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\fnote{\copyright{} Christian Urban, King's College London, 2019, 2023}

% CPS translations

% https://felleisen.org/matthias/4400-s20/lecture17.html

%% pattern matching resources

%% https://www.reddit.com/r/ProgrammingLanguages/comments/g1vno3/beginner_resources_for_compiling_pattern_matching/

\section*{Handout 9 (LLVM, SSA and CPS)}

Reflecting on our two tiny compilers targetting the JVM, the code

generation part was actually not so hard, no? Pretty much just some

post-traversal of the abstract syntax tree. Yes? One of the reasons

for this ease is that the JVM is a stack-based virtual machine and it

is therefore not hard to translate deeply-nested arithmetic

expressions into a sequence of instructions manipulating the

stack. That is pretty much the whole point of the JVM. The problem is

that ``real'' CPUs, although supporting stack operations, are not

really designed to be \emph{stack machines}.  The design of CPUs is

more like: Here are some instructions and a chunk of

memory---compiler, or better compiler writers, do something with

them. Consequently, modern compilers need to go the extra mile in

order to generate code that is much easier and faster to process by

actual CPUs, rather than running code on virtual machines that

introduce an additional layer of indirection. To make this all

tractable for this module, we target the \emph{LLVM Intermediate

  Language} (LLVM-IR). In this way we can take advantage of the tools

coming with LLVM.\footnote{\url{http://llvm.org}} For example we do

not have to worry about things like register allocations or peephole

optimisations. By using the LLVM-IR, however, we also have to pay

price in the sense that generating code gets

harder\ldots{}unfor\-tunately nothing comes for free in life.

\subsection*{LLVM and the LLVM-IR}

\noindent LLVM is a beautiful example

that projects from Academia can make a difference in the Real World. LLVM

started in 2000 as a project by two researchers at the  University of

Illinois at Urbana-Champaign. At the time the behemoth of compilers was

gcc with its myriad of front-ends for different programming languages (C++, Fortran,

Ada, Go, Objective-C, Pascal etc). The problem was that gcc morphed over

time into a monolithic gigantic piece of m\ldots ehm complicated

software, which you

could not mess about in an afternoon. In contrast, LLVM is designed to

be a modular suite of tools with which you can play around easily and

try out something new. LLVM became a big player once Apple hired one of

the original developers (I cannot remember the reason why Apple did not

want to use gcc, but maybe they were also just disgusted by gcc's big

monolithic codebase). Anyway, LLVM is now the big player and gcc is more

or less legacy. This does not mean that programming languages like C and

C++ are dying out any time soon---they are nicely supported by LLVM.

We will target the LLVM Intermediate Language, or LLVM Intermediate

Representation (short LLVM-IR). The LLVM-IR looks very similar to the

assembly language of Jasmin and Krakatau. Targetting LLVM-IR will also

allow us to benefit from the modular structure of the LLVM compiler

and we can let the compiler generate code for different CPUs, for

example X86 or ARM.  That means we can be agnostic about where our

code is actually going to run.\footnote{Anybody want to try to run our

  programs on Android phones?}  We can also be somewhat ignorant about

optimising our code and about allocating memory efficiently---the LLVM

tools will take care of all this.

However, what we have to do in order to make LLVM to play ball is to

generate code in \emph{Static Single-Assignment} format (short SSA). A

reason why LLVM uses the SSA-format, rather than JVM-like stack

instructions, is that stack instructions are difficult to

optimise---you cannot just re-arrange instructions without messing

about with what is calculated on the stack. Have a look at the

expression $((a + b) * 4) - (3 * (a + b))$ and the corresponding JVM

instructions:

\begin{lstlisting}[language=JVMIS, numbers=none,mathescape]

iload a

iload b

iadd

ldc 4

imul

ldc 3

iload a

iload b

iadd

imul

isub

\end{lstlisting}

\noindent

and try to reorganise the code such that you calculate the expression

$(a + b)$ only once. This requires either quite a bit of

math-understanding from the compiler or you need to add ``copy-and-fetch''

of a result from a local variable.  Also it is hard to find out if all

the calculations on the stack are actually necessary and not by chance

dead code. The JVM has for all these obstacles sophisticated machinery

to make such ``high-level'' code still run fast, but let's say that

for the sake of argument we do not want to rely on it. We want to

generate fast code ourselves. This means we have to work around the

intricacies of what instructions CPUs can actually process fast. This

is what the SSA format is designed for.

