\section{The proof}
In this section, we will present the correctness proof of the algorithm in more
detail.
The main correctness statement is as follows:
\begin{lstlisting}
definition sigma_policy_specification :=:
$\lambda$program: pseudo_assembly_program.
$\lambda$sigma: Word → Word.
$\lambda$policy: Word → bool.
sigma (zero $\ldots$) = zero $\ldots$ $\wedge$
$\forall$ppc: Word.$\forall$ppc_ok.
let instr_list := \snd program in
let pc ≝ sigma ppc in
let labels := \fst (create_label_cost_map (\snd program)) in
let lookup_labels :=
$\lambda$x.bitvector_of_nat ? (lookup_def ?? labels x 0) in
let instruction :=
\fst (fetch_pseudo_instruction (\snd program) ppc ppc_ok) in
let next_pc := \fst (sigma (add ? ppc (bitvector_of_nat ? 1))) in
(nat_of_bitvector $\ldots$ ppc ≤ |instr_list| →
next_pc = add ? pc (bitvector_of_nat $\ldots$
(instruction_size lookup_labels sigma policy ppc instruction)))
$\wedge$
((nat_of_bitvector $\ldots$ ppc < |instr_list| →
nat_of_bitvector $\ldots$ pc < nat_of_bitvector $\ldots$ next_pc)
$\vee$ (nat_of_bitvector $\ldots$ ppc = |instr_list| → next_pc = (zero $\ldots$))).
\end{lstlisting}
Informally, this means that when fetching a pseudo-instruction at $ppc$, the
translation by $\sigma$ of $ppc+1$ is the same as $\sigma(ppc)$ plus the size
of the instruction at $ppc$; i.e. an instruction is placed immediately
after the previous one, and there are no overlaps.
The other condition enforced is the fact that instructions are stocked in
order: the memory address of the instruction at $ppc$ should be smaller than
the memory address of the instruction at $ppc+1$. There is one exeception to
this rule: the instruction at the very end of the program, whose successor
address can be zero (this is the case where the program size is exactly equal
to the amount of memory).
And finally, we enforce that the program starts at address 0, i.e.
$\sigma(0) = 0$.
Since our computation is a least fixed point computation, we must prove
termination in order to prove correctness: if the algorithm is halted after
a number of steps without reaching a fixed point, the solution is not
guaranteed to be correct. More specifically, jumps might be encoded whose
displacement is too great for the instruction chosen.
Proof of termination rests on the fact that jumps can only increase, which
means that we must reach a fixed point after at most $2n$ iterations, with
$2n$ the number of jumps in the program. This worst case is reached if at every
iteration, we change the encoding of exactly one jump; since a jump can change
from {\tt short} to {\tt medium} and from {\tt medium} to {\tt long}, there
can be at most $2n$ changes.
This proof has been executed in the ``Russell'' style from~\cite{Sozeau2006}.
We have proven some invariants of the {\sc f} function from the previous
section; these invariants are then used to prove properties that hold for every
iteration of the fixed point computation; and finally, we can prove some
properties of the fixed point.
\subsection{Fold invariants}
These are the invariants that hold during the fold of {\sc f} over the program,
and that will later on be used to prove the properties of the iteration.
Note that during the fixed point computation, the $\sigma$ function is
implemented as a trie for ease access; computing $\sigma(x)$ is done by looking
up the value of $x$ in the trie. Actually, during the fold, the value we
pass along is a couple $\mathbb{N} \times \mathtt{ppc_pc_map}$. The natural
number is the number of bytes added to the program so far with respect to
the previous iteration, and {\tt ppc\_pc\_map} is a couple of the current
size of the program and our $\sigma$ function.
\begin{lstlisting}
definition out_of_program_none :=
$\lambda$prefix:list labelled_instruction.$\lambda$sigma:ppc_pc_map.
$\forall$i.i < 2^16 → (i > |prefix| $\leftrightarrow$
bvt_lookup_opt $\ldots$ (bitvector_of_nat ? i) (\snd sigma) = None ?).
