source: Papers/fopara2013/fopara13.tex @ 3422

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Basic restructuring of section 2.

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1\documentclass{llncs}
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42
43\title{Certified Complexity (CerCo)\thanks{The project CerCo acknowledges the
44financial support of the Future and Emerging Technologies (FET) programme within
45the Seventh Framework Programme for Research of the European Commission, under
46FET-Open grant number: 243881}}
47\author{
48%Roberto
49R.~M. Amadio$^{4}$ \and
50%Nicolas
51N.~Ayache$^{3,4}$ \and
52%François
53F.~Bobot$^{3,4}$ \and
54%Jacob
55J.~P. Boender$^1$ \and
56%Brian
57B.~Campbell$^2$ \and
58%Ilias
59I.~Garnier$^2$ \and
60%Antoine
61A.~Madet$^4$ \and
62%James
63J.~McKinna$^2$ \and
64%Dominic
65D.~P. Mulligan$^1$ \and
66%Mauro
67M.~Piccolo$^1$ \and
68%Randy
69R.~Pollack$^2$ \and
70%Yann
71Y.~R\'egis-Gianas$^{3,4}$ \and
72%Claudio
73C.~Sacerdoti Coen$^1$ \and
74%Ian
75I.~Stark$^2$ \and
76%Paolo
77P.~Tranquilli$^1$}
78\institute{Dipartimento di Informatica - Scienza e Ingegneria, Universit\'a di
79Bologna \and
80LFCS, School of Informatics, University of Edinburgh
81\and INRIA (Team $\pi$r2 )
82\and
83Universit\`e Paris Diderot
84}
85
86\bibliographystyle{splncs03}
87
88\begin{document}
89
90\maketitle
91
92% Brian: I've changed "without loss of accuracy" because we discuss
93% important precision/complexity tradeoffs for advanced architectures
94% in the dependent labelling section.
95\begin{abstract}
96This paper provides an overview of the FET-Open Project CerCo
97(`Certified Complexity'). The project's main achievement is
98the development of a technique for analysing non-functional
99properties of programs (time, space) at the source level, with little or no loss of accuracy
100and with a small, trusted code base. The main software component
101developed is a certified compiler producing cost annotations. The compiler
102translates source code to object code and produces an instrumented copy of the
103source code. This instrumentation exposes at
104the source level---and tracks precisely---the actual (non-asymptotic)
105computational cost of the input program. Untrusted invariant generators and trusted theorem provers
106can then be used to compute and certify the parametric execution time of the
107code.
108\end{abstract}
109
110% ---------------------------------------------------------------------------- %
111% SECTION                                                                      %
112% ---------------------------------------------------------------------------- %
113\section{Introduction}
114
115\paragraph{Problem statement.} Computer programs can be specified with both
116functional constraints (what a program must do) and non-functional constraints
117(e.g. what resources---time, space, etc.---the program may use).  In the current
118state of the art, functional properties are verified for high-level source code
119by combining user annotations (e.g. preconditions and invariants) with a
120multitude of automated analyses (invariant generators, type systems, abstract
121interpretation, theorem proving, and so on). By contrast, non-functional properties
122are generally checked on low-level object code, but also demand information
123about high-level functional behaviour that must somehow be recreated.
124
125This situation presents several problems: 1) it can be hard to infer this
126high-level structure in the presence of compiler optimisations; 2) techniques
127working on object code are not useful in early development, yet problems
128detected later are more expensive to tackle; 3) parametric cost analysis is very
129hard: how can we reflect a cost that depends on the execution state (e.g. the
130value of a register or a carry bit) to a cost that the user can understand
131looking at source code? 4) functional analysis performed only on object code
132makes any contribution from the programmer hard, leading to less precision in
133the estimates.
134
135\paragraph{Vision and approach.} We want to reconcile functional and
136non-functional analyses: to share information and perform both at the same time
137on source code.
138%
139What has previously prevented this approach is the lack of a uniform and precise
140cost model for high-level code: 1) each statement occurrence is compiled
141differently and optimisations may change control flow; 2) the cost of an object
142code instruction may depend on the runtime state of hardware components like
143pipelines and caches, all of which are not visible in the source code.
144
145To solve the issue, we envision a new generation of compilers able to keep track
146of program structure through compilation and optimisation, and to exploit that
147information to define a cost model for source code that is precise, non-uniform,
148and accounts for runtime state. With such a source-level cost model we can
149reduce non-functional verification to the functional case and exploit the state
150of the art in automated high-level verification~\cite{survey}. The techniques
151currently used by the Worst Case Execution Time (WCET) community, which perform the analysis on object code,
152are still available but can now be coupled with an additional source-level
153analysis. Where the approach produces precise cost models too complex to reason
154about, safe approximations can be used to trade complexity with precision.
155Finally, source code analysis can be made during early development stages, when
156components have been specified but not implemented: source code modularity means
157that it is enough to specify the non-functional behaviour of unimplemented
158components.
