vm: key data stack frame reuse on the exact frame size

push_frame and pop_frame compared 16-byte-aligned sizes, so two frames whose sizes differ by less than the alignment matched as a reuse. The caller reads that as "every slot was cleared by the previous frame", but the larger frame's tail slots were never touched by it. Assisted-by: Claude
youknowone · Aug 16, 2026 · Aug 13, 2026 · Aug 13, 2026 · Aug 13, 2026 · Aug 13, 2026
commit c7c1452a4f83d2e8528d6953ef392c07a456708c
diff --git a/crates/vm/src/datastack.rs b/crates/vm/src/datastack.rs
@@ -107,11 +107,13 @@ impl DataStack {
     /// a just-cleared frame block.
     #[inline(always)]
     pub fn push_frame(&mut self, size: usize) -> (*mut u8, bool) {
-        let aligned_size = (size + ALIGN - 1) & !(ALIGN - 1);
         let reusable_frame = self.reusable_frame.take();
         let ptr = self.push_inner(size);
-        let reused =
-            reusable_frame.is_some_and(|(base, old_size)| base == ptr && old_size == aligned_size);
+        // Exact sizes, not aligned ones: the caller reads "reused" as "every
+        // slot of this frame was cleared by the last one", and two frames whose
+        // sizes differ by less than ALIGN share an aligned size while the
+        // larger one's tail slots were never touched, let alone cleared.
+        let reused = reusable_frame.is_some_and(|(base, old_size)| base == ptr && old_size == size);
         (ptr, reused)
     }
 
@@ -177,8 +179,7 @@ impl DataStack {
     #[inline(always)]
     pub unsafe fn pop_frame(&mut self, base: *mut u8, size: usize) {
         unsafe { self.pop_inner(base) };
-        let aligned_size = (size + ALIGN - 1) & !(ALIGN - 1);
-        self.reusable_frame = Some((base, aligned_size));
+        self.reusable_frame = Some((base, size));
     }
 
     #[inline(always)]