabled items, they cannot be matched anyway.
-                if !matches!(item.perm, Permission::Disabled) {
+                if !matches!(item.perm(), Permission::Disabled) {
                     // We are looking for a strict upper bound, so add 1 to this tag.
-                    max = cmp::max(item.tag.0.checked_add(1).unwrap(), max);
+                    max = cmp::max(item.tag().0.checked_add(1).unwrap(), max);
                 }
             }
             if let Some(unk) = self.unknown_bottom() {
@@ -467,6 +551,7 @@ fn dealloc(
         global: &GlobalStateInner,
         alloc_history: &mut AllocHistory,
         exposed_tags: &FxHashSet<SbTag>,
+        threads: &ThreadManager<'_, 'tcx>,
     ) -> InterpResult<'tcx> {
         // Step 1: Make sure there is a granting item.
         self.find_granting(AccessKind::Write, tag, exposed_tags).map_err(|_| {
@@ -482,7 +567,7 @@ fn dealloc(
         // Step 2: Consider all items removed. This checks for protectors.
         for idx in (0..self.len()).rev() {
             let item = self.get(idx).unwrap();
-            Stack::item_popped(&item, None, global, alloc_history)?;
+            Stack::item_popped(&item, None, global, alloc_history, threads)?;
         }
         Ok(())
     }
@@ -502,10 +587,11 @@ fn grant(
         current_span: &mut CurrentSpan<'_, '_, 'tcx>,
         alloc_history: &mut AllocHistory,
         exposed_tags: &FxHashSet<SbTag>,
+        threads: &ThreadManager<'_, 'tcx>,
     ) -> InterpResult<'tcx> {
         // Figure out which access `perm` corresponds to.
         let access =
-            if new.perm.grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read };
+            if new.perm().grants(AccessKind::Write) { AccessKind::Write } else { AccessKind::Read };
 
         // Now we figure out which item grants our parent (`derived_from`) this kind of access.
         // We use that to determine where to put the new item.
@@ -517,7 +603,7 @@ fn grant(
         // Compute where to put the new item.
         // Either way, we ensure that we insert the new item in a way such that between
         // `derived_from` and the new one, there are only items *compatible with* `derived_from`.
-        let new_idx = if new.perm == Permission::SharedReadWrite {
+        let new_idx = if new.perm() == Permission::SharedReadWrite {
             assert!(
                 access == AccessKind::Write,
                 "this case only makes sense for stack-like accesses"
@@ -550,6 +636,7 @@ fn grant(
                 current_span,
                 alloc_history,
                 exposed_tags,
+                threads,
             )?;
 
             // We insert "as far up as possible": We know only compatible items are remaining
@@ -571,7 +658,7 @@ fn grant(
 impl<'tcx> Stacks {
     /// Creates new stack with initial tag.
     fn new(size: Size, perm: Permission, tag: SbTag) -> Self {
-        let item = Item { perm, tag, protector: None };
+        let item = Item::new(tag, perm, false);
         let stack = Stack::new(item);
 
         Stacks {
@@ -637,6 +724,7 @@ pub fn memory_read<'tcx>(
         range: AllocRange,
         state: &GlobalState,
         mut current_span: CurrentSpan<'_, '_, 'tcx>,
+        threads: &ThreadManager<'_, 'tcx>,
     ) -> InterpResult<'tcx> {
         trace!(
             "read access with tag {:?}: {:?}, size {}",
@@ -654,6 +742,7 @@ pub fn memory_read<'tcx>(
                 &mut current_span,
                 history,
                 exposed_tags,
+                threads,
             )
         })
     }
@@ -666,6 +755,7 @@ pub fn memory_written<'tcx>(
         range: AllocRange,
         state: &GlobalState,
         mut current_span: CurrentSpan<'_, '_, 'tcx>,
+        threads: &ThreadManager<'_, 'tcx>,
     ) -> InterpResult<'tcx> {
         trace!(
             "write access with tag {:?}: {:?}, size {}",
@@ -683,6 +773,7 @@ pub fn memory_written<'tcx>(
                 &mut current_span,
                 history,
                 exposed_tags,
+                threads,
             )
         })
     }
@@ -694,11 +785,12 @@ pub fn memory_deallocated<'tcx>(
         tag: SbTagExtra,
         range: AllocRange,
         state: &GlobalState,
+        threads: &ThreadManager<'_, 'tcx>,
     ) -> InterpResult<'tcx> {
         trace!("deallocation with tag {:?}: {:?}, size {}", tag, alloc_id, range.size.bytes());
         let state = state.borrow();
         self.for_each(range, |offset, stack, history, exposed_tags| {
-            stack.dealloc(tag, (alloc_id, range, offset), &state, history, exposed_tags)
+            stack.dealloc(tag, (alloc_id, range, offset), &state, history, exposed_tags, threads)
         })?;
         Ok(())
     }
@@ -801,7 +893,6 @@ fn reborrow(
             });
         }
 
