Terra is a new low-level system programming language that is designed to interoperate seamlessly with the Lua programming language:
-- This top-level code is plain Lua code.function printhello() -- This is a plain Lua function print("Hello, Lua!")endprinthello()-- Terra is backwards compatible with C-- we'll use C's io library in our example.C = terralib.includec("stdio.h")-- The keyword 'terra' introduces-- a new Terra function.terra hello(argc : int, argv : &rawstring) -- Here we call a C function from Terra C.printf("Hello, Terra!\n") return 0end-- You can call Terra functions directly from Luahello(0,nil)-- Or, you can save them to disk as executables or .o-- files and link them into existing programsterralib.saveobj("helloterra",{ main = hello })
Like C, Terra is a simple, statically-typed, compiled language with manual memory management. But unlike C, it is designed from the beginning to interoperate with Lua. Terra functions are first-class Lua values created using the terra
keyword. When needed they are JIT-compiled to machine code. Try the online demo!
You can use Terra and Lua as…
A scripting-language with high-performance extensions. While the performance of Lua and other dynamic languages is always getting better, a low-level of abstraction gives you predictable control of performance when you need it. Terra programs use the same LLVM backend that Apple uses for its C compilers. This means that Terra code performs similarly to equivalent C code. For instance, our translations of the nbody
and fannhakunen
programs from the programming language shootout1 perform within 5% of the speed of their C equivalents when compiled with Clang, LLVM’s C frontend. Terra also includes built-in support for SIMD operations, and other low-level features like non-temporal writes and prefetches. You can use Lua to organize and configure your application, and then call into Terra code when you need controllable performance.
An embedded JIT-compiler for building languages. We use techniques from multi-stage programming2 to make it possible to meta-program Terra using Lua. Terra expressions, types, and functions are all first-class Lua values, making it possible to generate arbitrary programs at runtime. This allows you to compile domain-specific languages (DSLs) written in Lua into high-performance Terra code. Furthermore, since Terra is built on the Lua ecosystem, it is easy to embed Terra-Lua programs in other software as a library. This design allows you to add a JIT-compiler into your existing software. You can use it to add a JIT-compiled DSL to your application, or to auto-tune high-performance code dynamically.
A stand-alone low-level language. Terra was designed so that it can run independently from Lua. In fact, if your final program doesn’t need Lua, you can save Terra code into a .o file or executable. In addition to ensuring a clean separation between high- and low-level code, this design lets you use Terra as a stand-alone low-level language. In this use-case, Lua serves as a powerful meta-programming language. You can think of it as a replacement for C++ template metaprogramming3 or C preprocessor X-Macros4 with better syntax and nicer properties such as hygiene5. Since Terra exists only as code embedded in a Lua meta-program, features that are normally built into low-level languages can be implemented as Lua libraries. This design keeps the core of Terra simple, while enabling powerful behavior such as conditional compilation, namespaces, templating, and even class systems implemented as libraries.
For more information about using Terra, see the getting started guide and API reference. Our publications provide a more in-depth look at its design.
[1] http://benchmarksgame.alioth.debian.org
[2] http://www.cs.rice.edu/~taha/MSP/
[3] http://en.wikipedia.org/wiki/Template_metaprogramming
[4] http://en.wikipedia.org/wiki/X_Macro
[5] http://en.wikipedia.org/wiki/Hygienic_macro
Terra entities such as functions, types, variables and expressions are first-class Lua values — they can be stored in Lua variables and passed to or returned from Lua functions. Using constructs from multi-stage programming2, you can write Lua code to programmatically generate arbitrary Terra code.