The main idea behind the SSA-format is to have sequences of very

simple variable assignments where every (tmp)variable is assigned only

once. The assignments need to be simple in the sense that they can be

just be primitive operations like addition, multiplication, jumps,

comparisons, function calls and so on.  Say, we have an expression

$((1 + a) + (foo(3 + 2) + (b * 5)))$, then the corresponding sequence

of assignments in SSA-format are:

\begin{lstlisting}[language=LLVMIR,numbers=left]

let tmp0 = add 1 a in

let tmp1 = add 3 2 in  

let tmp2 = call foo(tmp1)  

let tmp3 = mul b 5 in 

let tmp4 = add tmp2 tmp3 in 

let tmp5 = add tmp0 tmp4 in

  return tmp5 

\end{lstlisting}

\noindent where every tmpX-variable is used only once (we could, for

example, not write \texttt{tmp1 = add tmp2 tmp3} in Line 5 even if

this would not change the overall result). At the end we have a

return-instruction wich contains the final result of the

expression. As can be seen the task we have to solve for generating

SSA-code is to take apart compound expressions into its most basic

''particles'' and transfrom them into a sequence of simple assignments

that calculates the desired result. Note that this means we have to

fix the order in which the expression is calculated, like from the

left to right.

There are sophisticated algorithms for imperative languages, like C,

that efficiently transform high-level programs into SSA-format. But

we can ignore them here. We want to compile a functional language and

there things get much more interesting than just sophisticated. We

will need to have a look at CPS translations, where the CPS stands for

\emph{Continuation-Passing-Style}---basically black programming art or

abracadabra programming. So sit tight.

\subsection*{LLVM-IR}

Before we start, let's however first have a look at the \emph{LLVM Intermediate

Representation} in more detail. The LLVM-IR is in between the frontends

and backends of the LLVM framework. It allows compilation of multiple

source languages to multiple targets. It is also the place where most of

the target independent optimisations are performed. 

What is good about our toy Fun-language is that it basically only

contains expressions (be they arithmetic expressions, boolean

expressions or if-expressions). The exception are function definitions.

Luckily, for them we can use the mechanism of defining functions in the

LLVM-IR (this is similar to using JVM methods for functions in our

earlier compiler). For example the simple Fun-program 

\begin{lstlisting}[language=Scala,numbers=none]

def sqr(x) = x * x

\end{lstlisting}

\noindent

can be compiled to the following LLVM-IR function:

\begin{lstlisting}[language=LLVM]

define i32 @sqr(i32 %x) {

   %tmp = mul i32 %x, %x

   ret i32 %tmp

}    

\end{lstlisting}

\noindent First notice that all ``local'' variable names, in this case

\texttt{x} and \texttt{tmp}, are prefixed with \texttt{\%} in the

LLVM-IR.  Temporary variables can be named with an identifier, such as

\texttt{tmp}, or numbers. In contrast, function names, since they are

``global'', need to be prefixed with an @-symbol. Also, the LLVM-IR is

a fully typed language. The \texttt{i32} type stands for 32-bit

integers. There are also types for 64-bit integers (\texttt{i64}),

chars (\texttt{i8}), floats, arrays and even pointer types. In the

code above, \texttt{sqr} takes an argument of type \texttt{i32} and

produces a result of type \texttt{i32} (the result type is in front of

the function name, like in C). Each arithmetic operation, for example

addition and multiplication, are also prefixed with the type they

operate on. Obviously these types need to match up\ldots{} but since

we have in our programs only integers, for the moment \texttt{i32}

everywhere will do. We do not have to generate any other types, but

obviously this is a limitation in our Fun-language (and which we

are going to lift in the final CW).