\end{lstlisting}
This invariant states that every pseudo-address not yet treated cannot be
found in the lookup trie.
\begin{lstlisting}
definition not_jump_default ≝
$\lambda$prefix:list labelled_instruction.$\lambda$sigma:ppc_pc_map.
$\forall$i.i < |prefix| →
¬is_jump (\snd (nth i ? prefix $\langle$None ?, Comment []$\rangle$)) →
\snd (bvt_lookup $\ldots$ (bitvector_of_nat ? i) (\snd sigma)
$\langle$0,short_jump$\rangle$) = short_jump.
\end{lstlisting}
This invariant states that when we try to look up the jump length of a
pseudo-address where there is no jump, we will get the default value, a short
jump.
\begin{lstlisting}
definition jump_increase :=
λprefix:list labelled_instruction.λop:ppc_pc_map.λp:ppc_pc_map.
∀i.i ≤ |prefix| →
let $\langle$opc,oj$\rangle$ :=
bvt_lookup $\ldots$ (bitvector_of_nat ? i) (\snd op) $\langle$0,short_jump$\rangle$ in
let $\langle$pc,j$\rangle$ :=
bvt_lookup $\ldots$ (bitvector_of_nat ? i) (\snd p) $\langle$0,short_jump$\rangle$ in
jmpleq oj j.
\end{lstlisting}
This invariant states that between iterations ($op$ being the previous
iteration, and $p$ the current one), jump lengths either remain equal or
increase. It is needed for proving termination.
\begin{lstlisting}
definition sigma_compact_unsafe :=
λprogram:list labelled_instruction.λlabels:label_map.λsigma:ppc_pc_map.
∀n.n < |program| →
match bvt_lookup_opt $\ldots$ (bitvector_of_nat ? n) (\snd sigma) with
[ None ⇒ False
| Some x ⇒ let $\langle$pc,j$\rangle$ := x in
match bvt_lookup_opt $\ldots$ (bitvector_of_nat ? (S n)) (\snd sigma) with
[ None ⇒ False
| Some x1 ⇒ let $\langle$pc1,j1$\rangle$ ≝ x1 in
pc1 = pc + instruction_size_jmplen j
(\snd (nth n ? program $\langle$None ?, Comment []$\rangle$)))
]
].
\end{lstlisting}
This is a temporary formulation of the main property
({\tt sigma\_policy\_specification}); its main difference
with the final version is that it uses {\tt instruction\_size\_jmplen} to
compute the instruction size. This function uses $j$ to compute the size
of jumps (i.e. it uses the $\sigma$ function under construction), instead
of looking at the distance between source and destination. This is because
$\sigma$ is still under construction; later on we will prove that after the
final iteration, {\tt sigma\_compact\_unsafe} is equivalent to the main
property.
\begin{lstlisting}
definition sigma_safe :=
λprefix:list labelled_instruction.λlabels:label_map.λadded:$\mathbb{N}$.
λold_sigma:ppc_pc_map.λsigma:ppc_pc_map.
∀i.i < |prefix| → let $\langle$pc,j$\rangle$ :=
bvt_lookup $\ldots$ (bitvector_of_nat ? i) (\snd sigma) $\langle$0,short_jump$\rangle$ in
let pc_plus_jmp_length := bitvector_of_nat ? (\fst (bvt_lookup $\ldots$
(bitvector_of_nat ? (S i)) (\snd sigma) $\langle$0,short_jump$\rangle$)) in
let $\langle$label,instr$\rangle$ := nth i ? prefix $\langle$None ?, Comment [ ]$\rangle$ in
$\forall$dest.is_jump_to instr dest $\rightarrow$
let paddr := lookup_def $\ldots$ labels dest 0 in
let addr := bitvector_of_nat ? (if leb i paddr (* forward jump *)
then \fst (bvt_lookup $\ldots$ (bitvector_of_nat ? paddr) (\snd old_sigma)
$\langle$0,short_jump$\rangle$) + added
else \fst (bvt_lookup $\ldots$ (bitvector_of_nat ? paddr) (\snd sigma)
$\langle$0,short_jump$\rangle$)) in
match j with
[ short_jump $\Rightarrow$ $\neg$is_call instr $\wedge$
\fst (short_jump_cond pc_plus_jmp_length addr) = true
| medium_jump $\Rightarrow$ $\neg$is_relative_jump instr $\wedge$
\fst (medium_jump_cond pc_plus_jmp_length addr) = true $\wedge$
\fst (short_jump_cond pc_plus_jmp_length addr) = false
| long_jump $\Rightarrow$ \fst (short_jump_cond pc_plus_jmp_length addr) = false
$\wedge$ \fst (medium_jump_cond pc_plus_jmp_length addr) = false
].
