--- a/handouts/ho09.tex Tue May 30 13:27:54 2023 +0100
+++ b/handouts/ho09.tex Tue May 30 16:58:06 2023 +0100
@@ -25,7 +25,7 @@
expressions into a sequence of instructions manipulating the
stack. 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 operations and a chunk of
+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
@@ -34,11 +34,11 @@
tractable for this module, we target the LLVM Intermediate
Language. 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. By using LLVM, however,
-we also have to pay price in the sense that generating code gets
-harder\ldots{}unfortunately.
+worry about things like register allocations. By using the LLVM-IR,
+however, we also have to pay price in the sense that generating code
+gets harder\ldots{}unfor\-tunately.
-\subsection*{LLVM and LLVM-IR}
+\subsection*{LLVM and the LLVM-IR}
\noindent LLVM is a beautiful example
that projects from Academia can make a difference in the World. LLVM
@@ -61,8 +61,8 @@
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, for example, the compiler generate code for different
-CPUs, say X86 or ARM. That means we can be agnostic about where our
+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
@@ -71,7 +71,7 @@
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),
because that is what the LLVM-IR expects from us. A reason why LLVM
-uses the SSA format, rather than JVM-like stack instructions, is that
+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. Also it is hard to find out if all the calculations on
@@ -84,12 +84,13 @@
format is designed for.
-The main idea behind the SSA format is to use very simple variable
-assignments where every tmp-variable is assigned only once. The
-assignments also need to be primitive in the sense that they can be
-just simple operations like addition, multiplication, jumps,
+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 SSA format is
+$((1 + a) + (foo(3 + 2) + (b * 5)))$, then the corresponding sequence
+of assignments in SSA-format are:
\begin{lstlisting}[language=LLVMIR,numbers=left]
let tmp0 = add 1 a in
@@ -101,16 +102,17 @@
return tmp5
\end{lstlisting}
-\noindent where every tmp-variable is used only once (we could, for
+\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).
+would not change the overall result). At the end we have a return-instruction
+wich contains the final result of the expression.
There are sophisticated algorithms for imperative languages, like C,
-that efficiently transform a high-level program into SSA format. But
+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
-Continuation-Passing-Style---basically black programming art or
+\emph{Continuation-Passing-Style}---basically black programming art or
abracadabra programming. So sit tight.
\subsection*{LLVM-IR}
@@ -232,10 +234,10 @@
\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 sqr-function in \texttt{@main} with the argument \texttt{5}
+ 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. It stores
- the result of sqr in the variable called \texttt{\%i} and then
+ code for the \texttt{sqr}-function is in Lines 13 -- 16. It 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}
@@ -290,12 +292,12 @@
``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
+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:
+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 {
@@ -312,9 +314,9 @@
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 point we need to be careful, however, is that the
-arguments of the binary operations and function calls are in fact only
-variables or numbers. To encode this constraint into the type-system
-of Scala would make things too complicated---to satisfy this
+arguments of the binary operations and function calls can in fact only
+be variables or numbers. To encode this constraint into the type-system
+of Scala, however, would make things too complicated---to satisfy this
constraint is therefore on us. For our Fun-language, we will later on
consider some further K-values.
@@ -333,7 +335,7 @@
\noindent
By having in \texttt{KLet} taking first a string (standing for an
intermediate variable) and second a value, we can fulfil the SSA
-constraint in assignments ``by construction''---there is no way we
+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
@@ -343,21 +345,20 @@
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
+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 computationa---in teh
-K-language we only have linear sequence of instructions to need to be
-followed. Before we explain this, we have a look at some
-CPS-translation.
+main difficulty is that we need to order the computation---in the
+K-language we only have linear sequence of instructions. Before we
+explain this, we have a look at some CPS-translation.
\subsection*{CPS-Translations}
-CPS stands for Continuation-Passing-Style. It is a kind of programming
+CPS stands for \emph{Continuation-Passing-Style}. It is a kind of programming
technique often used in advanced functional programming. Before we delve
-into the CPS-translation for our Fun-language, let us look at
+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]
@@ -378,7 +379,7 @@
\end{lstlisting}
\noindent
-This function is called with the number, in this case 3, and the
+This function is called with a number, in this case 3, and the
identity-function (which returns just its input). The recursive
calls are:
@@ -435,7 +436,7 @@
\end{lstlisting}
\noindent
-The point of this section is that you are playing around with these
+The point of this section is that you should be playing around with these
simple definitions and make sure they calculate the expected result.