There are a few interesting instructions in the LLVM-IR which are quite

different than what we have seen in the JVM. Can you remember the

kerfuffle we had to go through with boolean expressions and negating the

condition? In the LLVM-IR, branching  if-conditions are implemented

differently: there is a separate \texttt{br}-instruction as follows:

\begin{lstlisting}[language=LLVM]

br i1 %var, label %if_br, label %else_br

\end{lstlisting}

\noindent

The type \texttt{i1} stands for booleans. If the variable is true,

then this instruction jumps to the if-branch, which needs an explicit

label; otherwise it jumps to the else-branch, again with its own

label. This allows us to keep the meaning of the boolean expression

``as is'' when compiling if's---thanks god no more negating of

booleans.

A value of type boolean is generated in the LLVM-IR by the

\texttt{icmp}-instruction. This instruction is for integers (hence the

\texttt{i}) and takes the comparison operation as argument. For example

\begin{lstlisting}[language=LLVM]

icmp eq  i32 %x, %y     ; for equal

icmp sle i32 %x, %y     ; signed less or equal

icmp slt i32 %x, %y     ; signed less than

icmp ult i32 %x, %y     ; unsigned less than 

\end{lstlisting}

\noindent

Note that in some operations the LLVM-IR distinguishes between signed and 

unsigned representations of integers. I assume you know what this means,

otherwise just ask.

It is also easy to call another function in LLVM-IR: as can be 

seen from Figure~\ref{lli} we can just call a function with the 

instruction \texttt{call} and can also assign the result to 

a variable. The syntax is as follows

\begin{lstlisting}[language=LLVM]

%var = call i32 @foo(...args...)

\end{lstlisting}

\noindent

where the arguments can only be simple variables and numbers, but not compound

expressions.

Conveniently, you can use the program \texttt{lli}, which comes with

LLVM, to interpret programs written in the LLVM-IR. Type on the command line

\begin{lstlisting}[language=bash,numbers=none]

lli sqr.ll

\end{lstlisting}

\noindent

and you can see the output of the pragram generated. 

This means you can easily check whether the code you produced actually

works. To get a running program that does something interesting you

need to add some boilerplate about printing out numbers and a

main-function that is the entry point for the program (see

Figure~\ref{lli} for a complete listing of the square function). Again

this is very similar to the boilerplate we needed to add in our JVM

compiler.

You can generate a binary for the program in Figure~\ref{lli} by using

the \texttt{llc}-compiler and then \texttt{gcc}/\texttt{clang}, whereby \texttt{llc} generates

an object file and \texttt{gcc} (that is actually \texttt{clang} on my Mac) generates the

executable binary:

\begin{lstlisting}[language=bash,numbers=none]

llc -filetype=obj sqr.ll

gcc sqr.o -o a.out

./a.out

> 25

\end{lstlisting}

\begin{figure}[t]\small 

\lstinputlisting[language=LLVM,numbers=left]{../progs/sqr.ll}

\caption{An LLVM-IR program for calculating the square function. It

  calls the \texttt{sqr}-function in \texttt{@main} with the argument \texttt{5}

  (Line 20). The

  code for the \texttt{sqr}-function is in Lines 13 -- 16. The main-function

  stores

  the result of sqr in the variable called \texttt{\%1} and then

  prints out the contents of this variable in Line 21. The other

  code is boilerplate for printing out integers.\label{lli}}

\end{figure}   

\subsection*{Our Own Intermediate Language}

Let's get back to our compiler: Remember compilers have to solve the

problem of bridging the gap between ``high-level'' programs and

``low-level'' hardware. If the gap is too wide for one step, then a

good strategy is to lay a stepping stone somewhere in between. The

LLVM-IR itself is such a stepping stone to make the task of generating

and optimising code easier. Like a real compiler we will use our own

stepping stone which I call the \emph{K-language}.

