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\title{
INFORMATION AND COMMUNICATION TECHNOLOGIES\\
(ICT)\\
PROGRAMME\\
\vspace*{1cm}Project FP7-ICT-2009-C-243881 \cerco{}}
\date{ }
\author{}
\begin{document}
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\includegraphics[width=0.6\textwidth]{../../style/cerco_logo.png}
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\maketitle
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\begin{LARGE}
\bf
Report n. D3.3\\
Executable Formal Semantics of front end intermediate languages\\
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\begin{center}
\begin{large}
Version 1.0
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\begin{center}
\begin{large}
Main Authors:\\
Brian~Campbell
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\vspace*{\fill}
\noindent
Project Acronym: \cerco{}\\
Project full title: Certified Complexity\\
Proposal/Contract no.: FP7-ICT-2009-C-243881 \cerco{}\\
\clearpage \pagestyle{myheadings} \markright{\cerco{}, FP7-ICT-2009-C-243881}
\newpage
\vspace*{7cm}
\paragraph{Abstract}
We report on the formalization of the front end intermediate languages for the
\cerco{} project's compiler using executable semantics in the \matita{} proof
assistant. This includes common definitions for fresh identifier handling,
$n$-bit arithmetic and operations and testing of the smeantics. We also
consider embedding invariants into the semantics for showing correctness
properties.
\newpage
\tableofcontents
% TODO: clear up any -ize vs -ise
% TODO: clear up any "front end" vs "front-end"
% TODO: clear up any mentions of languages that aren't textsf'd.
% TODO: fix unicode in listings
\section{Introduction}
This work formalizes the front end intermediate languages from the \cerco{}
prototype compiler, described in previous deliverables~\cite{d2.1,d2.2}. The
front end of the compiler is summarized in Figure~\ref{fig:summary} including
the intermediate languages and the compiler passes described in the
accompanying deliverable 3.2. We have also refined parts of the formal
development that are used for several of the languages in the compiler.
The input language to the formalized front end is the \textsf{Clight}
language. The executable semantics for this language were presented in a
previous deliverable~\cite{d3.1}. Here we will report on some
minor changes to its semantics made to better align it with the whole
development.
The formalization of each language takes the form of definitions for abstract
syntax and functions providing a small-step executable semantics. This is
done in the Calculus of Inductive Constructions (CIC), as implemented in the
\matita{} proof assistant. These definitions will be essential for the
correctness proofs of the compiler in task 3.4.
Finally, we will report on work to add several invariants to the
languages. This activity overlaps with task 3.4 on the correctness of the
compiler front end. However, the use of dependent types mean that this work
is tied closely to the definition of the languages and the transformations of
the front end in task 3.2. By considering it now we can experiment with and
judge its impact on the formal semantics, and how feasible retrofitting such
invariants is.
\begin{figure}
\begin{center}
\begin{minipage}{.8\linewidth}
\begin{tabbing}
\quad \= $\downarrow$ \quad \= \kill
\textsf{C} (unformalized)\\
\> $\downarrow$ \> CIL parser (unformalized \ocaml)\\
\textsf{Clight}\\
\> $\downarrow$ \> cast removal\\
\> $\downarrow$ \> add runtime functions\\
\> $\downarrow$ \> labelling\\
\> $\downarrow$ \> stack variable allocation and control structure
simplification\\
\textsf{Cminor}\\
\> $\downarrow$ \> generate global variable initialisation code\\
\> $\downarrow$ \> transform to RTL graph\\
\textsf{RTLabs}\\
\> $\downarrow$ \> start of target specific backend\\
\>\quad \vdots
\end{tabbing}
\end{minipage}
\end{center}
\caption{Front end languages and transformations}
\label{fig:summary}
\end{figure}
\subsection{Revisions to the prototype compiler}
Ongoing work to maintain and improve the prototype compiler has resulted in
several changes, mostly minor. The most important change is that the
transformations to replace 16 and 32 bit types have been moved from the
\textsf{Clight} language to the target-specific stage between \textsf{RTLabs}
and \textsf{RTL} to help generate better code, and the addition of a
\textsf{Clight} cast removal transformation to reduce the number of 16 and 32
bit operations.
The formalized semantics have tracked these changes, and in this report we
describe them as they currently stand.
\section{Definitions common to several languages}
The semantics for many of the languages in the compiler share some core parts:
the memory model, environments, runtime values, definitions of operations and
a monad for encapsulating failure and I/O (using resumptions). The executable
memory model was ported from CompCert as part of the work on \textsf{Clight}
and was reused for the front end languages\footnote{However, it is likely that
we will revise the memory model to make it better suited for describing the
back-end of our compiler.}. The failure and I/O monad was introduced in the
previous deliverable on \textsf{Clight}~\cite[\S4.2]{d3.1}. In all of the
languages except \textsf{Clight} we have a basic form of type, \lstinline'typ'
identifying integers and pointers along with their sizes. The other parts
are discussed below, with the only change to the runtime values being the
representation of integers.
