From 8c2080886fa46434e097c503d3d1ee9309eadc7d Mon Sep 17 00:00:00 2001
From: Mark Rousskov <mark.simulacrum@gmail.com>
Date: Sat, 15 May 2021 21:39:45 -0400
Subject: [PATCH] Write primitive types via array buffers

This allows a more efficient implementation (avoiding a fallback to memmove,
which is not optimal for short writes).

This saves 0.29% on diesel.
---
 library/proc_macro/src/bridge/buffer.rs | 17 ++++++++++++++++-
 library/proc_macro/src/bridge/rpc.rs    |  2 +-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/library/proc_macro/src/bridge/buffer.rs b/library/proc_macro/src/bridge/buffer.rs
index a2030b9b8bf..717201aef10 100644
--- a/library/proc_macro/src/bridge/buffer.rs
+++ b/library/proc_macro/src/bridge/buffer.rs
@@ -78,8 +78,23 @@ impl<T: Copy> Buffer<T> {
         mem::take(self)
     }
 
+    // We have the array method separate from extending from a slice. This is
+    // because in the case of small arrays, codegen can be more efficient
+    // (avoiding a memmove call). With extend_from_slice, LLVM at least
+    // currently is not able to make that optimization.
+    pub(super) fn extend_from_array<const N: usize>(&mut self, xs: &[T; N]) {
+        if xs.len() > (self.capacity - self.len) {
+            let b = self.take();
+            *self = (b.reserve)(b, xs.len());
+        }
+        unsafe {
+            xs.as_ptr().copy_to_nonoverlapping(self.data.add(self.len), xs.len());
+            self.len += xs.len();
+        }
+    }
+
     pub(super) fn extend_from_slice(&mut self, xs: &[T]) {
-        if xs.len() > self.capacity.wrapping_sub(self.len) {
+        if xs.len() > (self.capacity - self.len) {
             let b = self.take();
             *self = (b.reserve)(b, xs.len());
         }
diff --git a/library/proc_macro/src/bridge/rpc.rs b/library/proc_macro/src/bridge/rpc.rs
index ee9a2cf9a97..588e6ded0f4 100644
--- a/library/proc_macro/src/bridge/rpc.rs
+++ b/library/proc_macro/src/bridge/rpc.rs
@@ -27,7 +27,7 @@ macro_rules! rpc_encode_decode {
     (le $ty:ty) => {
         impl<S> Encode<S> for $ty {
             fn encode(self, w: &mut Writer, _: &mut S) {
-                w.write_all(&self.to_le_bytes()).unwrap();
+                w.extend_from_array(&self.to_le_bytes());
             }
         }