In the next step, you should understand \emph{how} these functions
calculate the result with the continuations.
@@ -443,11 +444,13 @@
\section*{Worked Example}
-Let us now come back to the CPS-translations for the Fun-language. The
-main difficulty of generating instructions in SSA format is that large
-compound expressions need to be broken up into smaller pieces and
-intermediate results need to be chained into later instructions. To do
-this conveniently, we use the CPS-translation mechanism. What continuations
+Let us now come back to the CPS-translations for the Fun-language.
+Though we will start with a simpler subset containing only numbers,
+arithmetic expressions and function calls. The main difficulty of
+generating instructions in SSA-format is that large compound
+expressions need to be broken up into smaller pieces and intermediate
+results need to be chained into later instructions. To do this
+conveniently, we use the CPS-translation mechanism. What continuations
essentially solve is the following problem: Given an expressions
\begin{equation}\label{exp}
@@ -455,20 +458,23 @@
\end{equation}
\noindent
-which of the subexpressions should be calculated first? We just arbitrarily
-going to decide that it will be from left to right. This means we have
-to tear the expression shown in \eqref{exp} apart as follows:
+which of the subexpressions should be calculated first? We just
+arbitrarily going to decide that it will be from left to right. Other
+languages make different choices---C famously is right to left. In our
+case this means we have to tear the expression shown in \eqref{exp}
+apart as follows:
\[
(1 + 2) \qquad\text{and}\qquad \Box * (3 + 4)
\]
\noindent
-The first one will give us a result, but the second one is not
-really an expression that makes sense. It will only make sense
-as the next step in the computation when we fill-in the result of
-$1+2$ into the ``hole'' indicated by $\Box$. Such incomplete
-expressions can be represented in Scala as functions
+The first subexpression can be calculated and will give us a result,
+but the second one is not really an expression that makes sense. It
+will only make sense as the next step in the computation when we
+fill-in the result of $1+2$ into the ``hole'' indicated by
+$\Box$. Such incomplete expressions can be represented in Scala as
+functions
\begin{lstlisting}[language=Scala, numbers=none]
r => r * (3 + 4)
@@ -476,7 +482,7 @@
\noindent
where \texttt{r} is any result that has been computed earlier. By the
-way in lambda-calculus notation we would write
+way, in lambda-calculus notation we would write
$\lambda r.\, r * (3 + 4)$ for the same function. To sum up, we use
functions (``continuations'') to represent what is coming next in a
sequence of instructions. In our case, continuations are functions of
@@ -498,7 +504,7 @@
apply an argument to the continuation (remember they are functions)
we essentially fill something into the corresponding hole.
-Lets look at concrete code. To simplify matters first,
+Lets look at some concrete code. To simplify matters,
suppose our source language consists just of three kinds of expressions
\begin{lstlisting}[language=Scala,numbers=none]
@@ -519,18 +525,23 @@
\noindent
where \code{k} is the continuation and \code{e} is the expression to
-be compiled. In case we have numbers, we can just pass them to the
-continuations because numbers need not be further teared apart as they
-are already atomic. Passing the number to the continuation means we
-apply the continuation like
+be compiled. The result of the function is a K-expression, which later
+can be compiled into LLVM-IR code.
+
+In case we have numbers, then we can just pass them in CPS-translation
+to the continuations because numbers need not be further teared apart
+as they are already atomic. Passing the number to the continuation
+means we apply the continuation like
\begin{lstlisting}[language=Scala,numbers=none]
case Num(i) => k(KNum(i))
\end{lstlisting}
-\noindent This would just fill in the $\Box$ in a \code{KLet}-expression.
-There is no need to create a temporary variable for simple numbers.
-More interesting is the case for arithmetic operations.
+\noindent This would fill in the $\Box$ in a \code{KLet}-expression.
+Since \texttt{k} is a function from \texttt{KVar} to \texttt{KExp} we
+also optain the correct type for CPS, namely \texttt{KExp}. There is
+no need to create a temporary variable for simple numbers. More
+interesting is the case for arithmetic operations.
\begin{lstlisting}[language=Scala,numbers=none,xleftmargin=0mm]
case Aop(op, e1, e2) => {
@@ -557,7 +568,7 @@
subexpression and one for the right subexpression. We can fill both
holes by calling the CPS function twice---this is a bit of the black
art in CPS. We can use the second call of CPS as the continuation
-of the first call: The reason is that
+of the first call. The reason is that
\begin{lstlisting}[language=Scala,numbers=none,xleftmargin=0mm]
r1 => CPS(e2)(r2 => KLet(z, KAop(op, r1, r2), k(KVar(z))))
@@ -569,11 +580,12 @@
computation of the arithmetic expression is stored in \texttt{z} to the
computations that follow. In this way we apply the continuation \texttt{k} with this
new variable (essentially we are plugging in a hole further down the line).
