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Product & Tech
Introduction
The Roblox engine is written in a combination of C++ and Lua, with the code that performs computationally intensive operations written in optimized C++, while game logic and scripts are written in Lua, for ease of development. For this model to be effective, it requires that the transitions between Lua and C++ be as fast as possible, as any time spent in this no man’s land is essentially just wasted milliseconds.
Over the past couple of months, we’ve been rolling out various improvements to this part of the system. One part specifically—the actual invocation of C++ methods from Lua—was particularly interesting, as it led to considerable speed improvements and required digging around in the guts of Lua to understand how things worked under the hood.
We ended up modifying the Lua VM itself, but before we get to that, we need to lay some groundwork.
Compilers, VM, and bytecode
When Lua source code is compiled, it’s compiled into Lua bytecode, which the Lua VM then runs. Lua bytecode has around 35 instructions in total, for things like reading/writing tables, calling functions, performing binary operations, jumps and conditionals, and so on. The Lua VM is register-based, as opposed to being stack-based like many other VMs, so part of what the compiler does when it generates bytecode is determine which registers each instruction should use.
Each instruction has the form “OP_CODE A B,” or “OP_CODE A B C,” where “OP_CODE” is the opcode (for example, CALL for calling a function) and A/B/C are the opcode arguments. The arguments (or registers) aren’t actual values. Instead, they are indices that point into one of two tables: the constant table (written Kst(.)) or the register table (written R(.)).
Interlude
For a detailed description of Lua bytecode, see “A No-Frills introduction to Lua 5.1 VM Instructions.” It’s a lot more exciting than it sounds; I promise!
To give you a feel for what Lua bytecode looks like, we’re going to go over some simple programs first and then progress to some more relevant examples.
Interlude
Using the Chunkspy utility, we can disassemble Lua bytecode into Lua assembly and get a listing of the code, as well as the constant table, so we can essentially see what bytecode gets generated for any given Lua source code.
Basic bytecode examples
A simple program like “x = 10” compiles into:
The first two lines show the constant table (with the string value “x” in slot 0 and the integer value 10 in slot 1), and the following two lines are the disassembled opcodes.
[Line 1] Looking up the LOADK opcode in “No Frills,” we see that it has the form “LOADK A Bx — R(A) := Kst(Bx).” So, LOADK has two arguments (registers A and B) and its operation is to assign the value found in the constant table at the slot given by the second register, Kst(Bx), to the register table in the slot given by the first argument, R(A). “Bx” just means that because the opcode only has two arguments, the B register is extended and assigned more bits.
[Line 2] SETGLOBAL has the form “SETGLOBAL A Bx — Gbl[Kst(Bx)] := R(A).” It assigns a value to the global table using the key given by the constant table at the slot of the second argument. Because the second argument is 0 and the value of the constant table at 0 is “x,” it writes something to the global table using the key “x.” Whatever is in the register table at the slot given by the first argument is what’s being written, which the previous instruction loaded with the value 10.
Let’s look at a slightly more complicated example, “x = 10; y = x.” I’ll leave the manual execution of the code as an exercise for the reader. ?
Bytecode for function calls
Let’s look at the code generated for “foo(10):” Samsung frp unlock tool pro gsm jony 2018.
To execute function calls, the function must be loaded into the first register and arguments into subsequent ones. The semantics for “CALL A B C” are such that A holds the function, B is the number of arguments (actually, it’s the number of arguments +1, due to the way “…” is implemented), and C is the number of return values (again, it’s the number of return values +1, to handle multiple return values).
Molho shimeji light. We’re familiar with the first two lines; they load a value into register table slot 0 and the value 10 into register table slot 1. The third line is what performs the function call, using the value in register A (register table slot 0, which was loaded with “foo”), with B specifying the number of arguments, and C the number of return values (remember, both B and C should have 1 added to their values). Before the function can be called, the VM also verifies that the value in R(A) is in fact callable.
Interlude — Metafunctions, metatables, and userdata
Lua has a mechanism that allows users to extend the functionality of tables by associating a metatable with an existing table. The metatable contains fallback methods that are invoked if a certain method or operation can’t be performed on the main table (see https://www.lua.org/pil/13.html for a thorough description).