159
160\paragraph{Contributions.} We have developed what we refer to as \emph{the labeling approach} \cite{labeling}, a
161technique to implement compilers that induce cost models on source programs by
162very lightweight tracking of code changes through compilation. We have studied
163how to formally prove the correctness of compilers implementing this technique.
164We have implemented such a compiler from C to object binaries for the 8051
165micro-controller, and verified it in an interactive theorem prover. We have
166implemented a Frama-C plugin \cite{framac} that invokes the compiler on a source
167program and uses this to generate invariants on the high-level source that
168correctly model low-level costs. Finally, the plugin certifies that the program
169respects these costs by calling automated theorem provers, a new and innovative
170technique in the field of cost analysis. As a case study, we show how the
171plugin can automatically compute and certify the exact reaction time of Lustre
172\cite{lustre} data flow programs compiled into C.
173
174\section{Project context and objectives}
175Formal methods for verification of functional properties of programs have
176now reached a level of maturity and automation that is facilitating a slow but
177increasing adoption in production environments. For safety critical code, it is
178becoming commonplace to combine rigorous software engineering methodologies and testing
179with static analysis, taking the strong points of each approach and mitigating
180their weaknesses. Of particular interest are open frameworks
181for the combination of different formal methods, where the programs can be
182progressively specified and are continuously enriched with new safety
183guarantees: every method contributes knowledge (e.g. new invariants) that
184becomes an assumption for later analysis.
185
186The scenario for the verification of non-functional properties (time spent,
187memory used, energy consumed) is bleaker and the future seems to be getting even
188worse. Most industries verify that real time systems meet their deadlines
189by simply performing many runs of the system and timing their execution,  computing the
190maximum time and adding an empirical safety margin, claiming the result to be a
191bound for the WCET of the program. Formal methods and software to statically
192analyse the WCET of programs exist, but they often produce bounds that are too
193pessimistic to be useful. Recent advancements in hardware architectures are all
194focused on the improvement of the average case performance, not the
195predictability of the worst case. Execution time is becoming increasingly
196dependent on execution history and the internal state of
197hardware components like pipelines and caches. Multi-core processors and non-uniform
198memory models are drastically reducing the possibility of performing
199static analysis in isolation, because programs are less and less time
200composable. Clock-precise hardware models are necessary for static analysis, and
201obtaining them is becoming harder as a consequence of the increased sophistication
202of hardware design.
203
204Despite the aforementioned problems, the need for reliable real time systems and programs is
205increasing, and there is increasing pressure from the research community towards
206the differentiation of hardware. The aim is to introduce alternative
207hardware with more predictable behaviour and hence more suitability for static
208analyses, for example, the decoupling of execution time from execution history
209by introducing randomisation \cite{proartis}.
210
211In the CerCo project \cite{cerco} we do not try to address this problem, optimistically
212assuming that static analysis of non-functional properties of programs will
213become feasible in the longer term. The main objective of our work is
214instead to bring together static analysis of functional and non-functional
215properties, which, according to the current state of the art, are completely
216independent activities with limited exchange of information: while the
217functional properties are verified on the source code, the analysis of
218non-functional properties is entirely performed on the object code to exploit
219clock-precise hardware models.
220
221\subsection{Current object-code methods}
222
223Analysis currently takes place on object code for two main reasons.
224First, there cannot be a uniform, precise cost model for source
225code instructions (or even basic blocks). During compilation, high level
226instructions are torn apart and reassembled in context-specific ways so that
227identifying a fragment of object code and a single high level instruction is
228infeasible. Even the control flow of the object and source code can be very
229different as a result of optimisations, for example aggressive loop
230optimisations may completely transform source level loops. Despite the lack of a uniform, compilation- and
231program-independent cost model on the source language, the literature on the
232analysis of non-asymptotic execution time on high level languages that assumes
233such a model is growing and gaining momentum. However, unless we can provide a
234replacement for such cost models, this literature's future practical impact looks
235to be minimal. Some hope has been provided by the EmBounded project \cite{embounded}, which
236compositionally compiles high-level code to a byte code that is executed by an
237emulator with guarantees on the maximal execution time spent for each byte code
238instruction. That approach provides a uniform model at the price of the model's
239precision (each cost is a pessimistic upper bound) and performance of the
240executed code (because the byte code is emulated  compositionally instead of
241performing a fully non-compositional compilation).
242
243The second reason to perform the analysis on the object code is that bounding
244the worst case execution time of small code fragments in isolation (e.g. loop
245bodies) and then adding up the bounds yields very poor estimates because no
246knowledge on the hardware state can be assumed when executing the fragment. By
247analysing longer runs the bound obtained becomes more precise because the lack
248of knowledge on the initial state has less of an effect on longer computations.
249
250The cost of an execution is the sum of the cost of basic blocks multiplied by
251the number of times they are executed, which is a functional property of the
252program. Therefore, in order to perform (parametric) time analysis of programs,
253it is necessary to combine a cost model with control and data flow analysis.
