Merge pull request #229 from overlookmotel/faster-push-str

Improve performance of `String::push_str`

benches/benches.rs

@@ -89,6 +89,14 @@ fn format_realloc(bump: &bumpalo::Bump, n: usize) {
     criterion::black_box(s);
 }
 
+#[cfg(feature = "collections")]
+fn string_push_str(bump: &bumpalo::Bump, str: &str) {
+    let str = criterion::black_box(str);
+    let mut s = bumpalo::collections::string::String::with_capacity_in(str.len(), bump);
+    s.push_str(str);
+    criterion::black_box(s);
+}
+
 const ALLOCATIONS: usize = 10_000;
 
 fn bench_alloc(c: &mut Criterion) {
@@ -202,6 +210,21 @@ fn bench_format_realloc(c: &mut Criterion) {
     }
 }
 
+fn bench_string_push_str(c: &mut Criterion) {
+    let len: usize = 16 * 1024; // 16 KiB
+
+    let mut group = c.benchmark_group("alloc");
+    group.throughput(Throughput::Elements(len as u64));
+    group.bench_function("push_str", |b| {
+        let mut bump = bumpalo::Bump::with_capacity(len);
+        let str = "x".repeat(len);
+        b.iter(|| {
+            bump.reset();
+            string_push_str(&bump, &*str);
+        });
+    });
+}
+
 criterion_group!(
     benches,
     bench_alloc,
@@ -212,6 +235,7 @@ criterion_group!(
     bench_try_alloc_with,
     bench_try_alloc_try_with,
     bench_try_alloc_try_with_err,
-    bench_format_realloc
+    bench_format_realloc,
+    bench_string_push_str
 );
 criterion_main!(benches);

src/collections/string.rs

@@ -925,7 +925,31 @@ impl<'bump> String<'bump> {
     /// ```
     #[inline]
     pub fn push_str(&mut self, string: &str) {
-        self.vec.extend_from_slice(string.as_bytes())
+        // Reserve space in the Vec for the string to be added
+        let old_len = self.vec.len();
+        self.vec.reserve(string.len());
+
+        let new_len = old_len + string.len();
+        debug_assert!(new_len <= self.vec.capacity());
+
+        // Copy string into space just reserved
+        // SAFETY:
+        // * `src` is valid for reads of `string.len()` bytes by virtue of being an allocated `&str`.
+        // * `dst` is valid for writes of `string.len()` bytes as `self.vec.reserve(string.len())`
+        //   above guarantees that.
+        // * Alignment is not relevant as `u8` has no alignment requirements.
+        // * Source and destination ranges cannot overlap as we just reserved the destination
+        //   range from the bump.
+        unsafe {
+            let src = string.as_ptr();
+            let dst = self.vec.as_mut_ptr().add(old_len);
+            ptr::copy_nonoverlapping(src, dst, string.len());
+        }
+
+        // Update length of Vec to include string just pushed
+        // SAFETY: We reserved sufficent capacity for the string above.
+        // The elements at `old_len..new_len` were initialized by `copy_nonoverlapping` above.
+        unsafe { self.vec.set_len(new_len) };
     }
 
     /// Returns this `String`'s capacity, in bytes.

tests/all/string.rs

@@ -17,3 +17,20 @@ fn trailing_comma_in_format_macro() {
     let v = format![in &b, "{}{}", 1, 2, ];
     assert_eq!(v, "12");
 }
+
+#[test]
+fn push_str() {
+    let b = Bump::new();
+    let mut s = String::new_in(&b);
+    s.push_str("abc");
+    assert_eq!(s, "abc");
+    s.push_str("def");
+    assert_eq!(s, "abcdef");
+    s.push_str("");
+    assert_eq!(s, "abcdef");
+    s.push_str(&"x".repeat(4000));
+    assert_eq!(s.len(), 4006);
+    s.push_str("ghi");
+    assert_eq!(s.len(), 4009);
+    assert_eq!(&s[s.len() - 5..], "xxghi");
+}