Inside Terra code, you can use an escape operator ([]
) to splice the result of a Lua expression into the Terra code:
local a = 5terra sin5() return [ math.sin(a) ]end
An escape is evaluated when a Terra function is compiled, and the result is spliced into the Terra code. In this example, this means that math.sin(5)
will be evaluated once and the code that implements the Terra function will return a constant. This can be verified by printing out the compiled version of the sin5
function:
--output a prettified representation of what this function doessin5:printpretty() > output:> sin50 = terra() : {double}> return -0.95892427466314> end
Escapes can also return other Terra entities such as a function:
add4 = terra(a : int) return a + 4 endterra example() return [add4](3) -- 7end
In this case, Terra will insert a call to the Terra function stored in the add4
variable:
example:printpretty()> output:> example4 = terra() : {int32}> return <extract0> #add43(3)#> end
In fact, any name used in Terra code such as add4
or foo.bar
is treated as if it were escaped by default.
Inside an escape, you can refer to variables defined in Terra:
--a function to be called inside an escapefunction choosesecond(a,b) -- prints false, 'a' is not a number: print(a == 1) -- prints true, 'a' is a Terra symbol: print(terralib.issymbol(a)) return bendterra example(input : int) var a = input var b = input+1 --create an escape that refers to 'a' and 'b' return [ choosesecond(a,b) ] --returns the value of bendexample(1) --returns 2
Since escapes are evaluated before a Terra function is compiled, variables a
and b
will not have concrete integer values inside the escape. Instead, inside the Lua code a
and b
are Terra symbols that represent references to Terra values. Since choosesecond
returns the symbol b
, the example
function will return the value of Terra variable b
when called.
The quotation operator (a backtick) allows you to generate Terra statements and expression in Lua. They can then be spliced into Terra code using the escape operator.
function addtwo(a,b) return `a + bendterra example(input : int) var a = input var b = input+1 return [ addtwo(a,b) ]endexample(1) -- returns 3
To generate statements rather than expressions you can use the quote
operator:
local printtwice = quote C.printf("hello\n") C.printf("hello\n")endterra print4() [printtwice] [printtwice]end
With these two operators, you can use Lua to generate arbitrary Terra code at compile-time. This makes the combination of Lua/Terra well suited for writing compilers for high-performance domain-specific languages. For instance, we can implement a compiler for BF, a minimal language that emulates a Turing machine. The Lua function compile
will take a string of BF code, and a maximum tape size N
. It then generates a Terra function that implements the BF code. Here is a skeleton that sets up the BF program:
local function compile(code,N) local function body(data,ptr) --<<implementation of body>> end return terra() --an array to hold the tape var data : int[N] --clear the tape initially for i = 0, N do data[i] = 0 end var ptr = 0 --generate the code for the body [ body(data,ptr) ] endend
The function body
is responsible for generating body of the BF program given the code string:
local function body(data,ptr) --the list of terra statements that make up the BF program local stmts = terralib.newlist() --loop over each character in the BF code for i = 1,#code do local c = code:sub(i,i) local stmt --generate the corresponding Terra statement --for each BF operator if c == ">" then stmt = quote ptr = ptr + 1 end elseif c == "<" then stmt = quote ptr = ptr - 1 end elseif c == "+" then stmt = quote data[ptr] = data[ptr] + 1 end elseif c == "-" then stmt = quote data[ptr] = data[ptr] - 1 end elseif c == "." then stmt = quote C.putchar(data[ptr]) end elseif c == "," then stmt = quote data[ptr] = C.getchar() end elseif c == "[" then error("Implemented below") elseif c == "]" then error("Implemented below") else error("unknown character "..c) end stmts:insert(stmt) end return stmtsend
It loops over the code string, and generates the corresponding Terra code for each character of BF (e.g. ”>” shifts the tape over by 1 and is implemented by the Terra code ptr = ptr + 1
). We can now compile a BF function:
add3 = compile(",+++.")