254Current state of the art WCET technology
255\cite{stateart} performs the analysis on the object code, where the logic of the
256program is harder to reconstruct and most information available at the source
257code level has been lost. Imprecision in the analysis leads to useless bounds. To
258augment precision, the tools ask the user to provide constraints on the object
259code control flow, usually in the form of bounds on the number of iterations of
260loops or linear inequalities on them. This requires the user to manually link
261the source and object code, translating his assumptions on the source code
262(which may be wrong) to object code constraints. The task is error prone and
263hard, especially in the presence of complex compiler optimisations.
264
265Traditional techniques for WCET that work on object code are also affected by
266another problem: they cannot be applied before the generation of the object
267code. Functional properties can be analysed in early development stages, while
268analysis of non-functional properties may come too late to avoid expensive
269changes to the program architecture.
270
271\subsection{CerCo}
272
273In CerCo we propose a radically new approach to the problem: we reject the idea
274of a uniform cost model and we propose that the compiler, which knows how the
275code is translated, must return the cost model for basic blocks of high level
276instructions. It must do so by keeping track of the control flow modifications
277to reverse them and by interfacing with static analysers.
278
279By embracing compilation, instead of avoiding it like EmBounded did, a CerCo
280compiler can at the same time produce efficient code and return costs that are
281as precise as the static analysis can be. Moreover, we allow our costs to be
282parametric: the cost of a block can depend on actual program data or on a
283summary of the execution history or on an approximated representation of the
284hardware state. For example, loop optimizations may assign to a loop body a cost
285that is a function of the number of iterations performed. As another example,
286the cost of a block may be a function of the vector of stalled pipeline states,
287which can be exposed in the source code and updated at each basic block exit. It
288is parametricity that allows one to analyse small code fragments without losing
289precision: in the analysis of the code fragment we do not have to ignore the
290initial hardware state. On the contrary, we can assume that we know exactly which
291state (or mode, as the WCET literature calls it) we are in.
292
293The CerCo approach has the potential to dramatically improve the state of the
294art. By performing control and data flow analyses on the source code, the error
295prone translation of invariants is completely avoided. It is in fact performed
296by the compiler itself when it induces the cost model on the source language.
297Moreover, any available technique for the verification of functional properties
298can be immediately reused and multiple techniques can collaborate together to
299infer and certify cost invariants for the program. Parametric cost analysis
300becomes the default one, with non parametric bounds used as last resorts when
301trading the complexity of the analysis with its precision. \emph{A priori}, no
302technique previously used in traditional WCET is lost: they can just be applied
303at the source code level.
304 Our approach can also work in the early
305stages of development by axiomatically attaching costs to unimplemented components.
306
307
308All software used to verify properties of programs must be as bug free as
309possible. The trusted code base for verification is made by the code that needs
310to be trusted to believe that the property holds. The trusted code base of
311state-of-the-art WCET tools is very large: one needs to trust the control flow
312analyser and the linear programming libraries it uses and also the formal models
313of the hardware. In CerCo we are moving the control flow analysis to the source
314code and we are introducing a non-standard compiler too. To reduce the trusted
315code base, we implemented a prototype and a static analyser in an interactive
316theorem prover, which was used to certify that the cost computed on the source
317code is indeed the one actually spent by the hardware. Formal models of the
318hardware and of the high level source languages were also implemented in the
319interactive theorem prover. Control flow analysis on the source code has been
320obtained using invariant generators, tools to produce proof obligations from
321generated invariants and automatic theorem provers to verify the obligations. If
322the automatic provers are able to generate proof traces that can be
323independently checked, the only remaining component that enters the trusted code
324base is an off-the-shelf invariant generator which, in turn, can be proved
325correct using an interactive theorem prover. Therefore we achieve the double
326objective of allowing the use of more off-the-shelf components (e.g. provers and
327invariant generators) whilst reducing the trusted code base at the same time.
328
329%\paragraph{Summary of the CerCo objectives.} To summarize, the goal of CerCo is
330% to reconcile functional with non-functional analysis by performing them together
331% on the source code, sharing common knowledge about execution invariants. We want
332% to achieve the goal implementing a new generation of compilers that induce a
333% parametric, precise cost model for basic blocks on the source code. The compiler
334% should be certified using an interactive theorem prover to minimize the trusted
335% code base of the analysis. Once the cost model is induced, off-the-shelf tools
336% and techniques can be combined together to infer and prove parametric cost
337% bounds.
338%The long term benefits of the CerCo vision are expected to be:
339%1. the possibility to perform static analysis during early development stages
340%2.  parametric bounds made easier
341%3.  the application of off-the-shelf techniques currently unused for the
342% analysis of non-functional properties, like automated proving and type systems
343%4. simpler and safer interaction with the user, that is still asked for
344% knowledge, but on the source code, with the additional possibility of actually
345% verifying the provided knowledge
346%5. a reduced trusted code base
347%6. the increased accuracy of the bounds themselves.
