[wip] Add JIT optimization tool: lib.specialize(fn)

src/README.md

@@ -909,6 +909,19 @@ lib.parse({foo=42, bar=43}, {foo={required=true}, bar={}, baz={default=44}})
   => {foo=42, bar=43, baz=44}
 ```
 
+— Function **lib.specialize** *function*
+
+Returns a clone of the given function with a shallow copy of its
+function definition. The clone function has a separate bytecode
+definition and any JIT traces that start in the clone will be
+aggressively specialized by LuaJIT for the environment of the clone.
+
+This can lead to especially efficient machine code when:
+- The function being specialized contains a loop (`for` or `while` or `repeat`) directly in its source code (not in a subroutine because the bytecode cloning is shallow).
+- The function will benefit from being compiled separately from other uses, for example because the way the clone will be called is expected to lead to a peculiar flow of control.
+- The function refers to values in its closure environment, which the JIT will treat more like constants than variables.
+
+See background information at [LuaJIT/LuaJIT#208](https://github.com/LuaJIT/LuaJIT/issues/208).
 
 ## Main
 

src/core/app.lua

@@ -196,6 +196,7 @@ function apply_config_actions (actions, conf)
          app.shm.dtime = {counter, C.get_unix_time()}
          app.shm = shm.create_frame("apps/"..name, app.shm)
       end
+      specialize(app)
    end
    function ops.restart (name)
       ops.stop(name)
@@ -209,6 +210,7 @@ function apply_config_actions (actions, conf)
          new_app_table[name] = app
          table.insert(new_app_array, app)
          app_name_to_index[name] = #new_app_array
+         specialize(app)
       else
          ops.restart(name)
       end
@@ -252,6 +254,13 @@ function apply_config_actions (actions, conf)
    end
 end
 
+-- Specialize an app so that its pull and push methods can be JITed
+-- into instance-specific machine code.
+function specialize (app)
+   if app.pull then app.pull = lib.specialize(app.pull) end
+   if app.push then app.push = lib.specialize(app.push) end
+end
+
 -- Call this to "run snabb switch".
 function main (options)
    options = options or {}

src/core/lib.lua

@@ -738,6 +738,24 @@ function parse (arg, config)
    return ret
 end
 
+-- Return a clone of the given function with separate bytecode.
+-- (Code taken from http://leafo.net/guides/function-cloning-in-lua.html)
+function specialize (fn)
+  local dumped = string.dump(fn)
+  local cloned = loadstring(dumped)
+  local i = 1
+  while true do
+    local name = debug.getupvalue(fn, i)
+    if not name then
+      break
+    end
+    debug.upvaluejoin(cloned, i, fn, i)
+    i = i + 1
+  end
+  return cloned
+end
+
+
 function selftest ()
    print("selftest: lib")
    print("Testing equal")