From 6eead1f95cda85863b0476bb4129b71e94aefccf Mon Sep 17 00:00:00 2001
From: Joe Wilm <joe@jwilm.com>
Date: Sun, 20 May 2018 16:04:14 -0700
Subject: [PATCH] Specialize Storage::swap for Row<T>

Removes 4 movaps instructions from generated assembly.
---
 src/grid/mod.rs     |  2 +-
 src/grid/row.rs     |  1 -
 src/grid/storage.rs | 70 ++++++++++++++++++++++++++++++++++++---------
 src/lib.rs          |  1 +
 4 files changed, 59 insertions(+), 15 deletions(-)

diff --git a/src/grid/mod.rs b/src/grid/mod.rs
index 0998d640..252733f3 100644
--- a/src/grid/mod.rs
+++ b/src/grid/mod.rs
@@ -27,7 +27,7 @@ pub use self::row::Row;
 mod tests;
 
 mod storage;
-use self::storage::Storage;
+use self::storage::{Storage, Swap};
 
 /// Bidirection iterator
 pub trait BidirectionalIterator: Iterator {
diff --git a/src/grid/row.rs b/src/grid/row.rs
index a0805822..d23e9045 100644
--- a/src/grid/row.rs
+++ b/src/grid/row.rs
@@ -58,7 +58,6 @@ impl<T: Copy + Clone> Row<T> {
     }
 
     /// Resets contents to the contents of `other`
-    #[inline(never)]
     pub fn reset(&mut self, other: &T) {
         let occ = self.occ;
         for item in &mut self.inner[..occ] {
diff --git a/src/grid/storage.rs b/src/grid/storage.rs
index ea220159..a6d0d2a8 100644
--- a/src/grid/storage.rs
+++ b/src/grid/storage.rs
@@ -15,10 +15,67 @@ use std::ops::{Index, IndexMut};
 use std::slice;
 
 use index::Line;
+use grid::Row;
 
 /// Maximum number of invisible lines before buffer is resized
 const TRUNCATE_STEP: usize = 100;
 
+pub trait Swap {
+    fn swap(&mut self, _: usize, _: usize);
+}
+
+impl<T> Swap for Storage<T> {
+    /// Swap two lines in raw buffer
+    ///
+    /// # Panics
+    ///
+    /// `swap` will panic if either `a` or `b` are out-of-bounds of the
+    /// underlying storage.
+    default fn swap(&mut self, a: usize, b: usize) {
+        let a = self.compute_index(a);
+        let b = self.compute_index(b);
+
+        self.inner.swap(a, b);
+    }
+}
+
+impl<T> Swap for Storage<Row<T>> {
+    /// Custom swap implementation for Row<T>.
+    ///
+    /// Exploits the known size of Row<T> to produce a slightly more efficient
+    /// swap than going through slice::swap.
+    ///
+    /// The default implementation from swap generates 8 movups and 4 movaps
+    /// instructions. This implementation only uses 8 movups instructions.
+    fn swap(&mut self, a: usize, b: usize) {
+        use std::mem::{size_of, uninitialized};
+        use ::libc::memcpy;
+
+        debug_assert!(size_of::<Row<T>>() == 32);
+
+        let a = self.compute_index(a);
+        let b = self.compute_index(b);
+
+        unsafe {
+            // Cast to a u64 array of size 4 to pretend that the data is copy
+            let a_ptr = self.inner.as_mut_ptr().offset(a as isize) as *mut u64;
+            let b_ptr = self.inner.as_mut_ptr().offset(b as isize) as *mut u64;
+
+            // Swap space
+            let mut tmp: u64;
+
+            // Copy 1 qword at a time
+            //
+            // The optimizer unrolls this loop and vectorizes it.
+            for i in 0..4 {
+                tmp = *a_ptr.offset(i);
+                *a_ptr.offset(i) = *b_ptr.offset(i);
+                *b_ptr.offset(i) = tmp;
+            }
+        }
+    }
+}
+
 #[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct Storage<T> {
     inner: Vec<T>,
@@ -205,19 +262,6 @@ impl<T> Storage<T> {
         self.inner.swap(a, b);
     }
 
-    /// Swap two lines in raw buffer
-    ///
-    /// # Panics
-    ///
-    /// `swap` will panic if either `a` or `b` are out-of-bounds of the
-    /// underlying storage.
-    pub fn swap(&mut self, a: usize, b: usize) {
-        let a = self.compute_index(a);
-        let b = self.compute_index(b);
-
-        self.inner.swap(a, b);
-    }
-
     /// Iterator over *logical* entries in the storage
     ///
     /// This *does not* iterate over hidden entries.
diff --git a/src/lib.rs b/src/lib.rs
index a8e18451..45734eba 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,6 +13,7 @@
 // limitations under the License.
 //
 //! Alacritty - The GPU Enhanced Terminal
+#![feature(specialization)]
 #![cfg_attr(feature = "clippy", feature(plugin))]
 #![cfg_attr(feature = "clippy", plugin(clippy))]
 #![cfg_attr(feature = "clippy", deny(clippy))]