348%
349%The long term success of the project is hindered by the increased complexity of
350% the static prediction of the non-functional behaviour of modern hardware. In the
351% time frame of the European contribution we have focused on the general
352% methodology and on the difficulties related to the development and certification
353% of a cost model inducing compiler.
354
355\section{The typical CerCo workflow}
356\begin{figure}[!t]
357\begin{tabular}{l@{\hspace{0.2cm}}|@{\hspace{0.2cm}}l}
358\begin{lstlisting}
359char a[] = {3, 2, 7, 14};
360char threshold = 4;
361
362int count(char *p, int len) {
363  char j;
364  int found = 0;
365  for (j=0; j < len; j++) {
366    if (*p <= threshold)
367      found++;
368    p++;
369    }
370  return found;
371}
372
373int main() {
374  return count(a,4);
375}
376\end{lstlisting}
377&
378%  $\vcenter{\includegraphics[width=7.5cm]{interaction_diagram.pdf}}$
379\begin{tikzpicture}[
380    baseline={([yshift=-.5ex]current bounding box)},
381    element/.style={draw, text width=1.6cm, on chain, text badly centered},
382    >=stealth
383    ]
384{[start chain=going below, node distance=.5cm]
385\node [element] (cerco) {CerCo\\compiler};
386\node [element] (cost)  {CerCo\\cost plugin};
387{[densely dashed]
388\node [element] (ded)   {Deductive\\platform};
389\node [element] (check) {Proof\\checker};
390}
391}
392\coordinate [left=3.5cm of cerco] (left);
393{[every node/.style={above, text width=3.5cm, text badly centered,
394                     font=\scriptsize}]
395\draw [<-] ([yshift=-1ex]cerco.north west) coordinate (t) --
396    node {C source}
397    (t-|left);
398\draw [->] (cerco) -- (cost);
399\draw [->] ([yshift=1ex]cerco.south west) coordinate (t) --
400    node {C source+\color{red}{cost annotations}}
401    (t-|left) coordinate (cerco out);
402\draw [->] ([yshift=1ex]cost.south west) coordinate (t) --
403    node {C source+\color{red}{cost annotations}\\+\color{blue}{synthesized assertions}}
404    (t-|left) coordinate (out);
405{[densely dashed]
406\draw [<-] ([yshift=-1ex]ded.north west) coordinate (t) --
407    node {C source+\color{red}{cost annotations}\\+\color{blue}{complexity assertions}}
408    (t-|left) coordinate (ded in) .. controls +(-.5, 0) and +(-.5, 0) .. (out);
409\draw [->] ([yshift=1ex]ded.south west) coordinate (t) --
410    node {complexity obligations}
411    (t-|left) coordinate (out);
412\draw [<-] ([yshift=-1ex]check.north west) coordinate (t) --
413    node {complexity proof}
414    (t-|left) .. controls +(-.5, 0) and +(-.5, 0) .. (out);
415\draw [dash phase=2.5pt] (cerco out) .. controls +(-1, 0) and +(-1, 0) ..
416    (ded in);
417}}
418%% user:
419% head
420\draw (current bounding box.west) ++(-.2,.5)
421    circle (.2) ++(0,-.2) -- ++(0,-.1) coordinate (t);
422% arms
423\draw (t) -- +(-.3,-.2);
424\draw (t) -- +(.3,-.2);
425% body
426\draw (t) -- ++(0,-.4) coordinate (t);
427% legs
428\draw (t) -- +(-.2,-.6);
429\draw (t) -- +(.2,-.6);
430\end{tikzpicture}
431\end{tabular}
432\caption{On the left: code to count the array's elements
433 that are less than or equal to the threshold. On the right: CerCo's interaction
434 diagram. CerCo's components are drawn solid.}
435\label{test1}
436\end{figure}
437We illustrate the workflow we envisage (on the right of~\autoref{test1})
438on an example program (on the left of~\autoref{test1}).
439The user writes the program and feeds it to the CerCo compiler, which outputs
440an instrumented version of the same program that updates global variables
441that record the elapsed execution time and the stack space usage.
442The red lines in \autoref{itest1} are the instrumentation introduced by the
443compiler. The annotated program can then be enriched with complexity assertions
444in the style of Hoare logic, that are passed to a deductive platform (in our
445case Frama-C). We provide as a Frama-C cost plugin a simple automatic
446synthesiser for complexity assertions (the blue lines in \autoref{itest1}),
447which can be overridden by the user to increase or decrease accuracy. From the
448assertions, a general purpose deductive platform produces proof obligations
449which in turn can be closed by automatic or interactive provers, ending in a
450proof certificate.
