Optimize Plug.Conn.Utils.validate_utf8!/3 (#1311)

Author: preciz

lib/plug/conn/utils.ex

@@ -286,25 +286,44 @@ defmodule Plug.Conn.Utils do
         do: stripped
   end
 
+  # 56-bit SWAR guard: all 7 bytes are ASCII (< 128)
+  defguardp ascii_swar?(w)
+            when Bitwise.band(w, 0x80808080808080) == 0
+
   @doc """
   Validates the given binary is valid UTF-8.
   """
   @spec validate_utf8!(binary, module, binary) :: :ok | no_return
   def validate_utf8!(binary, exception, context)
 
   def validate_utf8!(<<binary::binary>>, exception, context) do
-    do_validate_utf8!(binary, exception, context)
+    if byte_size(binary) < 12 do
+      do_validate_utf8_small!(binary, exception, context)
+    else
+      do_validate_utf8_swar!(binary, exception, context)
+    end
+  end
+
+  # SWAR loop
+  defp do_validate_utf8_swar!(<<w::56, b, rest::bits>>, exception, context)
+       when b <= 127 and ascii_swar?(w) do
+    do_validate_utf8_swar!(rest, exception, context)
+  end
+
+  defp do_validate_utf8_swar!(rest, exception, context) do
+    do_validate_utf8_small!(rest, exception, context)
   end
 
-  defp do_validate_utf8!(<<_::utf8, rest::bits>>, exception, context) do
-    do_validate_utf8!(rest, exception, context)
+  # Small loop (identical to original character loop)
+  defp do_validate_utf8_small!(<<_::utf8, rest::bits>>, exception, context) do
+    do_validate_utf8_small!(rest, exception, context)
   end
 
-  defp do_validate_utf8!(<<byte, _::bits>>, exception, context) do
+  defp do_validate_utf8_small!(<<byte, _::bits>>, exception, context) do
     raise exception, "invalid UTF-8 on #{context}, got byte #{byte}"
   end
 
-  defp do_validate_utf8!(<<>>, _exception, _context) do
+  defp do_validate_utf8_small!(<<>>, _exception, _context) do
     :ok
   end