-        let protector = if protect { Some(this.frame().extra.call_id) } else { None };
         trace!(
             "reborrow: {} reference {:?} derived from {:?} (pointee {}): {:?}, size {}",
             kind,
@@ -812,6 +903,13 @@ fn reborrow(
             size.bytes()
         );
 
+        if protect {
+            this.frame_mut().extra.protected_tags.push(new_tag);
+            this.machine.stacked_borrows.as_mut().unwrap().get_mut().protected_tags.insert(new_tag);
+        }
+        // FIXME: can't hold the current span handle across the borrows of self above
+        let current_span = &mut this.machine.current_span();
+
         // Update the stacks.
         // Make sure that raw pointers and mutable shared references are reborrowed "weak":
         // There could be existing unique pointers reborrowed from them that should remain valid!
@@ -848,15 +946,16 @@ fn reborrow(
                     } else {
                         Permission::SharedReadWrite
                     };
-                    let protector = if frozen {
+                    let protected = if frozen {
-                        protector
+                        protect
                     } else {
                         // We do not protect inside UnsafeCell.
                         // This fixes https://github.com/rust-lang/rust/issues/55005.
-                        None
+                        false
                     };
-                    let item = Item { perm, tag: new_tag, protector };
+                    let item = Item::new(new_tag, perm, protected);
                     let mut global = this.machine.stacked_borrows.as_ref().unwrap().borrow_mut();
+                    let threads = &this.machine.threads;
                     stacked_borrows.for_each(range, |offset, stack, history, exposed_tags| {
                         stack.grant(
                             orig_tag,
@@ -866,6 +965,7 @@ fn reborrow(
                             current_span,
                             history,
                             exposed_tags,
+                            threads,
                         )
                     })
                 })?;
@@ -881,9 +981,10 @@ fn reborrow(
             .as_mut()
             .expect("we should have Stacked Borrows data")
             .borrow_mut();
-        let item = Item { perm, tag: new_tag, protector };
+        let item = Item::new(new_tag, perm, protect);
         let range = alloc_range(base_offset, size);
         let mut global = machine.stacked_borrows.as_ref().unwrap().borrow_mut();
+        let threads = &machine.threads;
         let current_span = &mut machine.current_span(); // `get_alloc_extra_mut` invalidated our old `current_span`
         stacked_borrows.for_each(range, |offset, stack, history, exposed_tags| {
             stack.grant(
@@ -894,6 +995,7 @@ fn reborrow(
                 current_span,
                 history,
                 exposed_tags,
+                threads,
             )
         })?;
 

src/stacked_borrows/diagnostics.rs

@@ -141,7 +141,7 @@ pub fn grant_error<'tcx>(
     ) -> InterpError<'tcx> {
         let action = format!(
             "trying to reborrow {derived_from:?} for {new_perm:?} permission at {alloc_id:?}[{offset:#x}]",
-            new_perm = new.perm,
+            new_perm = new.perm(),
             offset = error_offset.bytes(),
         );
         err_sb_ub(
@@ -185,7 +185,7 @@ fn error_cause(stack: &Stack, tag: SbTagExtra) -> &'static str {
     if let SbTagExtra::Concrete(tag) = tag {
         if (0..stack.len())
             .map(|i| stack.get(i).unwrap())
-            .any(|item| item.tag == tag && item.perm != Permission::Disabled)
+            .any(|item| item.tag() == tag && item.perm() != Permission::Disabled)
         {
             ", but that tag only grants SharedReadOnly permission for this location"
         } else {

src/stacked_borrows/stack.rs

@@ -37,7 +37,7 @@ pub struct Stack {
 }
 
 /// A very small cache of searches of the borrow stack
-/// This maps tags to locations in the borrow stack. Any use of this still needs to do a
+/// This maps items to locations in the borrow stack. Any use of this still needs to do a
 /// probably-cold random access into the borrow stack to figure out what `Permission` an
 /// `SbTag` grants. We could avoid this by also storing the `Permission` in the cache, but
 /// most lookups into the cache are immediately followed by access of the full borrow stack anyway.
@@ -48,7 +48,7 @@ pub struct Stack {
 #[cfg(feature = "stack-cache")]
 #[derive(Clone, Debug)]
 struct StackCache {
-    tags: [SbTag; CACHE_LEN], // Hot in find_granting
+    items: [Item; CACHE_LEN], // Hot in find_granting
     idx: [usize; CACHE_LEN],  // Hot in grant
 }
 