451
452% NB: if you try this example with the live CD you should increase the timeout
453
454Twelve proof obligations are generated from~\autoref{itest1} (to prove
455that the loop invariant holds after one execution if it holds before,
456to prove that the whole program execution takes at most 1358 cycles,
457etc.).  Note that the synthesised time bound for \lstinline'count',
458$178+214*(1+\lstinline'len')$ cycles, is parametric in the length of
459the array. The CVC3 prover
460closes all obligations within half a minute on routine commodity
461hardware.  A simpler non-parametric version can be solved in a few
462seconds.
463%
464\fvset{commandchars=\\\{\}}
465\lstset{morecomment=[s][\color{blue}]{/*@}{*/},
466        moredelim=[is][\color{blue}]{$}{$},
467        moredelim=[is][\color{red}]{|}{|}}
468\begin{figure}[!p]
469\begin{lstlisting}
470|int __cost = 33, __stack = 5, __stack_max = 5;|
471|void __cost_incr(int incr) { __cost += incr; }|
472|void __stack_incr(int incr) {
473  __stack += incr;
474  __stack_max = __stack_max < __stack ? __stack : __stack_max;
475}|
476
477char a[4] = {3, 2, 7, 14};  char threshold = 4;
478
479/*@ behavior stack_cost:
480      ensures __stack_max <= __max(\old(__stack_max), 4+\old(__stack));
481      ensures __stack == \old(__stack);
482    behavior time_cost:
483      ensures __cost <= \old(__cost)+(178+214*__max(1+\at(len,Pre), 0));
484*/
485int count(unsigned char *p, int len) {
486  char j;  int found = 0;
487  |__stack_incr(4);  __cost_incr(111);|
488  $__l: /* internal */$
489  /*@ for time_cost: loop invariant
490        __cost <= \at(__cost,__l)+
491                  214*(__max(\at((len-j)+1,__l), 0)-__max(1+(len-j), 0));
492      for stack_cost: loop invariant
493        __stack_max == \at(__stack_max,__l);
494      for stack_cost: loop invariant
495        __stack == \at(__stack,__l);
496      loop variant len-j;
497  */
498  for (j = 0; j < len; j++) {
499    |__cost_incr(78);|
500    if (*p <= threshold) { |__cost_incr(136);| found ++; }
501    else { |__cost_incr(114);| }
502    p ++;
503  }
504  |__cost_incr(67);  __stack_incr(-4);|
505  return found;
506}
507
508/*@ behavior stack_cost:
509      ensures __stack_max <= __max(\old(__stack_max), 6+\old(__stack));
510      ensures __stack == \old(__stack);
511    behavior time_cost:
512      ensures __cost <= \old(__cost)+1358;
513*/
514int main(void) {
515  int t;
516  |__stack_incr(2);  __cost_incr(110);|
517  t = count(a,4);
518  |__stack_incr(-2);|
519  return t;
520}
521\end{lstlisting}
522\caption{The instrumented version of the program in \autoref{test1},
523 with instrumentation added by the CerCo compiler in red and cost invariants
524 added by the CerCo Frama-C plugin in blue. The \texttt{\_\_cost},
525 \texttt{\_\_stack} and \texttt{\_\_stack\_max} variables hold the elapsed time
526in clock cycles and the current and maximum stack usage. Their initial values
527hold the clock cycles spent in initialising the global data before calling
528\texttt{main} and the space required by global data (and thus unavailable for
529the stack).
530}
531\label{itest1}
532\end{figure}
533\section{Main scientific and technical results}
534We will now review the main results that the CerCo project has achieved.
535
536% \emph{Dependent labeling~\cite{?}.} The basic labeling approach assumes a
537% bijective mapping between object code and source code O(1) blocks (called basic
538% blocks). This assumption is violated by many program optimizations (e.g. loop
539% peeling and loop unrolling). It also assumes the cost model computed on the
540% object code to be non parametric: every block must be assigned a cost that does
541% not depend on the state. This assumption is violated by stateful hardware like
542% pipelines, caches, branch prediction units. The dependent labeling approach is
543% an extension of the basic labeling approach that allows to handle parametric
544% cost models. We showed how the method allows to deal with loop optimizations and
545% pipelines, and we speculated about its applications to caches.
546%
547% \emph{Techniques to exploit the induced cost model.} Every technique used for
548% the analysis of functional properties of programs can be adapted to analyse the
549% non-functional properties of the source code instrumented by compilers that
550% implement the labeling approach. In order to gain confidence in this claim, we
551% showed how to implement a cost invariant generator combining abstract
552% interpretation with separation logic ideas \cite{separation}. We integrated
553% everything in the Frama-C modular architecture, in order to compute proof
554% obligations from functional and cost invariants and to use automatic theorem
555% provers on them. This is an example of a new technique that is not currently
556% exploited in WCET analysis. It also shows how precise functional invariants
557% benefits the non-functional analysis too. Finally, we show how to fully
558% automatically analyse the reaction time of Lustre nodes that are first compiled
559% to C using a standard Lustre compiler and then processed by a C compiler that
560% implements the labeling approach.