For our purposes, the most relevant entries in the metatable are the “__index” and “__call” fields. __index is used when looking up an element in a table, so the code “local x = my_table[10]” would first call the __index method on my_table. If that failed, it would instead try to call __index on my_table’s metatable. __call is similarly used when you try to treat something as a function and call it “local x = foo(42),” for example
In order for Lua and C++ to interoperate, they need some way to be able to share functions and data. Lua facilitates this by providing a data type called UserData. UserData objects can be created in C++ land, and because they are native Lua data types, they can be adorned with metatables that allows Lua code to interact with them as if they were just regular Lua objects.
Member function calls
Okay, back to looking at some bytecode! This next example is a bit more interesting because it shows what happens when you have code like “foo:bar(10),” which is calling the bar method on the instance foo (an instance of the class Foo).
The new thing here is the self instruction [line 2], which we haven’t seen before. Self has the syntax “SELF A B C — R(A) := R(B)[RK(C)]; R(A+1) := R(B),” so let’s break this down. In the register table at slot R(A), it will place the result of looking up the table in register slot R(B) using the key in slot RK(C). It will also copy whatever was in slot R(B) into slot R(A+1), but more on this later. You might notice that the value of the C register is 257. This is valid because Lua is using RK(C) to look up the value, and RK will either use the register table or the constant table, depending on the value of 9:th bit. If it’s a 1, which it is in this case, then the constant table is used; otherwise, the lookup goes to the register table (after masking out the highest bit).
Line 3 places 10 in slot 2, and finally line 4 will execute the function call.
The SELF instruction serves two purposes. First, it looks for the “bar” method on the Foo class, and places it in R(A). Secondly, because foo is an instance method and we need the instance of the class that we’re invoking the method on when doing the call, it places this instance in R(A+1). If you’re familiar with classes in Python, then you might recognize the concept: methods are usually written as “def my_method(self, arg1, arg2.),” where self is the class instance.
We’ll need to dig into this a little deeper and take a look at what happens when the foo instance is a C++ object, represented in Lua as a UserData object.
The SELF call can be seen as a table lookup, i.e. Foo[“bar”] (capital Foo represents the class Foo, as opposed to foo, the instance), and we know that lookups will use the __index method. When the foo instance was created in C++ land, a metatable was associated with the instance, and the metatable had its __index field set to a piece of C++ code that will get called when __index is invoked.
Interlude — lua_State
When C/C++ gets called from Lua, the only data that gets passed is a lua_State object. This object contains everything related to the currently running Lua thread. The most important information in the state object is the Lua stack, which contains the function arguments (accessed via the lua_tointeger/tostring etc family of functions) and is also used to return values back to Lua.
In pseudo-C++, our __index function looks something like this:
Lots of internals are glossed over, but here’s the gist of it. Given that the UserData object passed as the first argument on the Lua stack, we’re able to find a descriptor that describes the actual C++ class, and via the descriptor we can see if this class has a method with the given name. If it does, a function pointer to a method invoker is pushed on the Lua stack, and we return success.
After this call, the Lua VM will place the rest of the arguments in the register table, and then call the function we returned from the metaIndex method, which will again call into C++, and land in the invoker function:
The methodInvoker also uses the ClassDescriptor, but this time it’s able to invoke the member function, and pop the correct arguments from the stack.
The home stretch!
Now that we can clearly see the two round trips from Lua to C++, we can try to figure out how to optimize this.
Our end goal is to do a single function call from Lua to C++ and have all the pieces we need on the Lua stack to be able to do method lookup and invocation at once. The problem seems to be that we have one register too few. When we call our combined lookup/invoker function, we want the Lua stack to look like [self, method name, arg1, arg2, …], but looking at SELF, we see that it uses its first slot for the result of looking up the method function and the second slot for storing the instance.
A key realization came when we looked at the way the __call metamethod worked. If an object has the __call metamethod, then before the _call function gets invoked, the object itself is pushed on the stack and all arguments are shifted up. By piggy-backing on this functionality, there was a way of getting “self” on the stack without having to explicitly store it in a register.