\end{lstlisting}
This is a more direct safety property: it states that jump instructions are
encoded properly, and that no wrong jump instructions are chosen.
Note that we compute the distance using the memory address of the instruction
plus its size: this is due to the behaviour of the MCS-51 architecture, which
increases the program counter directly after fetching, and only then executes
the jump.
\begin{lstlisting}
\fst (bvt_lookup $\ldots$ (bitvector_of_nat ? 0) (\snd policy)
$\langle$0,short_jump$\rangle$) = 0)
\fst policy = \fst (bvt_lookup $\ldots$
(bitvector_of_nat ? (|prefix|)) (\snd policy) $\langle$0,short_jump$\rangle$)
\end{lstlisting}
These two properties give the values of $\sigma$ for the start and end of the
program; $\sigma(0) = 0$ and $\sigma(n)$, where $n$ is the number of
instructions up until now, is equal to the maximum memory address so far.
\begin{lstlisting}
(added = 0 → policy_pc_equal prefix old_sigma policy))
(policy_jump_equal prefix old_sigma policy → added = 0))
\end{lstlisting}
And finally, two properties that deal with what happens when the previous
iteration does not change with respect to the current one. $added$ is the
variable that keeps track of the number of bytes we have added to the program
size by changing jumps; if this is 0, the program has not changed and vice
versa.
We need to use two different formulations, because the fact that $added$ is 0
does not guarantee that no jumps have changed: it is possible that we have
replaced a short jump with a medium jump, which does not change the size.
Therefore {\tt policy\_pc\_equal} states that $old\_sigma_1(x) = sigma_1(x)$,
whereas {\tt policy\_jump\_equal} states that $old\_sigma_2(x) = sigma_2(x)$.
This formulation is sufficient to prove termination and compactness.
Proving these invariants is simple, usually by induction on the prefix length.
\subsection{Iteration invariants}
These are invariants that hold after the completion of an iteration. The main
difference between these invariants and the fold invariants is that after the
completion of the fold, we check whether the program size does not supersede
65 Kbytes (the maximum memory size the MCS-51 can address).
The type of an iteration therefore becomes an option type: {\tt None} in case
the program becomes larger than 65 KBytes, or $\mathtt{Some}\ \sigma$
otherwise. We also no longer use a natural number to pass along the number of
bytes added to the program size, but a boolean that indicates whether we have
changed something during the iteration or not.
If an iteration returns {\tt None}, we have the following invariant:
\begin{lstlisting}
definition nec_plus_ultra :=
λprogram:list labelled_instruction.λp:ppc_pc_map.
¬(∀i.i < |program| →
is_jump (\snd (nth i ? program $\langle$None ?, Comment []$\rangle$)) →
\snd (bvt_lookup $\ldots$ (bitvector_of_nat 16 i) (\snd p) $\langle$0,short_jump$\rangle$) =
long_jump).
\end{lstlisting}
This invariant is applied to $old\_sigma$; if our program becomes too large
for memory, the previous iteration cannot have every jump encoded as a long
jump. This is needed later on in the proof of termination.