561
562% \emph{The CerCo compiler.} This is a compiler from a large subset of the C
563% program to 8051/8052 object code,
564% integrating the labeling approach and a static analyser for 8051 executables.
565% The latter can be implemented easily and does not require dependent costs
566% because the 8051 microprocessor is a very simple one, with constant-cost
567% instruction. It was chosen to separate the issue of exact propagation of the
568% cost model from the orthogonal problem of the static analysis of object code
569% that may require approximations or dependent costs. The compiler comes in
570% several versions: some are prototypes implemented directly in OCaml, and they
571% implement both the basic and dependent labeling approaches; the final version
572% is extracted from a Matita certification and at the moment implements only the
573% basic approach.
574
575\subsection{The (basic) labeling approach}
576The labeling approach is the foundational insight that underlies all the developments in CerCo.
577It allows the tracking of the evolution of
578basic blocks during the compilation process in order to propagate the cost model from the
579object code to the source code without losing precision in the process.
580\paragraph{Problem statement.} Given a source program $P$, we want to obtain an
581instrumented source program $P'$,  written in the same programming language, and
582the object code $O$ such that: 1) $P'$ is obtained by inserting into $P$ some
583additional instructions to update global cost information like the amount of
584time spent during execution or the maximal stack space required; 2) $P$ and $P'$ 
585must have the same functional behaviour, i.e.\ they must produce that same output
586and intermediate observables; 3) $P$ and $O$ must have the same functional
587behaviour; 4) after execution and in interesting points during execution, the
588cost information computed by $P'$ must be an upper bound of the one spent by $O$ 
589to perform the corresponding operations (soundness property); 5) the difference
590between the costs computed by $P'$ and the execution costs of $O$ must be
591bounded by a program-dependent constant (precision property).
592
593\paragraph{The labeling software components.} We solve the problem in four
594stages \cite{labeling}, implemented by four software components that are used
595in sequence.
596
597The first component labels the source program $P$ by injecting label emission
598statements in appropriate positions to mark the beginning of basic blocks.
599% The
600% set of labels with their positions is called labeling.
601The syntax and semantics
602of the source programming language is augmented with label emission statements.
603The statement ``EMIT $\ell$'' behaves like a NOP instruction that does not affect the
604program state or control flow, but its execution is observable.
605% Therefore the observables of a run of a program becomes a stream
606% of label emissions: $\ell_1,\ldots,\ell_n$, called the program trace. We clarify the conditions
607% that the labeling must respect later.
608
609The second component is a labeling preserving compiler. It can be obtained from
610an existing compiler by adding label emission statements to every intermediate
611language and by propagating label emission statements during compilation. The
612compiler is correct if it preserves both the functional behaviour of the program
613and the traces of observables.
614% We may also ask that the function that erases the cost
615% emission statements commute with compilation. This optional property grants that
616% the labeling does not interfere with the original compiler behaviour. A further
617% set of requirements will be added later.
618
619The third component is a static object code analyser. It takes as input a labeled
620object code and it computes the scope of each of its label emission statements,
621i.e.\ the tree of instructions that may be executed after the statement and before
622a new label emission is encountered.
623It is a tree and not a sequence because the scope
624may contain a branching statement. In order to grant that such a finite tree
625exists, the object code must not contain any loop that is not broken by a label
626emission statement. This is the first requirement of a sound labeling. The
627analyser fails if the labeling is unsound. For each scope, the analyser
628computes an upper bound of the execution time required by the scope, using the
629maximum of the costs of the two branches in case of a conditional statement.
630Finally, the analyser computes the cost of a label by taking the maximum of the
631costs of the scopes of every statement that emits that label.
632
633The fourth and last component takes in input the cost model computed at step 3
634and the labelled code computed at step 1. It outputs a source program obtained
635by replacing each label emission statement with a statement that increments the
636global cost variable with the cost associated to the label by the cost model. 
637The obtained source code is the instrumented source code.
638
639\paragraph{Correctness.} Requirements 1, 2 and 3 of the problem statement
640obviously hold, with 2 and 3 being a consequence of the definition of a correct
641labeling preserving compiler. It is also obvious that the value of the global
642cost variable of an instrumented source code is at any time equal to the sum of
643the costs of the labels emitted by the corresponding labelled code. Moreover,
644because the compiler preserves all traces, the sum of the costs of the labels
645emitted in the source and target labelled code are the same. Therefore, to
646satisfy the fourth requirement, we need to grant that the time taken to execute
647the object code is equal to the sum of the costs of the labels emitted by the
648object code. We collect all the necessary conditions for this to happen in the
649definition of a sound labeling: a) all loops must be broken by a cost emission
650statement;  b) all program instructions must be in the scope of some cost
651emission statement. To satisfy also the fifth requirement, additional
652requirements must be imposed on the object code labeling to avoid all uses of
653the maximum in the cost computation: the labeling is precise if every label is
654emitted at most once and both branches of a conditional jump start with a label
655emission statement.
