Split Vec::dedup_by into 2 cycles

library/alloc/benches/vec.rs

@@ -658,73 +658,161 @@ fn random_sorted_fill(mut seed: u32, buf: &mut [u32]) {
     buf.sort();
 }
 
-fn bench_vec_dedup_old(b: &mut Bencher, sz: usize) {
+// Measures performance of slice dedup impl.
+// This was used to justify separate implementation of dedup for Vec.
+// This algorithm was used for Vecs prior to Rust 1.52.
+fn bench_dedup_slice_truncate(b: &mut Bencher, sz: usize) {
     let mut template = vec![0u32; sz];
     b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
     random_sorted_fill(0x43, &mut template);
 
     let mut vec = template.clone();
     b.iter(|| {
+        let vec = black_box(&mut vec);
         let len = {
             let (dedup, _) = vec.partition_dedup();
             dedup.len()
         };
         vec.truncate(len);
 
         black_box(vec.first());
+        let vec = black_box(vec);
         vec.clear();
         vec.extend_from_slice(&template);
     });
 }
 
-fn bench_vec_dedup_new(b: &mut Bencher, sz: usize) {
+// Measures performance of Vec::dedup on random data.
+fn bench_vec_dedup_random(b: &mut Bencher, sz: usize) {
     let mut template = vec![0u32; sz];
     b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
     random_sorted_fill(0x43, &mut template);
 
     let mut vec = template.clone();
     b.iter(|| {
+        let vec = black_box(&mut vec);
         vec.dedup();
         black_box(vec.first());
+        let vec = black_box(vec);
+        vec.clear();
+        vec.extend_from_slice(&template);
+    });
+}
+
+// Measures performance of Vec::dedup when there is no items removed
+fn bench_vec_dedup_none(b: &mut Bencher, sz: usize) {
+    let mut template = vec![0u32; sz];
+    b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
+    template.chunks_exact_mut(2).for_each(|w| {
+        w[0] = black_box(0);
+        w[1] = black_box(5);
+    });
+
+    let mut vec = template.clone();
+    b.iter(|| {
+        let vec = black_box(&mut vec);
+        vec.dedup();
+        black_box(vec.first());
+        // Unlike other benches of `dedup`
+        // this doesn't reinitialize vec
+        // because we measure how efficient dedup is
+        // when no memory written
+    });
+}
+
+// Measures performance of Vec::dedup when there is all items removed
+fn bench_vec_dedup_all(b: &mut Bencher, sz: usize) {
+    let mut template = vec![0u32; sz];
+    b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
+    template.iter_mut().for_each(|w| {
+        *w = black_box(0);
+    });
+
+    let mut vec = template.clone();
+    b.iter(|| {
+        let vec = black_box(&mut vec);
+        vec.dedup();
+        black_box(vec.first());
+        let vec = black_box(vec);
         vec.clear();
         vec.extend_from_slice(&template);
     });
 }
 
 #[bench]
-fn bench_dedup_old_100(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 100);
+fn bench_dedup_slice_truncate_100(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 100);
 }
 #[bench]
-fn bench_dedup_new_100(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 100);
+fn bench_dedup_random_100(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 100);
 }
 
 #[bench]
-fn bench_dedup_old_1000(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 1000);
+fn bench_dedup_none_100(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 100);
 }
+
+#[bench]
+fn bench_dedup_all_100(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 100);
+}
+
+#[bench]
+fn bench_dedup_slice_truncate_1000(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 1000);
+}
+#[bench]
+fn bench_dedup_random_1000(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 1000);
+}
+
+#[bench]
+fn bench_dedup_none_1000(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 1000);
+}
+
 #[bench]
-fn bench_dedup_new_1000(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 1000);
+fn bench_dedup_all_1000(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 1000);
 }
 
 #[bench]
-fn bench_dedup_old_10000(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 10000);
+fn bench_dedup_slice_truncate_10000(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 10000);
 }
 #[bench]
-fn bench_dedup_new_10000(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 10000);
+fn bench_dedup_random_10000(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 10000);
 }
 