If the iteration returns $\mathtt{Some}\ \sigma$, the invariants
{\tt out\_of\_program\_none}, {\tt not\_jump\_default}, {\tt jump\_increase},
and the two invariants that deal with $\sigma(0)$ and $\sigma(n)$ are
retained without change.
Instead of using {\tt sigma\_compact\_unsafe}, we can now use the proper
invariant:
\begin{lstlisting}
definition sigma_compact: list labelled_instruction → label_map → ppc_pc_map → Prop :=
λprogram.λlabels.λsigma.
∀n.n < |program| →
match bvt_lookup_opt $\ldots$ (bitvector_of_nat ? n) (\snd sigma) with
[ None ⇒ False
| Some x ⇒ let $\langle$pc,j$\rangle$ := x in
match bvt_lookup_opt $\ldots$ (bitvector_of_nat ? (S n)) (\snd sigma) with
[ None ⇒ False
| Some x1 ⇒ let $\langle$pc1,j1$\rangle$ := x1 in
pc1 = pc + instruction_size
(λid.bitvector_of_nat ? (lookup_def ?? labels id 0))
(λppc.bitvector_of_nat ?
(\fst (bvt_lookup $\ldots$ ppc (\snd sigma) $\langle$0,short_jump$\rangle$)))
(λppc.jmpeqb long_jump (\snd (bvt_lookup $\ldots$ ppc
(\snd sigma) $\langle$0,short_jump$\rangle$))) (bitvector_of_nat ? n)
(\snd (nth n ? program $\langle$None ?, Comment []$\rangle$))
]
].
\end{lstlisting}
This is the same invariant as ${\tt sigma\_compact\_unsafe}$, but instead it
computes the sizes of jump instructions by looking at the distance between
position and destination using $\sigma$.
In actual use, the invariant is qualified: $\sigma$ is compact if there have
been no changes (i.e. the boolean passed along is {\tt true}). This is to
reflect the fact that we are doing a least fixed point computation: the result
is only correct when we have reached the fixed point.
There is another, trivial, invariant if the iteration returns
$\mathtt{Some}\ \sigma$:
\begin{lstlisting}
\fst p < 2^16
\end{lstlisting}
The invariants that are taken directly from the fold invariants are trivial to
prove.
The proof of {\tt nec\_plus\_ultra} works as follows: if we return {\tt None},
then the program size must be greater than 65 Kbytes. However, since the
previous iteration did not return {\tt None} (because otherwise we would
terminate immediately), the program size in the previous iteration must have
been smaller than 65 Kbytes.
Suppose that all the jumps in the previous iteration are long jumps. This means
that all jumps in this iteration are long jumps as well, and therefore that
both iterations are equal in jumps. Per the invariant, this means that
$added = 0$, and therefore that all addresses in both iterations are equal.
But if all addresses are equal, the program sizes must be equal too, which
means that the program size in the current iteration must be smaller than
65 Kbytes. This contradicts the earlier hypothesis, hence not all jumps in
the previous iteration are long jumps.
The proof of {\tt sigma\_compact} follows from {\tt sigma\_compact\_unsafe} and
the fact that we have reached a fixed point, i.e. the previous iteration and
the current iteration are the same. This means that the results of
{\tt instruction\_size\_jmplen} and {\tt instruction\_size} are the same.
\subsection{Final properties}
These are the invariants that hold after $2n$ iterations, where $n$ is the
program size. Here, we only need {\tt out\_of\_program\_none},
{\tt sigma\_compact} and the fact that $\sigma(0) = 0$.
Termination can now be proven through the fact that there is a $k \leq 2n$, with
$n$ the length of the program, such that iteration $k$ is equal to iteration
$k+1$. There are two possibilities: either there is a $k < 2n$ such that this
property holds, or every iteration up to $2n$ is different. In the latter case,
since the only changes between the iterations can be from shorter jumps to
longer jumps, in iteration $2n$ every jump must be long. In this case,
iteration $2n$ is equal to iteration $2n+1$ and the fixpoint is reached.