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Coursework 5
This coursework is worth 25% and is due on 12 January at 16:00. You are asked
to implement a compiler targeting the LLVM-IR. Be careful that this CW needs
some material about the LLVM-IR that has not been shown in the lectures and
your own experiments and research might be required. You can find information about the LLVM-IR at
• https://bit.ly/3rheZYr
• https://llvm.org/docs/LangRef.html
You can do the implementation of your compiler in any programming language
you like, but you need to submit the source code with which you generated
the LLVM-IR files, otherwise a mark of 0% will be awarded. You are asked to
submit the code of your compiler, but also the generated .ll files. No PDF
is needed for this coursework. You should use the lexer and parser from the
previous courseworks, but you need to make some modifications to them for
the ‘typed’ version of the Fun-language. I will award up to 5% if a lexer and a
parser are correctly implemented.
You will be marked according to the input files
• sqr.fun
• fact.fun
• mand.fun
• mand2.fun
• hanoi.fun
which are uploaded to KEATS and Github.
Exclamation-Triangle Disclaimer
It should be understood that the work you submit represents your own effort.
You have not copied from anyone else. An exception is the Scala code I showed
during the lectures or uploaded to KEATS, which you can both use. You can
also use your own code from the CW 1 – CW 4. But do not be tempted to ask
Github Copilot for help or do any other shenanigans like this!
Task
The goal is to lex and parse 5 Fun-programs, including the Mandelbrot program
shown in Figure 1, and generate corresponding code for the LLVM-IR. Unfortunately the calculations for the Mandelbrot Set require floating point arithmetic
and therefore we cannot be as simple-minded about types as we have been so
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far (remember the LLVM-IR is a fully-typed language and needs to know the
exact types of each expression). The idea is to deal appropriately with three
types, namely Int, Double and Void (they are represented in the LLVM-IR as
i32, double and void). You need to extend the lexer and parser accordingly
in order to deal with type annotations. The Fun-language includes global constants, such as
val Ymin: Double = -1.3;
val Maxiters: Int = 1000;
where you can assume that they are ‘normal’ identifiers, just starting with a
capital letter—all other identifiers should have lower-case letters. Function definitions can take arguments of type Int or Double, and need to specify a return
type, which can be Void, for example
def foo(n: Int , x: Double) : Double = ...
def id(n: Int) : Int = ...
def bar () : Void = ...
The idea is to record all typing information that is given in the Fun-program,
but then delay any further typing inference to after the CPS-translation. That
means the parser should generate ASTs given by the Scala dataypes:
abstract class Exp
abstract class BExp
abstract class Decl
case class Def(name: String , args: List [( String , String )],
ty: String , body: Exp) extends Decl
case class Main(e: Exp) extends Decl
case class Const(name: String , v: Int) extends Decl
case class FConst(name: String , x: Double) extends Decl
case class Call(name: String , args: List[Exp ]) extends Exp
case class If(a: BExp , e1: Exp , e2: Exp) extends Exp
case class Var(s: String) extends Exp
case class Num(i: Int) extends Exp // integer numbers
case class FNum(i: Double) extends Exp // floating numbers
case class ChConst(c: Int) extends Exp // char constants
case class Aop(o: String , a1: Exp , a2: Exp) extends Exp
case class Sequence(e1: Exp , e2: Exp) extends Exp
case class Bop(o: String , a1: Exp , a2: Exp) extends BExp
This datatype distinguishes whether the global constant is an integer constant
or floating constant. Also a function definition needs to record the return type
of the function, namely the argument ty in Def, and the arguments consist of
an pairs of identifier names and types (Int or Double). The hard part of the CW
is to design the K-intermediate language and infer all necessary types in order
to generate LLVM-IR code. You can check your LLVM-IR code by running it
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with the interpreter lli.
Also note that the second version of the Mandelbrot program and also the
Tower of Hanoi program use character constants, like 'a', '1', '\n' and so on.
