Move LazySpan rows through one retained offset

core/src/util/dirty.rs

@@ -45,6 +45,26 @@ impl Dirty {
         self.words[word] |= 1 << (i % 64);
     }
 
+    pub fn contains(&self, i: usize) -> bool {
+        self.all
+            || self
+                .words
+                .get(i / 64)
+                .is_some_and(|word| word & (1 << (i % 64)) != 0)
+    }
+
+    /// Clear one entry that was restored to the value already on the GPU.
+    /// `all` has no per-entry representation and is used only when every
+    /// byte must be uploaded regardless of later writes, so it stays set.
+    pub fn unmark(&mut self, i: usize) {
+        if self.all {
+            return;
+        }
+        if let Some(word) = self.words.get_mut(i / 64) {
+            *word &= !(1 << (i % 64));
+        }
+    }
+
     /// Everything must be written: the buffer was reallocated (its
     /// contents are undefined), or the array was cleared.
     pub fn mark_all(&mut self) {
@@ -114,6 +134,17 @@ mod tests {
         assert_eq!(marked(&[3, 4, 5], 64, 0), vec![3..6]);
     }
 
+    #[test]
+    fn a_restored_entry_can_be_unmarked() {
+        let mut dirty = Dirty::default();
+        dirty.mark(3);
+        dirty.mark(5);
+        assert!(dirty.contains(3));
+        dirty.unmark(3);
+        assert!(!dirty.contains(3));
+        assert_eq!(dirty.ranges(8, 0), vec![5..6]);
+    }
+
     #[test]
     fn a_run_that_crosses_a_word_boundary_is_one_range() {
         assert_eq!(marked(&[62, 63, 64, 65], 128, 0), vec![62..66]);