 #[bench]
-fn bench_dedup_old_100000(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 100000);
+fn bench_dedup_none_10000(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 10000);
 }
+
+#[bench]
+fn bench_dedup_all_10000(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 10000);
+}
+
+#[bench]
+fn bench_dedup_slice_truncate_100000(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 100000);
+}
+#[bench]
+fn bench_dedup_random_100000(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 100000);
+}
+
+#[bench]
+fn bench_dedup_none_100000(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 100000);
+}
+
 #[bench]
-fn bench_dedup_new_100000(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 100000);
+fn bench_dedup_all_100000(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 100000);
 }
 
 #[bench]

library/alloc/src/vec/mod.rs

@@ -1775,7 +1775,32 @@ impl<T, A: Allocator> Vec<T, A> {
             return;
         }
 
-        /* INVARIANT: vec.len() > read >= write > write-1 >= 0 */
+        // Check if we ever want to remove anything.
+        // This allows to use copy_non_overlapping in next cycle.
+        // And avoids any memory writes if we don't need to remove anything.
+        let mut first_duplicate_idx: usize = 1;
+        let start = self.as_mut_ptr();
+        while first_duplicate_idx != len {
+            let found_duplicate = unsafe {
+                // SAFETY: first_duplicate always in range [1..len)
+                // Note that we start iteration from 1 so we never overflow.
+                let prev = start.add(first_duplicate_idx.wrapping_sub(1));
+                let current = start.add(first_duplicate_idx);
+                // We explicitly say in docs that references are reversed.
+                same_bucket(&mut *current, &mut *prev)
+            };
+            if found_duplicate {
+                break;
+            }
+            first_duplicate_idx += 1;
+        }
+        // Don't need to remove anything.
+        // We cannot get bigger than len.
+        if first_duplicate_idx == len {
+            return;
+        }
+
+        /* INVARIANT: vec.len() > read > write > write-1 >= 0 */
         struct FillGapOnDrop<'a, T, A: core::alloc::Allocator> {
             /* Offset of the element we want to check if it is duplicate */
             read: usize,
@@ -1821,31 +1846,39 @@ impl<T, A: Allocator> Vec<T, A> {
             }
         }
 
-        let mut gap = FillGapOnDrop { read: 1, write: 1, vec: self };
-        let ptr = gap.vec.as_mut_ptr();
-
         /* Drop items while going through Vec, it should be more efficient than
          * doing slice partition_dedup + truncate */
 
+        // Construct gap first and then drop item to avoid memory corruption if `T::drop` panics.
+        let mut gap =
+            FillGapOnDrop { read: first_duplicate_idx + 1, write: first_duplicate_idx, vec: self };
+        unsafe {
+            // SAFETY: we checked that first_duplicate_idx in bounds before.
+            // If drop panics, `gap` would remove this item without drop.
+            ptr::drop_in_place(start.add(first_duplicate_idx));
+        }
+
         /* SAFETY: Because of the invariant, read_ptr, prev_ptr and write_ptr
          * are always in-bounds and read_ptr never aliases prev_ptr */
         unsafe {
             while gap.read < len {
-                let read_ptr = ptr.add(gap.read);
-                let prev_ptr = ptr.add(gap.write.wrapping_sub(1));
+                let read_ptr = start.add(gap.read);
+                let prev_ptr = start.add(gap.write.wrapping_sub(1));
 
-                if same_bucket(&mut *read_ptr, &mut *prev_ptr) {
+                // We explicitly say in docs that references are reversed.
+                let found_duplicate = same_bucket(&mut *read_ptr, &mut *prev_ptr);
+                if found_duplicate {
                     // Increase `gap.read` now since the drop may panic.
                     gap.read += 1;
                     /* We have found duplicate, drop it in-place */
                     ptr::drop_in_place(read_ptr);
                 } else {
-                    let write_ptr = ptr.add(gap.write);
+                    let write_ptr = start.add(gap.write);
 
-                    /* Because `read_ptr` can be equal to `write_ptr`, we either
-                     * have to use `copy` or conditional `copy_nonoverlapping`.
-                     * Looks like the first option is faster. */
-                    ptr::copy(read_ptr, write_ptr, 1);
+                    /* read_ptr cannot be equal to write_ptr because at this point
+                     * we guaranteed to skip at least one element (before loop starts).
+                     */
+                    ptr::copy_nonoverlapping(read_ptr, write_ptr, 1);
 
                     /* We have filled that place, so go further */
                     gap.write += 1;