656
657The correctness and precision of the labeling approach only rely on the
658correctness and precision of the object code labeling. The simplest
659way to achieve them is to impose correctness and precision
660requirements also on the initial labeling produced by the first software
661component, and to ask the compiler to preserve these
662properties too. The latter requirement imposes serious limitations on the
663compilation strategy and optimizations: the compiler may not duplicate any code
664that contains label emission statements, like loop bodies. Therefore several
665loop optimisations like peeling or unrolling are prevented. Moreover, precision
666of the object code labeling is not sufficient \emph{per se} to obtain global
667precision: we implicitly assumed that a precise constant cost can be assigned
668to every instruction. This is not possible in
669the presence of stateful hardware whose state influences the cost of operations,
670like pipelines and caches. In the next subsection we will see an extension of the
671basic labeling approach to cover this situation.
672
673The labeling approach described in this section can be applied equally well and
674with minor modifications to imperative and functional languages
675\cite{labeling2}. We have tested it on a simple imperative language without
676functions (a `while' language), on a subset of C and on two compilation chains for
677a purely functional higher-order language. The two main changes required to deal
678with functional languages are: 1) because global variables and updates are not
679available, the instrumentation phase produces monadic code to `update' the
680global costs; 2) the requirements for a sound and precise labeling of the
681source code must be changed when the compilation is based on CPS translations.
682In particular, we need to introduce labels emitted before a statement is
683executed and also labels emitted after a statement is executed. The latter capture
684code that is inserted by the CPS translation and that would escape all label
685scopes.
686
687% Brian: one of the reviewers pointed out that standard Prolog implementations
688% do have some global state that apparently survives backtracking and might be
689% used.  As we haven't experimented with this, I think it's best to elide it
690% entirely.
691
692% Phases 1, 2 and 3 can be applied as well to logic languages (e.g. Prolog).
693% However, the instrumentation phase cannot: in standard Prolog there is no notion
694% of (global) variable whose state is not retracted during backtracking.
695% Therefore, the cost of executing computations that are later backtracked would
696% not be correctly counted in. Any extension of logic languages with
697% non-backtrackable state could support our labeling approach.
698
699\subsection{Dependent labeling}
700The core idea of the basic labeling approach is to establish a tight connection
701between basic blocks executed in the source and target languages. Once the
702connection is established, any cost model computed on the object code can be
703transferred to the source code, without affecting the code of the compiler or
704its proof. In particular, it is immediate that we can also transport cost models
705that associate to each label functions from hardware state to natural numbers.
706However, a problem arises during the instrumentation phase that replaces cost
707emission statements with increments of global cost variables. The global cost
708variable must be incremented with the result of applying the function associated
709to the label to the hardware state at the time of execution of the block.
710The hardware state comprises both the functional state that affects the
711computation (the value of the registers and memory) and the non-functional
712state that does not (the pipeline and cache contents for example). The former is
713in correspondence with the source code state, but reconstructing the
714correspondence may be hard and lifting the cost model to work on the source code
715state is likely to produce cost expressions that are too complex to understand and reason about.
716Luckily enough, in all modern architectures the cost of executing single
717instructions is either independent of the functional state or the jitter---the
718difference between the worst and best case execution times---is small enough
719to be bounded without losing too much precision. Therefore we can concentrate
720on dependencies over the `non-functional' parts of the state only.
721
722The non-functional state has no correspondence in the high level state and does
723not influence the functional properties. What can be done is to expose the
724non-functional state in the source code. We present here the basic
725intuition in a simplified form: the technical details that allow us to handle the
726general case are more complex and can be found in~\cite{paolo}. We add
727to the source code an additional global variable that represents the
728non-functional state and another one that remembers the last labels emitted. The
729state variable must be updated at every label emission statement, using an
730update function which is computed during static analysis too. The update
731function associates to each label a function from the recently emitted labels
732and old state to the new state. It is computed composing the semantics of every
733instruction in a basic block and restricting it to the non-functional part of
734the state.
735
736Not all the details of the non-functional state needs to be exposed, and the
737technique works better when the part of state that is required can be summarized
738in a simple data structure. For example, to handle simple but realistic
739pipelines it is sufficient to remember a short integer that encodes the position
740of bubbles (stuck instructions) in the pipeline. In any case, it is not necessary
741for the user to understand the meaning of the state to reason over the properties
742of the
743program. Moreover, at any moment the user, or the invariant generator tools that
744analyse the instrumented source code produced by the compiler, can decide to
745trade precision of the analysis for simplicity by approximating the parametric
746cost with safe non parametric bounds. Interestingly, the functional analysis of
747the code can determine which blocks are executed more frequently in order to
748approximate more aggressively the ones that are executed less.
