Miri subtree update

src/tools/miri/bench-cargo-miri/slice-chunked/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "slice-chunked"
+version = "0.1.0"

src/tools/miri/bench-cargo-miri/slice-chunked/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "slice-chunked"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/tools/miri/bench-cargo-miri/slice-chunked/src/main.rs

@@ -0,0 +1,26 @@
+//! This is a small example using slice::chunks, which creates a very large Tree Borrows tree.
+//! Thanks to ##3837, the GC now compacts the tree, so this test can be run in a reasonable time again.
+//! The actual code is adapted from tiny_skia, see https://github.com/RazrFalcon/tiny-skia/blob/master/src/pixmap.rs#L121
+//! To make this benchmark demonstrate the effectiveness, run with MIRIFLAGS="-Zmiri-tree-borrows -Zmiri-provenance-gc=100"
+
+const N: usize = 1000;
+
+fn input_vec() -> Vec<u8> {
+    vec![0; N]
+}
+
+fn main() {
+    let data_len = 2 * N;
+    let mut rgba_data = Vec::with_capacity(data_len);
+    let img_data = input_vec();
+    for slice in img_data.chunks(2) {
+        let gray = slice[0];
+        let alpha = slice[1];
+        rgba_data.push(gray);
+        rgba_data.push(gray);
+        rgba_data.push(gray);
+        rgba_data.push(alpha);
+    }
+
+    assert_eq!(rgba_data.len(), data_len);
+}

src/tools/miri/rust-version

@@ -1 +1 @@
-fdf61d499c8a8421ecf98e7924bb87caf43a9938
+0d634185dfddefe09047881175f35c65d68dcff1

src/tools/miri/src/borrow_tracker/mod.rs

@@ -71,6 +71,12 @@ pub struct FrameState {
 
 impl VisitProvenance for FrameState {
     fn visit_provenance(&self, visit: &mut VisitWith<'_>) {
+        // Visit all protected tags. At least in Tree Borrows,
+        // protected tags can not be GC'd because they still have
+        // an access coming when the protector ends. Additionally,
+        // the tree compacting mechanism of TB's GC relies on the fact
+        // that all protected tags are marked as live for correctness,
+        // so we _have_ to visit them here.
         for (id, tag) in &self.protected_tags {
             visit(Some(*id), Some(*tag));
         }

src/tools/miri/src/borrow_tracker/tree_borrows/perms.rs

@@ -130,7 +130,7 @@ mod transition {
             Active =>
                 if protected {
                     // We wrote, someone else reads -- that's bad.
-                    // (If this is initialized, this move-to-protected will mean insta-UB.)
+                    // (Since Active is always initialized, this move-to-protected will mean insta-UB.)
                     Disabled
                 } else {
                     // We don't want to disable here to allow read-read reordering: it is crucial
@@ -267,6 +267,44 @@ impl Permission {
         transition::perform_access(kind, rel_pos, old_state, protected)
             .map(|new_state| PermTransition { from: old_state, to: new_state })
     }
+
+    /// During a provenance GC, we want to compact the tree.
+    /// For this, we want to merge nodes upwards if they have a singleton parent.
+    /// But we need to be careful: If the parent is Frozen, and the child is Reserved,
+    /// we can not do such a merge. In general, such a merge is possible if the parent
+    /// allows similar accesses, and in particular if the parent never causes UB on its
+    /// own. This is enforced by a test, namely `tree_compacting_is_sound`. See that
+    /// test for more information.
+    /// This method is only sound if the parent is not protected. We never attempt to
+    /// remove protected parents.
+    pub fn can_be_replaced_by_child(self, child: Self) -> bool {
+        match (self.inner, child.inner) {
+            // ReservedIM can be replaced by anything, as it allows all
+            // transitions.
+            (ReservedIM, _) => true,
+            // Reserved (as parent, where conflictedness does not matter)
+            // can be replaced by all but ReservedIM,
+            // since ReservedIM alone would survive foreign writes
+            (ReservedFrz { .. }, ReservedIM) => false,
+            (ReservedFrz { .. }, _) => true,
+            // Active can not be replaced by something surviving
+            // foreign reads and then remaining writable.
+            (Active, ReservedIM) => false,
+            (Active, ReservedFrz { .. }) => false,
+            // Replacing a state by itself is always okay, even if the child state is protected.
+            (Active, Active) => true,
+            // Active can be replaced by Frozen, since it is not protected.
+            (Active, Frozen) => true,
+            (Active, Disabled) => true,
+            // Frozen can only be replaced by Disabled (and itself).
+            (Frozen, Frozen) => true,
+            (Frozen, Disabled) => true,
+            (Frozen, _) => false,
+            // Disabled can not be replaced by anything else.
+            (Disabled, Disabled) => true,
+            (Disabled, _) => false,
+        }
+    }
 }
 
 impl PermTransition {