\subsection{Identifiers}
Each language features one or more kinds of names to represent variables, such
as registers, \texttt{goto} labels or RTL graph labels. We also need to
describe various maps whose domain is a set of identifiers when defining the
semantics and compilation.
Previous work on the executable semantics of the target machine
code included bit vectors and bit vector tries to define various integers in
the semantics, and give a low level view of memory~\cite{d4.1}. To keep the
size of the development down we have reused these data structures for
identifiers and maps respectively.
One difficulty with using fixed size bit vectors for identifiers is that
fresh name generation can fail if generate too many. While we use an error
monad to deal with failures, we wish to minimize its use in the compiler.
Thus we add a flag to detect overflows, and check it after the phase of the
compiler is complete to report exhaustion. The rest of the phase can then be
written as if name generation always succeeds. In practice this will never
occur on normal programs because more identifiers of each sort are available
than bytes of code memory on the target.
Given the wide variety of identifiers used in the compiler we also
wish to separate the different classes of identifier. Thus we encapsulate the
bit vector representing the identifier in a datatype that also carries a tag
identifying which class of identifier we are using:
\begin{lstlisting}
inductive identifier (tag:String) : Type[0] ≝
an_identifier : Word $\rightarrow$ identifier tag.
\end{lstlisting}
The tries are also tagged in the same manner. These tags have also proved
useful during testing by making the resulting terms more readable.
\subsection{Machine integers and arithmetic}
The bit vectors in~\cite{d4.1} also came equipped with some basic arithmetic
for the target semantics. The front end required these operations to be
generalized and extended. In particular, we required operations such as zero
and sign extension and translation between bit vectors and full integers. It
also became apparent that while the original definitions worked reasonably on
8-bit vectors, they did not scale up to 32-bit integers. The definitions were
then reworked to make them efficient enough to animate programs in the
front-end semantics.
\subsection{Front end operations}
The two front end intermediate languages, \textsf{Cminor} and \textsf{RTLabs},
share the same set of operations on values. They differ from
\textsf{Clight}'s operations by incorporating casts and by having a separate
operation for each type of data operated upon. For example, subtraction of
pointers is treated as a different operation from subtraction of integers.
A common semantics is given for these operations in the form of simple
CIC functions on the operation and runtime values.
\section{\textsf{Clight} modifications}
The \textsf{Clight} input language remained largely the same as in the
previous deliverable~\cite{d3.1}. The principal changes were to use the
identifiers and arithmetic described above in place of the arbitrarily large
integers used before. For the identifiers, this relieved us of the burden of
adding an efficient datatype for maps by reusing the bit vector tries instead.
The arithmetic replaced a dependent pair of an arbitrary integer and a proof
that it was in range of 32 bit integers by the exact bit vector for each
size of integer. This direct approach is closer to the implementation and
more obviously correct --- no extra precision can be left in by accident.
\section{\textsf{Cminor}}
The \textsf{Cminor} language does not store local variables in memory, and has
simpler control structures than \textsf{Clight}. It is similar in nature to
the \textsf{Cminor} language in CompCert, although the semantics have been
based on the \cerco{} prototype rather than ported from CompCert. The syntax
is similar to the prototype, except that the types attached to expressions are
restricted so that some corner cases are ruled out in the \textsf{Cminor} to
\textsf{RTLabs} stage (see the accompanying deliverable 3.2 for details):
\begin{lstlisting}
inductive expr : typ $\rightarrow$ Type[0] ≝
| Id : $\forall$t. ident $\rightarrow$ expr t
| Cst : $\forall$t. constant $\rightarrow$ expr t
| Op1 : $\forall$t,t'. unary_operation $\rightarrow$ expr t $\rightarrow$ expr t'
| Op2 : $\forall$t1,t2,t'. binary_operation $\rightarrow$ expr t1 $\rightarrow$ expr t2 $\rightarrow$ expr t'
| Mem : $\forall$t,r. memory_chunk $\rightarrow$ expr (ASTptr r) $\rightarrow$ expr t
| Cond : $\forall$sz,sg,t. expr (ASTint sz sg) $\rightarrow$ expr t $\rightarrow$ expr t $\rightarrow$ expr t
| Ecost : $\forall$t. costlabel $\rightarrow$ expr t $\rightarrow$ expr t.
\end{lstlisting}
For example, note that conditional expressions only switch on integer
expressions.
In principle we could extend this to statically ensure that only well-typed
\textsf{Cminor} expressions are considered, and we will consider this as part
of the work on correctness in task 3.4.
