RE: sleep() patch for Lua 4.0

["Eric Ries" <eries@...>]

This is terrific! Thanks for making this patch and posting it. 

Eric

> -----Original Message-----
> From: owner-lua-l@tecgraf.puc-rio.br
> [mailto:owner-lua-l@tecgraf.puc-rio.br]On Behalf Of Thatcher Ulrich
> Sent: Monday, December 10, 2001 8:27 AM
> To: Multiple recipients of list
> Subject: ANN: sleep() patch for Lua 4.0
> 
> 
> I've created a patch for Lua 4.0 that makes calls from Lua to Lua
> non-recursive (i.e. "stackless").  This allows the implementation of a
> "sleep()" call, which exits from the host program's lua_do* call with
> a LUA_SLEEP code, and leaves the lua_State in a condition where the
> script can be resumed later via a call to a new API function,
> lua_resume(lua_State*).
> 
> The patch is on the "Power Patches" page at lua-users.org :
> 
> http://lua-users.org/wiki/LuaPowerPatches
> 
> This is the core functionality that Bret Mogilefsky has described in
> the "sleep()/yield() for Lua" thread, and has been much bandied about
> in the archives.
> 
> As part of the VM changes, I also made it so OP_TAILCALL is properly
> non-recursive.  This lets you implement long iteration-like
> computations using recursive syntax (provided the compiler emits an
> OP_TAILCALL for the tail call -- I'm not totally sure what the right
> conditions for that are).  See the test program below for a contrived
> example.
> 
> The patched lua correctly executes all the test scripts in the 4.0
> distribution, but it's very possible there's a bug somewhere,
> especially with error handling and such.
> 
> Also, these changes need to be ported to 4.1, to take advantage of Lua
> threads.  I started with 4.0 in order to have a stable platform to
> work in.
> 
> The biggest known problem with this patch is that sleep() will not
> actually succeed if it's called from a tag method, or from a recursive
> call to lua_do* (i.e. script calls a C function, which then calls
> lua_do*).  Instead you get an error message and the script continues
> execution.  So the semantics of sleep() depend somewhat on the
> internal workings of the interpreter.  For practical use in a game
> scripting language, this shouldn't be a problem.
> 
> Feedback welcome!
> 
> -- 
> Thatcher Ulrich <tu@tulrich.com>
> http://tulrich.com
> 
> 
> 
> ------ test/sleep.lua -------
> -- test the sleep patch.  The sleep patch adds a sleep() function,
> -- which suspends script execution in a restartable state (via the API
> -- function lua_resume(L)).  The sleep patch also implements true tail
> -- recursion for the OP_TAILCALL opcode.
> 
> 
> function donothing_tailrec(x)
> 	return x
> end
> 
> function donothing_rec1(x)
> 	return donothing_tailrec(x)
> end
> 
> function donothing_rec2(x)
> 	for i = 1,1000 do
> 		donothing_rec1(x)
> 	end
> 	return donothing_rec1(x)
> end
> 
> function donothing_rec3(x)
> 	for i = 1,1000 do
> 		donothing_rec2(x)
> 
> 		-- exit script execution, returning LUA_SLEEP to the caller.
> 		-- The caller can resume the script via lua_resume(L).
> 		sleep()
> 	end
> 	return donothing_rec2(x)
> end
> 
> c = clock()
> r = donothing_rec3(10)
> t = clock() - c
> print("time = " .. t .. ", result = " .. r);
> 
> 
> count = 0
> 
> function increment_tailrec(ct)
> -- stupid recursive function to increment 'count'
> 	if ct <= 0 then
> 		return 0
> 	else
> 		count = count + 1
> 		return increment_tailrec(ct-1)	-- for some reason, 
> we must return a value for OP_TAILCALL to be generated.
> 	end
> end
> 
> 
> function test(x)
> 	count = 0
> 	increment_tailrec(x)
> 	if count == x then
> 		print("success --> count = " .. x);
> 	else
> 		print("failure --> count = " .. count .. " but x = " .. x);
> 	end
> end
> 
> 
> -- The sleep patch also implements proper tail recursion for the
> -- OP_TAILCALL opcode.  In unpatched Lua 4.0, one of the following
> -- calls will probably cause a stack overflow error.
> test(1)
> test(10)
> test(100)
> test(1000)
> test(10000)
> test(100000)
> test(1000000)
>