+Hope this makes sense.
The last case we need to consider in our small expression language are
function calls.
-\begin{lstlisting}[language=Scala,numbers=none,xleftmargin=0mm]
+\begin{lstlisting}[language=Scala,numbers=left,xleftmargin=0mm]
case Call(fname, args) => {
def aux(args: List[Expr], vs: List[KVal]): KExp = args match {
case Nil => {
@@ -591,15 +603,17 @@
function arguments---remember in our source language we can have calls
such as $foo(3 + 4, g(3))$ where we first have to create temporary
variables (and computations) for each argument. Therefore \texttt{aux}
-analyses the arguments from left to right. In case there is an argument
-\texttt{a} on the front of the list (the case \texttt{a::as}), we call
-CPS recursively for the corresponding subexpression. The temporary variable
-containing the result for this argument we add to the end of the K-values we
-have already analysed before. Again very conveniently we can use the
-recursive call to \texttt{aux} as the continuation for the computations
-that follow. If we reach the end of the argument list (the \texttt{Nil}-case),
-then we collect all the K-values \texttt{v1} to \texttt{vn} and call the
-function in the K-language like so
+analyses the argument list from left to right. In case there is an
+argument \texttt{a} on the front of the list (the case \texttt{a::as}
+in Line 7), we call CPS recursively for the corresponding
+subexpression. The temporary variable containing the result for this
+argument we add to the end of the K-values we have already analysed
+before. Again very conveniently we can use the recursive call to
+\texttt{aux} as the continuation for the computations that
+follow. When we reach the end of the argument list (the
+\texttt{Nil}-case in Lines 3--6), then we collect all the K-values
+\texttt{v1} to \texttt{vn} and call the function in the K-language
+like so
\begin{lstlisting}[language=LLVMIR,numbers=none,escapeinside={(*@}{@*)}]
@@ -626,14 +640,33 @@
should start CPS? It turns out that this initial continuation will be the last one that is
called. What should be the last step in the computation? Yes, we need to return the
temporary variable where the last result is stored (if it is a simple number, then we can
-just return this number). Therefore we cal CPS with the code
+just return this number). Therefore we call CPS with the code
\begin{lstlisting}[language=Scala,numbers=none]
def CPSi(e: Expr) : KExp = CPS(e)(KReturn(_))
\end{lstlisting}
\noindent
-where we give the function \code{KReturn(_)}.
+where we give the function \code{KReturn(_)} as the continuation. With
+this we completed the translation of expressions into our
+K-language. Figure~\ref{absfun} gives the complete datatypes for the
+ASTs of the Fun-language and the K-values and K-expressions for the
+K-language.
+
+Having obtained a K-expression, it is relatively straightforward to
+generate a valid program for the LLVM-IR. We leave this to the
+attentive reader. What else can we do? Well it should be relatively
+easy to apply some common optimisations to the K-expressions. One
+optimisations is called constant folding---for example if we have an
+expression $3 + 4$ then we can replace it by just $5$ instead of
+generating code to compute $5$. Now this information needs to be
+propagated to the next computation step to see whether any further
+constant foldings are possible. Another useful technique is common
+subexpression elimination, meaning if you have twice a calculation of
+a function $foo(a)$, then we want to call it only once and store the
+result in a temporary variable that can be used instead of the second,
+or third, call to $foo(a)$. Again I leave this to the attentive reader
+to work out and implement.
\begin{figure}[p]\small
@@ -652,7 +685,6 @@
case class Bop(o: String, a1: Exp, a2: Exp) extends BExp
-
// K-language (K-expressions, K-values)
abstract class KExp
abstract class KVal
@@ -667,7 +699,7 @@
case class KLet(x: String, v: KVal, e: KExp) extends KExp
case class KReturn(v: KVal) extends KExp
\end{lstlisting}
-\caption{Abstract syntax trees for the Fun-language.\label{absfun}}
+\caption{Abstract syntax trees for the Fun-language and the K-language.\label{absfun}}
\end{figure}