Use ptr increment instead of add (#2337)

This seems to make a large different to the throughput of extend

Author: joseph-isaacs

Cargo.toml

@@ -43,7 +43,7 @@ include = [
     "README.md",
 ]
 edition = "2021"
-rust-version = "1.82"
+rust-version = "1.84"
 readme = "README.md"
 categories = ["database-implementations", "data-structures", "compression"]
 

vortex-buffer/src/buffer_mut.rs

@@ -423,19 +423,25 @@ impl<T> Extend<T> for BufferMut<T> {
 
         let remaining = self.capacity() - self.len();
 
-        let dst: *mut T = self.bytes.spare_capacity_mut().as_mut_ptr().cast();
-        let mut consumed = 0;
-        while consumed < remaining {
+        let begin: *const T = self.bytes.spare_capacity_mut().as_mut_ptr().cast();
+        let mut dst: *mut T = begin.cast_mut();
+        let buffer_end: *const T = unsafe { dst.add(remaining) };
+        while dst.addr() < buffer_end.addr() {
             if let Some(item) = iterator.next() {
-                // SAFETY: We know we have enough capacity to write the item.
-                unsafe { dst.add(consumed).write(item) };
-                consumed += 1;
+                unsafe {
+                    // SAFETY: We know we have enough capacity to write the item.
+                    dst.write(item);
+                    // Note. we used to have dst.add(iteration).write(item), here.
+                    // however this was much slower than just incrementing dst.
+                    dst = dst.add(1);
+                }
             } else {
                 break;
             }
         }
 
-        self.length += consumed;
+        // TODO(joe): replace with ptr_sub when stable
+        self.length += unsafe { dst.byte_offset_from(begin) as usize / size_of::<T>() };
         unsafe { self.bytes.set_len(self.length * size_of::<T>()) };
 
         iterator.for_each(|item| self.push(item));