Triforce/SerialDevice: Add PassThroughTxBytes function and some minor cleanups.

Source/Core/Core/HW/Triforce/SerialDevice.cpp

@@ -16,9 +16,7 @@ void SerialDevice::WriteRxBytes(std::span<const u8> bytes)
 #if defined(__cpp_lib_containers_ranges)
   m_rx_buffer.append_range(bytes);
 #else
-  const auto prev_size = m_rx_buffer.size();
+  m_rx_buffer.insert(m_rx_buffer.end(), bytes.begin(), bytes.end());
-  m_rx_buffer.resize(prev_size + bytes.size());
-  std::ranges::copy(bytes, m_rx_buffer.begin() + prev_size);
 #endif
 }
 
@@ -42,12 +40,34 @@ void SerialDevice::WriteTxBytes(std::span<const u8> bytes)
 #if defined(__cpp_lib_containers_ranges)
   m_tx_buffer.append_range(bytes);
 #else
-  const auto prev_size = m_tx_buffer.size();
+  m_tx_buffer.insert(m_tx_buffer.end(), bytes.begin(), bytes.end());
-  m_tx_buffer.resize(prev_size + bytes.size());
-  std::ranges::copy(bytes, m_tx_buffer.begin() + prev_size);
 #endif
 }
 
+void SerialDevice::PassThroughTxBytes(SerialDevice& other)
+{
+  // The destination buffer will often be empty or near-empty.
+  if (m_tx_buffer.size() < other.m_tx_buffer.size()) [[likely]]
+  {
+#if defined(__cpp_lib_containers_ranges)
+    other.m_tx_buffer.prepend_range(m_tx_buffer);
+#else
+    other.m_tx_buffer.insert(other.m_tx_buffer.begin(), m_tx_buffer.begin(), m_tx_buffer.end());
+#endif
+    m_tx_buffer.swap(other.m_tx_buffer);
+  }
+  else
+  {
+#if defined(__cpp_lib_containers_ranges)
+    m_tx_buffer.append_range(other.m_tx_buffer);
+#else
+    m_tx_buffer.insert(m_tx_buffer.end(), other.m_tx_buffer.begin(), other.m_tx_buffer.end());
+#endif
+  }
+
+  other.m_tx_buffer.clear();
+}
+
 void SerialDevice::DoState(PointerWrap& p)
 {
   p.Do(m_rx_buffer);

Source/Core/Core/HW/Triforce/SerialDevice.h

@@ -45,6 +45,9 @@ protected:
   void WriteTxByte(u8 byte) { m_tx_buffer.emplace_back(byte); }
   void WriteTxBytes(std::span<const u8> bytes);
 
+  // Transfer the entirety of other's tx buffer to this tx buffer.
+  void PassThroughTxBytes(SerialDevice& other);
+
 private:
   // The stream of bytes from the baseboard to the device.
   // FYI: Current device implementations tend to empty the entire buffer in one go,