The second part involved getting the method name on the stack as well. For this, we had to be sneaky and alter the workings of the SELF opcode.
Remember that in the default case, SELF would try to find the member function and store this in R(A) together with the instance in R(A+1). We ended up skipping the lookup altogether and stored the actual object in R(A) and the method name in R(A+1).
If we now made sure that the object in R(A) had a __call metamethod, then we’d also end up pushing self on the stack. So, we’d have a stack that looked like [self, method name, args…] and did just a single call into C++. Perfect! Well, almost. ?
Before considering this done, we wanted to put some final polish on it. We didn’t want to overload the semantics of the __call metamethod, so instead we added a specific metamethod for this type of invocation--called __namecall--that was only available on UserData objects. We also modified the SELF opcode so it only uses the new semantics if the object has a __namecall metamethod.
The second thing we did was mostly make the new path and the old path able to share code easily. Instead of having the method-name as the second argument, we pushed it to the last argument. So, after it had been used to look up the method pointer, it could easily be popped off the stack and the stack looked like it did if the function had been invoked via the old path.
Conclusion
How much of an impact does this optimization have? Well, like most things in programming, the answer is “it depends.” For functions that are heavyweight--and you don’t call that often--you won’t see much of an improvement. But for smaller functions that you call often, the savings can be considerable.
People on the Developer Forum quickly noticed the appearance of this strange, new metamethod, and a table was presented that compared the speed of __namecall against both the old method of calling instance methods and against a workaround that developers had been using to optimize method calling:
The first loop uses the new __namecall code path, but because all the magic happens under the hood, there is no need for developers to change any existing code to benefit from the optimization.
The second loop emulates the old way of doing an instance method call; first doing a lookup to find the method and then invoking it.
And finally, the third loop shows a common optimization that developers were doing, where the method was first looked up, stored in a local variable, and then the variable was invoked.
The nice thing here is that it shows that with the __namecall optimization, it’s no longer necessary to explicitly cache instance functions, as it’s just as fast as the cached optimization, so the most straightforward code will also be the most performant.
Now that __namecall has been deployed, and we’re happy with the results we’re seeing, it’s time to turn our focus to memory usage, and see what we can do to improve the client in that area!
Recommended
Troubleshooters.Comand Code Corner Present
From a running Cprogram, you can call a Lua script. The C program can pass arguments tothe Lua script, and the Lua script can pass back a return. The Luascript can serve to initialize the C program, or the Lua script caninstall a callback routine in the C program.
Calling a Lua script from a C program can be daunting without the rightinformation. Different computers have different names and locations forlibraries and include files, and it's difficult to figure out what'scausing compile/link problems.
Further complicating things is that with every major version of Lua,the method of calling Lua from C changes. Much documentation on theInternet is specific to Lua 5.0, and won't work on 5.1. Likewise, thisdocumentation is specific to 5.1, and when 5.2 comes out (it's alpharight now), the documentation on this page won't work on the newversion.
The purpose of this document is to make it easy for you to do simple Lua calls from C programs.
A typical interaction from a C program to a Lua 5.1 script looks something like this:
- Create a Lua state variable
- Load Lua libraries
- Load but don't run the Lua script file
- Priming script run to create script's global vars
- Pass all arguments to the Lua script on the stack
- Run the loaded Lua script
- Retrieve the return from the Lua script
- Close the Lua state variable
Create a Lua state variable | lua_State *L; L = luaL_newstate(); | |
Load Lua libraries | luaL_openlibs(L); | |
Load but don't run the Lua script file | luaL_loadfile(L, 'script.lua') | |
Priming run of the loaded Lua script to create the script's global variables | ||
Pass all arguments to the Lua script on the stack | Various routines, too many to name. | |
Run the loaded Lua script | ||
Retrieve the return from the Lua script | Various pops and stack calls | |
Close the Lua state variable | lua_close(L); |
Header Files
Always put these three header files at the top of any C program calling Lua:Compiling and Linking
Nothing is tougher than compiling and linking your first C to Luaprogram. There are way too many variables, it seems like everything's ablack box, and you run around changing things trial and error, crossingyour fingers.But there's a method to the madness, and this subsection explains it.If you know all the content of this section, try not to be offended orinsulted -- the extra info is for those who don't know it yet.