@@ -59,11 +59,11 @@ impl StackCache {
     /// We use the position in the cache to represent how recently a tag was used; the first position
     /// is the most recently used tag. So an add shifts every element towards the end, and inserts
     /// the new element at the start. We lose the last element.
-    /// This strategy is effective at keeping the most-accessed tags in the cache, but it costs a
+    /// This strategy is effective at keeping the most-accessed items in the cache, but it costs a
     /// linear shift across the entire cache when we add a new tag.
-    fn add(&mut self, idx: usize, tag: SbTag) {
+    fn add(&mut self, idx: usize, item: Item) {
-        self.tags.copy_within(0..CACHE_LEN - 1, 1);
+        self.items.copy_within(0..CACHE_LEN - 1, 1);
-        self.tags[0] = tag;
+        self.items[0] = item;
         self.idx.copy_within(0..CACHE_LEN - 1, 1);
         self.idx[0] = idx;
     }
@@ -80,20 +80,20 @@ impl Eq for Stack {}
 
 impl<'tcx> Stack {
     /// Panics if any of the caching mechanisms have broken,
-    /// - The StackCache indices don't refer to the parallel tags,
+    /// - The StackCache indices don't refer to the parallel items,
-    /// - There are no Unique tags outside of first_unique..last_unique
+    /// - There are no Unique items outside of first_unique..last_unique
     #[cfg(feature = "expensive-debug-assertions")]
     fn verify_cache_consistency(&self) {
         // Only a full cache needs to be valid. Also see the comments in find_granting_cache
         // and set_unknown_bottom.
         if self.borrows.len() >= CACHE_LEN {
-            for (tag, stack_idx) in self.cache.tags.iter().zip(self.cache.idx.iter()) {
+            for (tag, stack_idx) in self.cache.items.iter().zip(self.cache.idx.iter()) {
-                assert_eq!(self.borrows[*stack_idx].tag, *tag);
+                assert_eq!(self.borrows[*stack_idx], *tag);
             }
         }
 