The result, add3
, is a Terra function that adds3 to an input character and then prints it out:
add3:printpretty()> bf_t_46_1 = terra() : {}> var data : int32[256]> ...> var ptr : int32 = 0> data[ptr] = <extract0> #getchar()#> data[ptr] = data[ptr] + 1> data[ptr] = data[ptr] + 1> data[ptr] = data[ptr] + 1> <extract0> #putchar(data[ptr])#> end
We can also use goto
statements (goto labelname
) and labels (::labelname::
) to implement BF’s looping construct:
local function body(data,ptr) local stmts = terralib.newlist() --add a stack to keep track of the beginning of each loop local jumpstack = {} for i = 1,#code do local c = code:sub(i,i) local stmt if ... elseif c == "[" then --generate labels to represent the beginning --and ending of the loop --the 'symbol' function generates a globally unique --name for the label local target = { before = symbol(), after = symbol() } table.insert(jumpstack,target) stmt = quote --label for beginning of the loop ::[target.before]:: if data[ptr] == 0 then goto [target.after] --exit the loop end end elseif c == "]" then --retrieve the labels that match this loop local target = table.remove(jumpstack) assert(target) stmt = quote goto [target.before] --loop back edge :: [target.after] :: --label for end of the loop end else error("unknown character "..c) end stmts:insert(stmt) end return stmtsend
We are using these generative programming constructs to implement domain-specific languages and auto-tuners. Our PLDI paper describes our implementation of Orion, a language for image processing kernels, and we are in the process of porting the Liszt language for mesh-based PDE’s to Terra.
Programming languages don’t exist in a vacuum, and the generative programming features of Terra can be useful even in projects that are primarily implemented in other programming languages. We make it possible to integrate Terra with other projects so you can use it to generate low-level code, while keeping most of your project in a well-established language.
First, we make it possible to pass values between Lua and Terra. Our implementation is built on top of LuaJIT’s foreign fuction interface. You can call Terra functions directly from Lua (or vice-versa), and access Terra objects directly from Lua (more details in the API reference).
Furthermore, Lua-Terra is backwards compatible with both pure Lua and C, which makes it easy to use preexisting code. In Lua-Terra, you can use require
or loadfile
and it will treat the file as a Lua program (use terralib.loadfile
to load a combined Lua-Terra file). You can use terralib.includec
to import C functions from already existing header files.
Finally, Lua-Terra can also be embedded in pre-existing applications by linking the application against libterra.a
and using Terra’s C API. The interface is very similar to that of the Lua interpreter. A simple example initializes Terra and then runs code from the file specified in each argument:
#include <stdio.h>#include "terra.h"int main(int argc, char ** argv) { lua_State * L = luaL_newstate(); //create a plain lua state luaL_openlibs(L); //initialize its libraries //initialize the terra state in lua terra_init(L); for(int i = 1; i < argc; i++) //run the terra code in each file if(terra_dofile(L,argv[i])) exit(1); return 0;}
The combination of a simple low-level language with a simple dynamic programming language means that many built-in features of statically-typed low-level languages can be implemented as libraries in the dynamic language. Here are just a few examples:
Normally conditional compilation is accomplished using preprocessor directives (e.g., #ifdef
), or custom build systems. Using Lua-Terra, we can write Lua code to decide how to construct a Terra function. Since Lua is a full programming language, it can do things that most preprocessors cannot, such as call external programs. In this example, we conditionally compile a Terra function differently on OSX and Linux by first calling uname
to discover the operating system, and then using an if
statement to instantiate a different version of the Terra function depending on the result:
--run uname to figure out what OS we are runninglocal uname = io.popen("uname","r"):read("*a")local C = terralib.includec("stdio.h")if uname == "Darwin\n" then terra reportos() C.printf("this is osx\n") endelseif uname == "Linux\n" then terra reportos() C.printf("this is linux\n") endelse error("OS Unknown")end--conditionally compiled to --the right version for this osreportos()
Statically-typed languages normally need constructs that specifically deal with the problem of namespaces (e.g., C++’s namespace
keyword, or Java’s import
constructs). For Terra, we just use Lua’s first-class tables as way to organize functions. When you use any “name” such as myfunctions.add
inside a Terra function, the Terra will resolve it at compile time to the Terra value it holds. Here is an example of placing a Terra function inside a Lua table, and then calling it from another Terra function:
local myfunctions = {}-- terra functions are first-class Lua values-- they can be stored in Lua tablesterra myfunctions.add(a : int, b : int) : int return a + bend-- and called from the tables as wellterra myfunctions.add3(a : int) return myfunctions.add(a,3)end--the declaration of myfunctions.add is just syntax sugar for:myfunctions["add"] = terra(a : int, b : int) : int return a + bendprint(myfunctions.add3(4))
In fact, you’ve already seen this behavior when we imported C functions:
C = terralib.includec("stdio.h")
The function includec
just returns a Lua table (C
) that contains the C functions. Since C
is a Lua table, you can iterate through it if you want:
for k,v in pairs(C) do print(k,v)end> seek <terra function>> asprintf <terra function>> gets <terra function>> size_t uint64> ...