Rare Lua C Scripts Copy And Paste
Here's a typical command to compile it:Let's break that down:
Your systems C compilation command. Could be cc, or gcc, or something else. | |
Show all warnings. You might choose not to insert this until all other errors and warnings have been fixed. | |
The name of the finished executable. | |
The name of the C file to compile. | |
Every C compiler checks for include files in certain places. But often lua.h, liblua.h and lauxlua.h aren't in those places, so you need to tell the compiler where to find them. | |
Every C compiler checks for libraries to link in certainplaces. But often the Lua libraries are elsewhere, so you need to tellthe compiler where to find them. | |
Link in the Lua library. The way you name this library in the compiler's -l option requires some thought and research, which will be explained later in this subsection. | |
Link in the math library (often not necessary) |
The Process of Finding the Right Compile Command
Finding the right compile command is a process that looks something like this:- Try to compile, no link, no -Wall, no -I. You might get lucky.
- Find the location of the Lua include files.
- Using that location, compile, no link, no -Wall.
- Compile, no link, use -Wall.
- Find the location and filenames of the Lua library files.
- Using the location and filenames, compile and link.
- Test the Executable.
Step 1: Try to compile, no link, no -Wall, no -I
Do this:Notice that the -c means 'compile only, do not link'. Sometimes it'snot all that clear whether an error or warning comes from compile orlink, but it's very clear if you only compile. If the preceding commanddoesn't error out, skip to step 4. However, in my case it errored out:
On to step 2..
Step 2: Find the location of the Lua include files
The three Ubiquitous Lua include files are lua.h, lauxlib.h, and lualib.h. Use your Linux box's locate command to find lua.h after running updatedb to make sure that locate finds everything. Make sure that the directory with lua.h also contains lauxlib.h, and lualib.h.Obviously the directory you want is /usr/include/lua5.1. Now see if that directory contains the other two:
OK, that's it. You know you need to add -I /usr/include/lua5.1 to the command.
Step 3: Using that location, compile, no link, no -Wall
In my case the preceding worked:If it hadn't worked, and you'd included all three header files in hello.c, and you're sure you got the additional include directory right, suspect wrong code in hello.c itself. If it doesn't produce errors or warnings, go on to the next step..
Step 4: Compile, no link, use -Wall
In my case, the preceding produced no errors or warnings:If it produces warnings, try your best to fix them. Warnings are oftena sign of problems that rear their ugly head a week after deployment.
Once you have no errors or warnings, go on to the next step..
Step 5: Find the location and filenames of the Lua library files
First a word about how the -l compiler option works. The -loption assumes every library file starts with 'lib'. Therefore, insteadof making you type -lliblua, it makes you type -llua and then out ofthe goodness of its heart prepends lib before the lua. For instance, if you put -llua, that means that it searches for a file called liblua.a or liblua.o or liblua.so, etc. So you need to find the liblua.* files. Do it as follows:Here's the partial result:
See the ones called liblua5.1.*? Those are the Lua libraries on my computer. So we have a pretty good idea that the -l argument should be lua5.1.We also know that they're located in /usr/lib, which probably isalready on the C compiler's library path, so we probably don't need a -L compiler option. If these files had been in /usr/lib/lua, we'd have needed an -L option.
Step 6: Using the location and filenames, compile and link
This step differs from step 4 in two ways:- We remove the -c so that it will attempt to link
- We add -llua5.1
It worked. If it hadn't, we'd have been forced to troubleshoot.
Step 7: Test the Executable
So let's run it:You haven't seen the code for hello.c and helloscript.lua yet, but take it from me that the preceding output was exactly what it was supposed to do.