\begin{center}

  \begin{tikzpicture}[scale=0.9,font=\bf,

                      node/.style={

                      rectangle,rounded corners=3mm,

                      ultra thick,draw=black!50,minimum height=16mm, 

                      minimum width=20mm,

                      top color=white,bottom color=black!20}]

  %\node (0) at (-3,0) {};  

  \node (A) at (0.0,0) [node,text width=1.6cm,text centered,label=above:{\small\it{}source}] {Fun-Language};

  \node (B) at (3.5,0) [node,text width=1.6cm,text centered,label=above:{\small\it{}stepping stone}] {K-Language};

  \node (C) at (7.0,0) [node,text width=1.6cm,text centered,label=above:{\small\it{}target}] {LLVM-IR};

  \draw [->,line width=2.5mm] (A) -- node [above,pos=0.35] {} (B); 

  \draw [->,line width=2.5mm] (B) -- node [above,pos=0.35] {} (C); 

  \end{tikzpicture}

  \end{center}

  \noindent

  To see why we need a stepping stone for generating code in SSA-format,

  considder again arithmetic expressions such as

  $((1 + a) + (3 + (b * 5)))$. They need to be broken up into smaller

  ``atomic'' steps, like so

\begin{lstlisting}[language=LLVMIR,numbers=none]

let tmp0 = add 1 a in   

let tmp1 = mul b 5 in 

let tmp2 = add 3 tmp1 in 

let tmp3 = add tmp0 tmp2 in

  return tmp3 

\end{lstlisting}

\noindent

Here \texttt{tmp3} will contain the result of what the whole

expression stands for. In each individual step we can only perform an

``atomic'' or ``trival'' operation, like addition or multiplication of

a number or a variable.  We are not allowed to have for example a

nested addition or an if-condition on the right-hand side of an

assignment. Such constraints are forced upon us because of how the

SSA-format works in the LLVM-IR. To simplify matters we represent

assignments with two kinds of syntactic entities, namely

\emph{K-values} and \emph{K-expressions}. K-values are all ``things''

that can appear on the right-hand side of an equal. Say we have the

following definition for K-values:

\begin{lstlisting}[language=Scala,numbers=none]

enum KVal {

  case KVar(s: String)

  case KNum(n: Int)

  case KAop(op: String, v1: KVal, v2: KVal)

  case KCall(fname: String, args: List[KVal])

}

\end{lstlisting}

\noindent

where a K-value can be a variable, a number or a ``trivial'' binary

operation, such as addition or multiplication. The two arguments of a

binary operation need to be K-values as well. Finally, we have

function calls, but again each argument of the function call needs to

be a K-value.  One constraint we need to be careful about is that the

arguments of the binary operations and function calls can only be

variables or numbers. For example

\begin{lstlisting}[language=Scala,numbers=none]

KAop("+", KAop("*", KNum(1), KNum(2)), KNum(3))

KCall("foo", List(KAop("+", KNum(4), KNum(5)))  

\end{lstlisting}

\noindent

while perfect instances of K-values according to the types, are not

allowed in the LLVM-IR. To encode this constraint into the type-system

of Scala, however, would make things overly complicated---to satisfy

this constraint is therefore on us. For the moment this will be all

K-values we are considdering. Later on, we will have some more for our

Fun-language.