Since Terra types and functions are first class values, you can get functionality similar to a C++ template by simply creating a Terra type and defining a Terra function inside of a Lua function. Here is an example where we define the Lua function MakeArray(T)
which takes a Terra type T
and generates an Array
object that can hold multiple T
objects (i.e. a simple version of C++’s std::vector
).
C = terralib.includec("stdlib.h")function MakeArray(T) --create a new Struct type that contains a pointer --to a list of T's and a size N local struct ArrayT { --&T is a pointer to T data : &T; N : int; } --add some methods to the type terra ArrayT:init(N : int) -- the syntax [&T](...) is a cast, -- the C equivalent is (T*)(...) self.data = [&T](C.malloc(sizeof(T)*N)) self.N = N end terra ArrayT:get(i : int) return self.data[i] end terra ArrayT:set(i : int, v : T) self.data[i] = v end --return the type as a return ArrayTendIntArray = MakeArray(int)DoubleArray = MakeArray(double)terra UseArrays() var ia : IntArray var da : DoubleArray ia:init(1) da:init(1) ia:set(0,3) da:set(0,4.5) return ia:get(0) + da:get(0)end
As shown in this example, Terra allows you to define methods on struct
types. Unlike other statically-typed languages with classes, there are no built-in mechanisms for inheritance or runtime polymorphism. Methods declarations are just a syntax sugar that associates table of Lua methods with each type. Here the get
method is equivalent to:
ArrayT.methods.get = terra(self : &T, i : int) return self.data[i]end
The object ArrayT.methods
is a Lua table that holds the methods for type ArrayT
.
Similarly an invocation such as ia:get(0)
is equivalent to T.methods.get(&ia,0)
.
By nesting a Terra function inside a Lua function, you can compile different versions of a function. Here we generate different versions of the power function (e.g. pow2, or pow3):
--generate a power function for a specific N (e.g. N = 3)function makePowN(N) local function emit(a,N) if N == 0 then return 1 else return `a*[emit(a,N-1)] end end return terra(a : double) return [emit(a,N)] endend--use it to fill in a table of functionslocal mymath = {}for n = 1,10 do mymath["pow"..n] = makePowN(n)endprint(mymath.pow3(2)) -- 8
As shown in the templating example, Terra allows you to define methods on struct
types but does not provide any built-in mechanism for inheritance or polymorphism. Instead, normal class systems can be written as libraries. For instance, a user might write:
J = terralib.require("lib/javalike")Drawable = J.interface { draw = {} -> {} }struct Square { length : int; }J.extends(Square,Shape)J.implements(Square,Drawable)terra Square:draw() : {} --draw implementationend
The functions J.extends
and J.implements
are Lua functions that generate the appropriate Terra code to implement a class system. More information is available in our PLDI Paper. The file lib/javalike.t has one possible implementation of a Java-like class system, while the file lib/golike.t is more similar to Google’s Go language.
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