When they are tokenised, such characters should be interpreted as the corresponding ASCII code (an integer), such that we can use them in calculations
like 'a' + 10 where the result should be 107. As usual, the character '\n' is
the ASCII code 10.
LLVM-IR
There are some subtleties in the LLVM-IR you need to be aware of:
• Global constants: While global constants such as
val Max : Int = 10;
can be easily defined in the LLVM-IR as follows
@Max = global i32 10
they cannot easily be referenced. If you want to use this constant then you
need to generate code such as
%tmp_22 = load i32 , i32* @Max
first, which treats @Max as an Integer-pointer (type i32*) that needs to be
loaded into a local variable, here %tmp_22.
• Void-Functions: While integer and double functions can easily be called
and their results can be allocated to a temporary variable:
%tmp_23 = call i32 @sqr (i32 %n)
void-functions cannot be allocated to a variable. They need to be called
just as
call void @print_int (i32 %tmp_23)
• Floating-Point Operations: While integer operations are specified in the
LLVM-IR as
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def compile_op (op: String) = op match {
case "+" => "add i32 "
case "*" => "mul i32 "
case "-" => "sub i32 "
case "==" => "icmp eq i32 "
case "!=" => "icmp ne i32 "
case "<=" => "icmp sle i32 " // signed less or equal
case "<" => "icmp slt i32 " // signed less than
}
the corresponding operations on doubles are
def compile_dop (op: String) = op match {
case "+" => "fadd double "
case "*" => "fmul double "
case "-" => "fsub double "
case "==" => "fcmp oeq double "
case "!=" => "fcmp one double "
case "<=" => "fcmp ole double "
case "<" => "fcmp olt double "
}
• Typing: In order to leave the CPS-translations as is, it makes sense to
defer the full type-inference to the K-intermediate-language. For this it is
good to define the KVar constructor as
case class KVar(s: String , ty: Ty = "UNDEF") extends KVal
where first a default type, for example UNDEF, is given. Then you need to
define two typing functions
Both functions require a typing-environment that updates the information about what type each variable, operation and so on receives. Once
the types are inferred, the LLVM-IR code can be generated. Since we are
dealing only with simple first-order functions, nothing on the scale as
the ‘Hindley-Milner’ typing-algorithm is needed. I suggest to just look
at what data is avaliable and generate all missing information by “simple
means”…rather than looking at the literature which solves the problem
with much heavier machinery.
• Build-In Functions: The ‘prelude’ comes with several build-in functions:
new_line(), skip, print_int(n), print_space(), print_star() and print_char(n).
You can find the ‘prelude’ for example in the file sqr.ll.
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// Mandelbrot program (without character constants)
val Ymin: Double = -1.3;
val Ymax: Double = 1.3;
val Ystep: Double = 0.05; //0.025;
val Xmin: Double = -2.1;
val Xmax: Double = 1.1;
val Xstep: Double = 0.02; //0.01;
val Maxiters: Int = 1000;
def m_iter(m: Int , x: Double , y: Double ,
zr: Double , zi: Double) : Void = {
if Maxiters <= m
then print_star ()
else {
if 4.0 <= zi*zi+zr*zr then print_space ()
else m_iter(m + 1, x, y, x+zr*zr -zi*zi , 2.0* zr*zi+y)
}
};
def x_iter(x: Double , y: Double) : Void = {
if x <= Xmax
then { m_iter (0, x, y, 0.0, 0.0) ; x_iter(x + Xstep , y) }
else skip ()
};
def y_iter(y: Double) : Void = {
if y <= Ymax
then { x_iter(Xmin , y) ; new_line () ; y_iter(y + Ystep) }
else skip ()
};
y_iter(Ymin)
Figure 1: The Mandelbrot program in the ‘typed’ Fun-language.
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Figure 2: Ascii output of the Mandelbrot program.
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