Our K-expressions will encode the \texttt{let} and the \texttt{return}

from the SSA-format (again for the moment we only consider these two

constructors---later on we will have if-branches as well).

\begin{lstlisting}[language=Scala,numbers=none]

enum KExp {

  case KLet(x: String, v: KVal, e: KExp)

  case KReturn(v: KVal)

}

\end{lstlisting}

\noindent

By having \texttt{KLet} taking first a string (standing for a

tmp-variable) and second a value, we can fulfil the SSA constraint in

assignments ``by con\-struction''---there is no way we could write

anything else than a K-value.  Note that the third argument of a

\texttt{KLet} is again a K-expression, meaning either another

\texttt{KLet} or a \texttt{KReturn}. In this way we can construct a

sequence of computations and indicate what the final result of the

computations is.  According to the SSA-format, we also have to ensure

that all intermediary computations are given (fresh) names---we will

use an (ugly) counter for this.

To sum up, K-values are the atomic operations that can be on the

right-hand side of assignemnts. The K-language is restricted such that

it is easy to generate the SSA-format for the LLVM-IR. What remains to

be done is a translation of our Fun-language into the K-language. The

main difficulty is that we need to order the computation---in the

K-language we only have linear sequences of instructions. Before we

explain this, we have a look at some programs in CPS-style.

\subsection*{CPS-Translations}

CPS stands for \emph{Continuation-Passing-Style}. It is a kind of

programming technique often used in advanced functional programming

and in particular in compilers. Before we delve into the

CPS-translation for our Fun-language compiler, let us look at

CPS-versions of some well-known functions. Consider

\begin{lstlisting}[language=Scala, numbers=none]

def fact(n: Int) : Int = 

  if (n == 0) 1 else n * fact(n - 1) 

\end{lstlisting}

\noindent 

This is clearly the usual factorial function. But now consider the

following slight variant of the factorial function:

\begin{lstlisting}[language=Scala, numbers=left]

def factC(n: Int, k: Int => Int) : Int = 

  if (n == 0) k(1) 

  else factC(n - 1, x => k(n * x)) 

factC(3, id)

\end{lstlisting}

\noindent

The function is is Lines 1--3. The function call is in Line 5.  We

call this function with a number, in this case 3, and the

identity-function (which returns just its input). The \texttt{k} is

the continuation in this function. The recursive calls are:

\begin{lstlisting}[language=Scala, numbers=none]

factC(2, x => id(3 * x))

factC(1, x => id(3 * (2 * x)))

factC(0, x => id(3 * (2 * (1 * x))))

\end{lstlisting}

\noindent

Having reached 0, we get out of the recursion and apply 1 to the

continuation (see if-branch above in Line 2). This gives

\begin{lstlisting}[language=Scala, numbers=none]

id(3 * (2 * (1 * 1)))

= 3 * (2 * (1 * 1))

= 6

\end{lstlisting}

\noindent

which is the expected result. If this looks somewhat familiar to you,

than this is because functions with continuations can be seen as a

kind of generalisation of tail-recursive functions.  So far, however,

we did not look at this generalisation in earnest.  Anyway notice how

the continuations is ``stacked up'' during the recursion and then

``unrolled'' when we apply 1 to the continuation. Interestingly, we

can do something similar to the Fibonacci function where in the

traditional version we have two recursive calls. Consider the

following function

\begin{lstlisting}[language=Scala, numbers=left]

def fibC(n: Int, k: Int => Int) : Int = 

  if (n == 0 || n == 1) k(1) 

  else fibC(n - 1,

             r1 => fibC(n - 2,

               r2 => k(r1 + r2)))

\end{lstlisting}

\noindent

Here the continuation (Lines 4--5) is a nested function essentially wrapping up 

the second recursive call plus the original continuation. Let us check how the recursion unfolds

when called with 3 and the identity function:

\begin{lstlisting}[language=Scala, numbers=none]

fibC(3, id)

fibC(2, r1 => fibC(1, r2 => id(r1 + r2)))

fibC(1, r1 => 

   fibC(0, r2 => fibC(1, r2a => id((r1 + r2) + r2a))))

fibC(0, r2 => fibC(1, r2a => id((1 + r2) + r2a)))

fibC(1, r2a => id((1 + 1) + r2a))

id((1 + 1) + 1)

(1 + 1) + 1

3

\end{lstlisting}