We also provide a variant of the syntax where the only initialization data is
the size of each global variable, for use after the initialization code has
been generated.
The definition of the semantics is routine: a functional definition of a
single small step of the machine is given, reusing the memory model,
environments, arithmetic and operations mentioned above.
\section{\textsf{RTLabs}}
The \textsf{RTLabs} language provides a target independent Register Transfer
Language, where programs are represented as control flow graphs. We use the
identifiers described above for the graph labels and the maps for the graph
itself. The tagging mechanism ensures that labels cannot be mixed up with
other identifiers in the program (in particular, we cannot accidentally reuse
a \texttt{goto} label from Cminor where a graph label should appear).
Otherwise, the syntax and semantics of \textsf{RTLabs} mirrors that of the
prototype compiler. Some of the syntax is shown below, including the type of
the control flow graphs. The same common elements are used as for \textsf{Cminor},
including the front end operations mentioned above.
\begin{lstlisting}[language=matita]
inductive statement : Type[0] ≝
| St_skip : label $\rightarrow$ statement
| St_cost : costlabel $\rightarrow$ label $\rightarrow$ statement
| St_const : register $\rightarrow$ constant $\rightarrow$ label $\rightarrow$ statement
| St_op1 : unary_operation $\rightarrow$ register $\rightarrow$ register $\rightarrow$ label $\rightarrow$ statement
...
| St_return : statement
.
definition graph : Type[0] $\rightarrow$ Type[0] ≝ identifier_map LabelTag.
record internal_function : Type[0] ≝
{ f_labgen : universe LabelTag
; f_reggen : universe RegisterTag
; f_result : option (register $\times$ typ)
; f_params : list (register $\times$ typ)
; f_locals : list (register $\times$ typ)
; f_stacksize : nat
; f_graph : graph statement
}.
\end{lstlisting}
\section{Testing}
To provide some assurance that the semantics were properly implemented, and to
support the testing described in the accompanying deliverable 3.2, we have
adapted the pretty printers in the prototype compiler to produce \matita{}
terms for the syntax of each language described above.
A few common definitions were added for animating the small step semantics
definitions for any of the front end languages in \matita. We then used a
small selection of test cases to ensure basic functionality. However, this is
still a time consuming process, so more testing will carried out once the
extraction of CIC terms to \ocaml{} programs is implemented in \matita.
\section{Embedding invariants}
Each phase of the prototype compiler can fail in a number of places if the
input language permits programs that are badly structured in some sense: a
missing label in a \texttt{goto} statement or CFG, an undefined variable name,
a \texttt{break} statement outside of a loop or \texttt{switch}, and so on.
We wish to restrict our intermediate languages using dependent types to remove
as many `junk' programs as possible to rule out such failures. We also hope
that such restrictions will help in other correctness proofs.
This goal lies in the overlap between several tasks in the project: it
involves manipulating the syntax and semantics of the intermediate languages
(the present work), the encoding of the front-end compiler phases in \matita{}
(task 3.2) and the correctness of the front-end (task 3.4). Thus this work is
rather experimental; it is being carried out on branches in our source code
repository and the final form will be decided and merged in during task 3.4.
So far we have tried adding two forms of invariant --- one using dependent
types to index statements in \textsf{Cminor} by their block depth, and the
other asserts that variables and labels are present in the appropriate
environments by adding a separate invariant to each function. Note that these
do not yet cover all of the properties that a program in these languages is
expected to enjoy; for example, there are currently no checks that references
to globals are well-defined.
\subsection{\textsf{Cminor} block depth}
The \textsf{Cminor} language has relatively simple control structures.
Statements are provided for infinite loops, non-looping blocks and
exiting an arbitrary number of blocks (for \texttt{break}, failing loop guards
and the switch statement\footnote{We are considering replacing the
\textsf{Cminor} switch statement with one that uses \texttt{goto}-like labels
in both the prototype and the formalized compilers, but for now we stick with
this CompCert-style arrangement.}).
However, this means that there are badly-formed \textsf{Cminor} programs such
as
\begin{lstlisting}[language={}]
int main() {
block {
loop {
exit 5
}
}
}
\end{lstlisting}
where we attempt to exit more blocks than exist. To rule these out (including
demonstrating that the previous phase of the compiler does not generate them)
we can index the statements of the language by the depth of the enclosing
blocks.
