[libc] Cache ownership of the shared buffer in the port

This patch adds another variable to cache cases where we know that we
own the buffer. This allows us to skip the atomic load on the inbox
because we already know its state. This is legal immediately after
opening a port, or when sending immediately after a recieve. This
caching nets a significant (~17%) speedup for the basic open, send,
recieve combination.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D150516

libc/src/__support/RPC/rpc.h

@@ -300,7 +300,7 @@ template <bool T> struct Port {
   LIBC_INLINE Port(Process<T> &process, uint64_t lane_mask, uint64_t index,
                    uint32_t out)
       : process(process), lane_mask(lane_mask), index(index), out(out),
-        receive(false) {}
+        receive(false), owns_buffer(true) {}
   LIBC_INLINE ~Port() = default;
 
 private:
@@ -329,9 +329,9 @@ template <bool T> struct Port {
   }
 
   LIBC_INLINE void close() {
-    // If the server last did a receive it needs to exchange ownership before
-    // closing the port.
-    if (receive && T)
+    // The server is passive, if it own the buffer when it closes we need to
+    // give ownership back to the client.
+    if (owns_buffer && T)
       out = process.invert_outbox(index, out);
     process.unlock(lane_mask, index);
   }
@@ -342,6 +342,7 @@ template <bool T> struct Port {
   uint64_t index;
   uint32_t out;
   bool receive;
+  bool owns_buffer;
 };
 
 /// The RPC client used to make requests to the server.
@@ -370,7 +371,7 @@ struct Server : public Process<true> {
 
 /// Applies \p fill to the shared buffer and initiates a send operation.
 template <bool T> template <typename F> LIBC_INLINE void Port<T>::send(F fill) {
-  uint32_t in = process.load_inbox(index);
+  uint32_t in = owns_buffer ? out ^ T : process.load_inbox(index);
 
   // We need to wait until we own the buffer before sending.
   while (Process<T>::buffer_unavailable(in, out)) {
@@ -382,17 +383,20 @@ template <bool T> template <typename F> LIBC_INLINE void Port<T>::send(F fill) {
   process.invoke_rpc(fill, process.get_packet(index));
   atomic_thread_fence(cpp::MemoryOrder::RELEASE);
   out = process.invert_outbox(index, out);
+  owns_buffer = false;
   receive = false;
 }
 
 /// Applies \p use to the shared buffer and acknowledges the send.
 template <bool T> template <typename U> LIBC_INLINE void Port<T>::recv(U use) {
   // We only exchange ownership of the buffer during a receive if we are waiting
   // for a previous receive to finish.
-  if (receive)
+  if (receive) {
     out = process.invert_outbox(index, out);
+    owns_buffer = false;
+  }
 
-  uint32_t in = process.load_inbox(index);
+  uint32_t in = owns_buffer ? out ^ T : process.load_inbox(index);
 
   // We need to wait until we own the buffer before receiving.
   while (Process<T>::buffer_unavailable(in, out)) {
@@ -404,6 +408,7 @@ template <bool T> template <typename U> LIBC_INLINE void Port<T>::recv(U use) {
   // Apply the \p use function to read the memory out of the buffer.
   process.invoke_rpc(use, process.get_packet(index));
   receive = true;
+  owns_buffer = true;
 }
 
 /// Combines a send and receive into a single function.