         for (idx, item) in self.borrows.iter().enumerate() {
-            if item.perm == Permission::Unique {
+            if item.perm() == Permission::Unique {
                 assert!(
                     self.unique_range.contains(&idx),
                     "{:?} {:?}",
@@ -128,7 +128,7 @@ pub(super) fn find_granting(
                     .rev() // search top-to-bottom
                     .find_map(|(idx, item)| {
                         // If the item fits and *might* be this wildcard, use it.
-                        if item.perm.grants(access) && exposed_tags.contains(&item.tag) {
+                        if item.perm().grants(access) && exposed_tags.contains(&item.tag()) {
                             Some(idx)
                         } else {
                             None
@@ -161,9 +161,9 @@ fn find_granting_tagged(&mut self, access: AccessKind, tag: SbTag) -> Option<usi
         // If we didn't find the tag in the cache, fall back to a linear search of the
         // whole stack, and add the tag to the cache.
         for (stack_idx, item) in self.borrows.iter().enumerate().rev() {
-            if tag == item.tag && item.perm.grants(access) {
+            if tag == item.tag() && item.perm().grants(access) {
                 #[cfg(feature = "stack-cache")]
-                self.cache.add(stack_idx, tag);
+                self.cache.add(stack_idx, *item);
                 return Some(stack_idx);
             }
         }
@@ -175,7 +175,7 @@ fn find_granting_cache(&mut self, access: AccessKind, tag: SbTag) -> Option<usiz
         // This looks like a common-sense optimization; we're going to do a linear search of the
         // cache or the borrow stack to scan the shorter of the two. This optimization is miniscule
         // and this check actually ensures we do not access an invalid cache.
-        // When a stack is created and when tags are removed from the top of the borrow stack, we
+        // When a stack is created and when items are removed from the top of the borrow stack, we
         // need some valid value to populate the cache. In both cases, we try to use the bottom
         // item. But when the stack is cleared in `set_unknown_bottom` there is nothing we could
         // place in the cache that is correct. But due to the way we populate the cache in
@@ -185,21 +185,23 @@ fn find_granting_cache(&mut self, access: AccessKind, tag: SbTag) -> Option<usiz
             return None;
         }
         // Search the cache for the tag we're looking up
-        let cache_idx = self.cache.tags.iter().position(|t| *t == tag)?;
+        let cache_idx = self.cache.items.iter().position(|t| t.tag() == tag)?;
         let stack_idx = self.cache.idx[cache_idx];
         // If we found the tag, look up its position in the stack to see if it grants
         // the required permission
-        if self.borrows[stack_idx].perm.grants(access) {
+        if self.cache.items[cache_idx].perm().grants(access) {
             // If it does, and it's not already in the most-recently-used position, re-insert it at
             // the most-recently-used position. This technically reduces the efficiency of the
             // cache by duplicating elements, but current benchmarks do not seem to benefit from
             // avoiding this duplication.
             // But if the tag is in position 1, avoiding the duplicating add is trivial.
+            // If it does, and it's not already in the most-recently-used position, move it there.
+            // Except if the tag is in position 1, this is equivalent to just a swap, so do that.
             if cache_idx == 1 {
-                self.cache.tags.swap(0, 1);
+                self.cache.items.swap(0, 1);
                 self.cache.idx.swap(0, 1);
             } else if cache_idx > 1 {
-                self.cache.add(stack_idx, tag);
+                self.cache.add(stack_idx, self.cache.items[cache_idx]);
             }
             Some(stack_idx)
         } else {
@@ -224,7 +226,7 @@ fn insert_cache(&mut self, new_idx: usize, new: Item) {
         if self.unique_range.end >= new_idx {
             self.unique_range.end += 1;
         }
-        if new.perm == Permission::Unique {
+        if new.perm() == Permission::Unique {
             // Make sure the possibly-unique range contains the new borrow
             self.unique_range.start = self.unique_range.start.min(new_idx);
             self.unique_range.end = self.unique_range.end.max(new_idx + 1);
@@ -233,7 +235,7 @@ fn insert_cache(&mut self, new_idx: usize, new: Item) {
         // The above insert changes the meaning of every index in the cache >= new_idx, so now
         // we need to find every one of those indexes and increment it.
         // But if the insert is at the end (equivalent to a push), we can skip this step because
-        // it didn't change the position of any other tags.
+        // it didn't change the position of any other items.
         if new_idx != self.borrows.len() - 1 {
             for idx in &mut self.cache.idx {
                 if *idx >= new_idx {
@@ -243,7 +245,7 @@ fn insert_cache(&mut self, new_idx: usize, new: Item) {
         }
 
         // This primes the cache for the next access, which is almost always the just-added tag.
-        self.cache.add(new_idx, new.tag);
+        self.cache.add(new_idx, new);
 
         #[cfg(feature = "expensive-debug-assertions")]
         self.verify_cache_consistency();
@@ -255,9 +257,9 @@ pub fn new(item: Item) -> Self {
             borrows: vec![item],
             unknown_bottom: None,
             #[cfg(feature = "stack-cache")]
-            cache: StackCache { idx: [0; CACHE_LEN], tags: [item.tag; CACHE_LEN] },
+            cache: StackCache { idx: [0; CACHE_LEN], items: [item; CACHE_LEN] },
             #[cfg(feature = "stack-cache")]
-            unique_range: if item.perm == Permission::Unique { 0..1 } else { 0..0 },
+            unique_range: if item.perm() == Permission::Unique { 0..1 } else { 0..0 },
         }
     }
 
