diff --git a/libs/@local/hashql/core/src/id/vec.rs b/libs/@local/hashql/core/src/id/vec.rs
--- a/libs/@local/hashql/core/src/id/vec.rs
+++ b/libs/@local/hashql/core/src/id/vec.rs
@@ -6,7 +6,7 @@
     fmt::{self, Debug},
     hash::{Hash, Hasher},
     marker::PhantomData,
-    ops::{Deref, DerefMut, RangeBounds},
+    ops::{Bound, Deref, DerefMut, RangeBounds},
     slice,
 };
 
@@ -510,8 +510,16 @@
     where
         T: Copy,
     {
-        let start = src.start_bound().copied().map(Id::as_usize);
-        let end = src.end_bound().copied().map(Id::as_usize);
+        let start = match src.start_bound() {
+            Bound::Included(&bound) => Bound::Included(bound.as_usize()),
+            Bound::Excluded(&bound) => Bound::Excluded(bound.as_usize()),
+            Bound::Unbounded => Bound::Unbounded,
+        };
+        let end = match src.end_bound() {
+            Bound::Included(&bound) => Bound::Included(bound.as_usize()),
+            Bound::Excluded(&bound) => Bound::Excluded(bound.as_usize()),
+            Bound::Unbounded => Bound::Unbounded,
+        };
 
         self.raw.copy_within((start, end), dst.as_usize());
     }
@@ -796,3 +804,23 @@
         Self::from_raw(Vec::from_iter_in(iter, alloc))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::IdVec;
+    use crate::id::{Id as _, newtype};
+
+    newtype!(#[id(crate = crate)] struct TestId(u32 is 0..=32));
+
+    #[test]
+    fn copy_within_preserves_inclusive_end_bound() {
+        let mut vec = IdVec::<TestId, i32>::from_raw(alloc::vec![10, 20, 30]);
+
+        vec.copy_within(
+            TestId::from_usize(1)..=TestId::from_usize(1),
+            TestId::from_usize(0),
+        );
+
+        assert_eq!(vec.as_slice().as_raw(), &[20, 20, 30]);
+    }
+}

diff --git a/libs/@local/hashql/mir/src/pass/execution/island/graph/mod.rs b/libs/@local/hashql/mir/src/pass/execution/island/graph/mod.rs
--- a/libs/@local/hashql/mir/src/pass/execution/island/graph/mod.rs
+++ b/libs/@local/hashql/mir/src/pass/execution/island/graph/mod.rs
@@ -20,7 +20,7 @@
 #[cfg(test)]
 pub(crate) mod tests;
 
-use alloc::alloc::Global;
+use alloc::{alloc::Global, collections::VecDeque};
 use core::{
     alloc::Allocator,
     ops::{Index, IndexMut},
@@ -29,11 +29,10 @@
 use hashql_core::{
     debug_panic,
     graph::{
-        DirectedGraph, EdgeId, LinkedGraph, NodeId, Predecessors, Successors, Traverse as _,
+        DirectedGraph, EdgeId, LinkedGraph, NodeId, Predecessors, Successors,
         algorithms::{Dominators, dominators},
         linked::Edge,
     },
-    heap::CollectIn as _,
     id::{
         HasId as _, Id as _,
         bit_vec::{BitMatrix, DenseBitSet},
@@ -243,13 +242,39 @@
     where
         S: Allocator + Clone,
     {
-        let mut topo: Vec<IslandId, _> = self
-            .inner
-            .depth_first_forest_post_order()
-            .map(|node| IslandId::new(node.as_u32()))
-            .collect_in(scratch.clone());
-        topo.reverse();
+        #[expect(clippy::cast_possible_truncation)]
+        let node_count = self.node_count();
+        let mut in_degree = IslandVec::from_elem_in(0_u32, node_count, scratch.clone());
+        for (island_id, _) in self.iter_nodes() {
+            in_degree[island_id] = self.predecessors(island_id).count() as u32;
+        }
 
+        let mut queue: VecDeque<IslandId, _> = VecDeque::new_in(scratch.clone());
+        for (island_id, _) in self.iter_nodes() {
+            if in_degree[island_id] == 0 {
+                queue.push_back(island_id);
+            }
+        }
+
+        let mut topo = Vec::with_capacity_in(node_count, scratch.clone());
+        while let Some(island_id) = queue.pop_front() {
+            topo.push(island_id);
+
+            for successor in self.successors(island_id) {
+                in_degree[successor] -= 1;
+
+                if in_degree[successor] == 0 {
+                    queue.push_back(successor);
+                }
+            }
+        }
+
+        assert_eq!(
+            topo.len(),
+            node_count,
+            "island requirement resolution requires acyclic control flow",
+        );
+
         let start = self.lookup[BasicBlockId::START];
 
         RequirementResolver::new(self, start, scratch).resolve(&topo);
@@ -379,7 +404,7 @@
     }
 
     fn resolve(mut self, topo: &[IslandId]) {
-        // Iterate in reverse for topological order
+        // Iterate in topological order so dominators are processed before dependents.
         for &island_id in topo {
             let island = &self.graph[island_id];
             let IslandKind::Exec(_) = &island.kind else {

diff --git a/libs/@local/hashql/mir/src/pass/execution/island/schedule/mod.rs b/libs/@local/hashql/mir/src/pass/execution/island/schedule/mod.rs
--- a/libs/@local/hashql/mir/src/pass/execution/island/schedule/mod.rs
+++ b/libs/@local/hashql/mir/src/pass/execution/island/schedule/mod.rs
@@ -122,6 +122,12 @@
             }
         }
 
+        assert_eq!(
+            entries.len(),
+            node_count,
+            "island schedule requires acyclic control flow",
+        );
+
         entries.sort_by_key(|entry| entry.level);
         IslandSchedule { entries }
     }