Summary
Here's how you build a C program that runs a Lua script:Create a Lua state variable | lua_State *L; L = luaL_newstate(); | |
Load Lua libraries | luaL_openlibs(L); | |
Load but don't run the Lua script file | luaL_loadfile(L, 'script.lua') | |
Priming run of the loaded Lua script to create the script's global variables | ||
Pass all arguments to the Lua script on the stack | Various routines, too many to name. | |
Run the loaded Lua script | ||
Retrieve the return from the Lua script | Various pops and stack calls | |
Close the Lua state variable | lua_close(L); |
You always include lua.h, lauxlib.h and lualib.h. You use the Linux locate command to find where those files are, and use that location in a -I compiler option. You always link with liblua, and you use locate commands to find where that library is, and use it in the -l compiler option and if its location isn't in a normal library directory, put its directory in a -L compiler option.
Getting Lua-calling C programs to compile and link can be difficult andfrustrating. To limit the difficulty and frustration, use this process:
- Try to compile, no link, no -Wall, no -I. You might get lucky.
- Find the location of the Lua include files.
- Using that location, compile, no link, no -Wall.
- Compile, no link, use -Wall.
- Find the location and filenames of the Lua library files.
- Using the location and filenames, compile and link.
- Test the Executable.
Let's start with the one line Lua script itself. Call it helloscript.lua:
That's it, one line. Notice it doesn't have the #!/usr/bin/lua line on top. It doesn't have to because the C Lua interface has a built-in Lua interpreter which the lua_pcall() function invokes automatically.
So if you run the C program and see the text 'This is coming from lua.' you know you've succeded.
Next let's review the steps covered in the Anatomy of a Lua Call article:
Create a Lua state variable | lua_State *L; L = luaL_newstate(); | |
Load Lua libraries | luaL_openlibs(L); | |
Load but don't run the Lua script file | luaL_loadfile(L, 'script.lua') | |
Priming run of the loaded Lua script to create the script's global variables | ||
Pass all arguments to the Lua script on the stack | Various routines, too many to name. | |
Run the loaded Lua script | ||
Retrieve the return from the Lua script | Various pops and stack calls | |
Close the Lua state variable | lua_close(L); |
Your Hello World program will implement all the above, in that same order, EXCEPT:
- Priming run of the loaded Lua script
- Passing arguments to the Lua script
- Retrieving the return from the Lua script
As you can see, it's pretty much the same as discussed in the list ofsteps, except no args are passed into Lua and no return variables arepassed out.
Remembering the discussion from the Anatomy of a Lua Call article, compile and link it, troubleshooting as necessary. Here's how it compiles and links on my computer:
Do what's needed til it compiles no errors, no warnings, with the -Wall compiler option. Then run it. It should look like this:
Study the program. Everything should be pretty obvious except lua_pcall(),and that will be discussed later. Once you've gotten output like thepreceding and really understand what's going on (with the possibleexception of lua_pcall() details), it's time to continue..
Remember the program from the Hello World article did everything except these:
- Priming run of the loaded Lua script
- Passing arguments to the Lua script
- Retrieving the return from the Lua script
Tourist Trap Alert All over the Internet, including on some of the Lua project's owndocumentation, you'll see hints basically telling you that to call a Csubroutine you do: lua_getglobals(L, 'subroutine_name') push_args_to_subroutine if(lua_pcall(L, num_of_args, num_of_returns, 0) error_routine(); These Internet instructions say nothing about doing a priming lua_pcall(), and if you do not do a priming lua_pcall(), you'll get an error message something like this: attempt to call a nil value If you get the preceding error message and don't know it's caused by lack of a priming lua_pcall() (or priming lua_call() or priming dofile()),you're about to have several hours or maybe even days of frustration.Mailing list threads old and new about this error basically tell youRTFM or RTFW, but of course we locals know it was reading the web thatgot you into this problem in the first place. I saw one guy who took the opportunity to tell the asker 'I'm trying tohelp you help yourself but you won't take the help, so now I'm puttingyou on my list of people I won't help.' All for what we locals know wasprobably a simple lack of a priming run THAT SEEMS NOT TO BE DOCUMENTEDANYWHERE! Before calling a function in a Lua script, do that priming run first!!!!!! |
OK, let's start by calling a specific function in a Lua script. Create this callfuncscript.lua file:
Lua C Scripts Pastebin
Now, noting that most of it is from the Hello World article you've already read, create the following callfunc.c:
The only change over the Hello World code is a priming run and pushing the tellme() Lua function via the lua_getglobal() call. Note that the second lua_pcall() still has its second arg, number_of_args, of 0, because you're not sending an argument to the Lua function tellme().