@@ -298,10 +300,15 @@ pub fn disable_uniques_starting_at<V: FnMut(Item) -> crate::InterpResult<'tcx>>(
             let lower = unique_range.start.max(disable_start);
             let upper = (unique_range.end + 1).min(self.borrows.len());
             for item in &mut self.borrows[lower..upper] {
-                if item.perm == Permission::Unique {
+                if item.perm() == Permission::Unique {
                     log::trace!("access: disabling item {:?}", item);
                     visitor(*item)?;
-                    item.perm = Permission::Disabled;
+                    item.set_disabled();
+                    for t in &mut self.cache.items {
+                        if t.tag() == item.tag() {
+                            t.set_disabled();
+                        }
+                    }
                 }
             }
         }
@@ -341,7 +348,7 @@ pub fn pop_items_after<V: FnMut(Item) -> crate::InterpResult<'tcx>>(
             // also possible that the whole cache is still valid. So we call this method to repair what
             // aspects of the cache are now invalid, instead of resetting the whole thing to a trivially
             // valid default state.
-            let base_tag = self.borrows[0].tag;
+            let base_tag = self.borrows[0];
             let mut removed = 0;
             let mut cursor = 0;
             // Remove invalid entries from the cache by rotating them to the end of the cache, then
@@ -350,7 +357,7 @@ pub fn pop_items_after<V: FnMut(Item) -> crate::InterpResult<'tcx>>(
             for _ in 0..CACHE_LEN - 1 {
                 if self.cache.idx[cursor] >= start {
                     self.cache.idx[cursor..CACHE_LEN - removed].rotate_left(1);
-                    self.cache.tags[cursor..CACHE_LEN - removed].rotate_left(1);
+                    self.cache.items[cursor..CACHE_LEN - removed].rotate_left(1);
                     removed += 1;
                 } else {
                     cursor += 1;
@@ -358,7 +365,7 @@ pub fn pop_items_after<V: FnMut(Item) -> crate::InterpResult<'tcx>>(
             }
             for i in CACHE_LEN - removed - 1..CACHE_LEN {
                 self.cache.idx[i] = 0;
-                self.cache.tags[i] = base_tag;
+                self.cache.items[i] = base_tag;
             }
 
             if start < self.unique_range.start.saturating_sub(1) {

src/thread.rs

@@ -281,6 +281,10 @@ fn active_thread_stack_mut(&mut self) -> &mut Vec<Frame<'mir, 'tcx, Tag, FrameDa
         &mut self.threads[self.active_thread].stack
     }
 
+    pub fn all_stacks(&self) -> impl Iterator<Item = &[Frame<'mir, 'tcx, Tag, FrameData<'tcx>>]> {
+        self.threads.iter().map(|t| &t.stack[..])
+    }
+
     /// Create a new thread and returns its id.
     fn create_thread(&mut self) -> ThreadId {
         let new_thread_id = ThreadId::new(self.threads.len());

tests/fail/stacked_borrows/aliasing_mut1.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
   --> $DIR/aliasing_mut1.rs:LL:CC
    |
 LL | pub fn safe(_x: &mut i32, _y: &mut i32) {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/aliasing_mut2.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG> (call ID)]
+error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG>] (call ID)
   --> $DIR/aliasing_mut2.rs:LL:CC
    |
 LL | pub fn safe(_x: &i32, _y: &mut i32) {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG> (call ID)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/aliasing_mut4.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG> (call ID)]
+error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG>] (call ID)
   --> $DIR/aliasing_mut4.rs:LL:CC
    |
 LL | pub fn safe(_x: &i32, _y: &mut Cell<i32>) {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG> (call ID)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/deallocate_against_barrier1.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: deallocating while item is protected: [Unique for <TAG> (call ID)]
+error: Undefined Behavior: deallocating while item is protected: [Unique for <TAG>] (call ID)
   --> RUSTLIB/alloc/src/alloc.rs:LL:CC
    |
 LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ deallocating while item is protected: [Unique for <TAG> (call ID)]
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ deallocating while item is protected: [Unique for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/deallocate_against_barrier2.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: deallocating while item is protected: [SharedReadWrite for <TAG> (call ID)]
+error: Undefined Behavior: deallocating while item is protected: [SharedReadWrite for <TAG>] (call ID)
   --> RUSTLIB/alloc/src/alloc.rs:LL:CC
    |
 LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ deallocating while item is protected: [SharedReadWrite for <TAG> (call ID)]
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ deallocating while item is protected: [SharedReadWrite for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/illegal_write6.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
   --> $DIR/illegal_write6.rs:LL:CC
    |
 LL |     unsafe { *y = 2 };
-   |              ^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+   |              ^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/invalidate_against_barrier1.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
   --> $DIR/invalidate_against_barrier1.rs:LL:CC
    |
 LL |     let _val = unsafe { *x };
-   |                         ^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+   |                         ^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/invalidate_against_barrier2.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG> (call ID)]
+error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG>] (call ID)
   --> $DIR/invalidate_against_barrier2.rs:LL:CC
    |
 LL |     unsafe { *x = 0 };
-   |              ^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG> (call ID)]
+   |              ^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [SharedReadOnly for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

tests/fail/stacked_borrows/newtype_retagging.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+error: Undefined Behavior: not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
   --> RUSTLIB/alloc/src/boxed.rs:LL:CC
    |
 LL |         Box(unsafe { Unique::new_unchecked(raw) }, alloc)
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG> (call ID)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item is protected: [Unique for <TAG>] (call ID)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information