The adaption of the syntax adds the depth to every statement, and uses bounded
integers in the exit and switch statements:
\begin{lstlisting}[language=matita]
inductive stmt : $\forall$blockdepth:nat. Type[0] ≝
| St_skip : $\forall$n. stmt n
...
| St_loop : $\forall$n. stmt n $\rightarrow$ stmt n
| St_block : $\forall$n. stmt (S n) $\rightarrow$ stmt n
| St_exit : $\forall$n. Fin n $\rightarrow$ stmt n
(* expr to switch on, table of , default *)
| St_switch : expr $\rightarrow$ $\forall$n. list (int $\times$ (Fin n)) $\rightarrow$ Fin n $\rightarrow$ stmt n
...
\end{lstlisting}
where \texttt{stmt n} is a statement enclosed in \texttt{n} blocks,
and \texttt{Fin n} is a standard construction for a natural number which is at
most \texttt{n}.
In the semantics the number of blocks is also added to the continuations and
state, and the function to find the continuation from an exit statement can be
made failure-free.
We note in passing that adding this parameter detected a small mistake in the
semantics concerning continuations and tail calls, although the mistake itself
was benign.
\subsection{Identifier invariants}
To show that the variables and labels occurring in the body of a function
are present in the relevant structures we add an additional invariant to the
function records.
For \textsf{Cminor} we use a higher-order predicate which recursively applies
a predicate to all substatements:
\begin{lstlisting}[language=matita]
let rec stmt_P (P:stmt $\rightarrow$ Prop) (s:stmt) on s : Prop ≝
match s with
[ St_seq s1 s2 $\Rightarrow$ P s $\wedge$ stmt_P P s1 $\wedge$ stmt_P P s2
| St_ifthenelse _ _ _ s1 s2 $\Rightarrow$ P s $\wedge$ stmt_P P s1 $\wedge$ stmt_P P s2
| St_loop s' $\Rightarrow$ P s $\wedge$ stmt_P P s'
| St_block s' $\Rightarrow$ P s $\wedge$ stmt_P P s'
| St_label _ s' $\Rightarrow$ P s $\wedge$ stmt_P P s'
| St_cost _ s' $\Rightarrow$ P s $\wedge$ stmt_P P s'
| _ $\Rightarrow$ P s
].
\end{lstlisting}
Dependent pattern matching on statements thus allows an accompanying
\lstinline'stmt_P' fact to be unfold to the predicate on the current statement
and the predicate applied to all substatements.
We require two properties to hold in \textsf{Cminor} functions:
\begin{enumerate}
\item All variables in the body are present in the list of parameters or the
list of variables for the function (this also uses a similar recursive
predicate on expressions).
\item All labels in \texttt{goto} statements appear in a label statement.
\end{enumerate}
The function definition thus becomes:
\begin{lstlisting}[language=matita]
record internal_function : Type[0] ≝
{ f_return : option typ
; f_params : list (ident $\times$ typ)
; f_vars : list (ident $\times$ typ)
; f_stacksize : nat
; f_body : stmt
; f_inv : stmt_P ($\lambda$s.stmt_vars ($\lambda$i.Exists ? ($\lambda$x.\fst x = i) (f_params @ f_vars)) s $\wedge$
stmt_labels ($\lambda$l.Exists ? ($\lambda$l'.l' = l) (labels_of f_body)) s) f_body
}.
\end{lstlisting}
where \lstinline'stmt_vars' and \lstinline'stmt_labels' constrain the
variables and labels that appear directly in a statement (but not
substatements) to appear in the given list, and
\lstinline'labels_of' returns a list of all the labels defined in a
statement.
The \textsf{Clight} semantics can be amended to use these invariants, although
the main benefit is for the compiler stages (see the accompanying deliverable
3.2 for details). The semantics require the invariants to be added to the
state and continuations. It was convenient to split the continuations between
the local continuation representing the rest of the code to be executed within
the function, and the stack of function calls because it becomes easier to
state the property on the local continuation alone.
The invariant for variables is
slightly different --- we require that every variable appear in the local
environment. We use \lstinline'f_inv' from the function to establish this
invariant when the environment is set up on function entry.
It is unclear whether changing the semantics is really worthwhile. It
witnesses that the invariants are those we wanted, but makes no difference to
the actual execution of the program, especially as the execution can still
fail due to genuine runtime errors. Moreover, it is unclear what effect the
presence of proof terms and more dependent pattern matching in the semantics
will have on the complexity of future correctness proofs. We plan to examine
this issue during task 3.4.
We use a similar method to specify the invariant that the \textsf{RTLabs}
graph is closed --- that is, any successor labels in a statement in the graph
are present in the graph. The definition is simpler in \textsf{RTLabs}
because the flat representation of the graph does not require recursive
definitions like \lstinline'stmt_P' above.
\section{Conclusion}
We have developed executable semantics for each of the front-end languages of
the \cerco{} compiler. These will form the basis of the correctness
statements for each stage of the compiler in task 3.4. We have also shown
that useful invariants can be added as dependent types, and intend to use
these in subsequent work.
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