Arguments and Return Value
The previous part of this article was written primarily to make sure you knew that to call a specific Lua function, you need to push that function on the stack with lua_getglobal()Lua C Scripts Roblox Pastebin
to do a priming run, because without it you're in for hours of frustrating debugging and detour ridden RTFW.There are several pieces of example code on the net showing the passingof arguments to entire Lua scripts rather than functions, and thosewhole Lua scripts returning arguments. They do this by, within C,setting a global variable to be used in Lua. I've chosen not toreproduce these techniques here because:
|
Now take your callfuncscript.lua file and add a function called square, so the whole script looks like this:
And change callfunc.c to look like the following:
Be sure to notice the following things in the preceding C code:
- Even though two functions are called, only one priming run isneeded. Each Lua script requires only one priming run in order tocreate its globals so its functions are visible to the C program.
- In order to fully pass an integer into callfuncscript.lua'ssquare() function and receive the function's return, the lua_pcall()'snargs argument must be 1 (one argument passed in) and its nreturnsargument must be 1 (1 argument passed back). If nargs is 0 you'll getthe dreaded ' attempt to call a number value' error, while if nreturnsis 0 it will always return 0. So remember, this lua_pcall() is lua_pcall(L, 1, 1, 0).
A use case that happens often is the passing of tables to and from Luafunctions. How is that handled? There are a few idioms you see over andover again to make it happen. Before discussing the idioms, here's thecode:
The Lua Program
Here's the Lua program. As you can see, function tweaktable receives atable argument, converts all values to upper case, counts the values,and puts all that info in a new table that's returned.The C Program
The C program is the same as all the others except stacking argumentsto Lua is a little different, and recovering the table passed back fromLua is a little different. The Lua tweaktable() function takes a tableas its one and only argument and passes back one table. It passes backa completely different table so there's absolutely no question of thechanges being made by reference to the args rather than passback.Start by looking and running the code, and then we'll discuss some of the idioms that make it work..
The preceding code yields the following output:
Rare Lua C Scripts Admin
Table Argument Passing Idioms
You start by placing an empty table on the stack, and build it it onekey/value pair at a time. The way you place an empty table on the stackis with the following command:The way you give the now stacked table a key/value pair is like this:
You can do that over and over again with different key/value pairs. Each time the numeric argument to lua_settable() is -3 because lua_settablepops the two stack items above it after incorporating them in itstable, so at its end it's at the same level that it was before youstacked the key and value.
Note that you might be stacking things other than literals. Here are the many pushers available:
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Other functions push or at least put things on the stack, including:
In spite of what it sounds like, this function actually puts the function corresponding to the fcnname argument on the top of the stack. |
This function puts an empty table at the top of the stack. |
Thisfunction replaces the previous key with the new one, and then pushesthe new value on top of that. You need to do a pop to stay even. |
And here are some poppers:
This pops off the stack, number levels. |
This pops twice after assigning the key/value pair in the top two stack positions to the table at level -level. |
Table Return Passing Idioms
When a Lua function called from C returns ends it puts its returnvalues on the stack. When a return value is a table, retrieving thetable's elements isn't obvious. Assuming the Lua function returned onlyone thing and that thing is a table, then that table is at the stack'stop when it comes back to C. Here's how you access it:In the preceding, you push a nil in order to make sure lua_next()starts with the first item. The reason you use -2 as an arg tolua_next() is because after you push the nil, the returned table is thesecond item down on the stack. On each iteration, lua_next() replacesthe existing key with the next one, and then pushes the next value ontop of it. Therefore to keep things in place you need one pop per cycle.
On every iteration immediately after lua_next(), the top of the stack is the new value, one down from that is the new key, and one down from that is the table.
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