749
750Dependent labeling can also be applied to allow the compiler to duplicate
751blocks that contain labels (e.g. in loop optimisations)~\cite{paolo}. The effect is to assign
752a different cost to the different occurrences of a duplicated label. For
753example, loop peeling turns a loop into the concatenation of a copy of the loop
754body (that executes the first iteration) with the conditional execution of the
755loop (for the successive iterations). Because of further optimisations, the two
756copies of the loop will be compiled differently, with the first body usually
757taking more time.
758
759By introducing a variable that keeps track of the iteration number, we can
760associate to the label a cost that is a function of the iteration number. The
761same technique works for loop unrolling without modifications: the function will
762assign a cost to the even iterations and another cost to the odd ones. The
763actual work to be done consists of introducing within the source code, and for each
764loop, a variable that counts the number of iterations. The loop optimisation code
765that duplicate the loop bodies must also modify the code to propagate correctly
766the update of the iteration numbers. The technical details are more complex and
767can be found in the CerCo reports and publications. The implementation, however,
768is quite simple and the changes to a loop optimising compiler are minimal.
769
770\subsection{Techniques to exploit the induced cost model}
771We review the cost synthesis techniques developed in the project.
772The starting hypothesis is that we have a certified methodology to annotate
773blocks in the source code with constants which provide a sound and sufficiently
774precise upper bound on the cost of executing the blocks after compilation to
775object code.
776
777The principle that we have followed in designing the cost synthesis tools is
778that the synthetic bounds should be expressed and proved within a general
779purpose tool built to reason on the source code. In particular, we rely on the
780Frama-C tool to reason on C code and on the Coq proof-assistant to reason on
781higher-order functional programs.
782
783This principle entails that: 1)
784The inferred synthetic bounds are indeed correct as long as the general purpose
785tool is; 2) there is no limitation on the class of programs that can be handled
786as long as the user is willing to carry on an interactive proof.
787
788Of course, automation is desirable whenever possible. Within this framework,
789automation means writing programs that give hints to the general purpose tool.
790These hints may take the form, say, of loop invariants/variants, of predicates
791describing the structure of the heap, or of types in a light logic. If these
792hints are correct and sufficiently precise the general purpose tool will produce
793a proof automatically, otherwise, user interaction is required.
794
795\paragraph{The Cost plugin and its application to the Lustre compiler.}
796Frama-C \cite{framac} is a set of analysers for C programs with a
797specification language called ACSL. New analyses can be dynamically added
798via a plugin system. For instance, the Jessie plugin allows deductive
799verification of C programs with respect to their specification in ACSL, with
800various provers as back-end tools.
801We developed the CerCo Cost plugin for the Frama-C platform as a proof of
802concept of an automatic environment exploiting the cost annotations produced by
803the CerCo compiler. It consists of an OCaml program which essentially
804takes the following actions: 1) it receives as input a C program, 2) it
805applies the CerCo compiler to produce a related C program with cost annotations,
8063) it applies some heuristics to produce a tentative bound on the cost of
807executing the C functions of the program as a function of the value of their
808parameters, 4) the user can then call the Jessie plugin to discharge the
809related proof obligations.
810In the following we elaborate on the soundness of the framework and the
811experiments we performed with the Cost tool on the C programs produced by a
812Lustre compiler.
813
814\paragraph{Soundness.} The soundness of the whole framework depends on the cost
815annotations added by the CerCo compiler, the synthetic costs produced by the
816cost plugin, the verification conditions (VCs) generated by Jessie, and the
817external provers discharging the VCs. The synthetic costs being in ACSL format,
818Jessie can be used to verify them. Thus, even if the added synthetic costs are
819incorrect (relatively to the cost annotations), the process as a whole is still
820correct: indeed, Jessie will not validate incorrect costs and no conclusion can
821be made about the WCET of the program in this case. In other terms, the
822soundness does not really depend on the action of the cost plugin, which can in
823principle produce any synthetic cost. However, in order to be able to actually
824prove a WCET of a C function, we need to add correct annotations in a way that
825Jessie and subsequent automatic provers have enough information to deduce their
826validity. In practice this is not straightforward even for very simple programs
827composed of branching and assignments (no loops and no recursion) because a fine
828analysis of the VCs associated with branching may lead to a complexity blow up.
829\paragraph{Experience with Lustre.} Lustre is a data-flow language for programming
830synchronous systems, with a compiler which targets C. We designed a
831wrapper for supporting Lustre files.
832The C function produced by the compiler implements the step function of the
833synchronous system and computing the WCET of the function amounts to obtain a
834bound on the reaction time of the system. We tested the Cost plugin and the
835Lustre wrapper on the C programs generated by the Lustre compiler. For programs
836consisting of a few hundred lines of code, the cost plugin computes a WCET and Alt-
837Ergo is able to discharge all VCs automatically.
838
839\paragraph{Handling C programs with simple loops.}
840The cost annotations added by the CerCo compiler take the form of C instructions
841that update by a constant a fresh global variable called the cost variable.
842Synthesizing a WCET of a C function thus consists in statically resolving an
843upper bound of the difference between the value of the cost variable before and
844after the execution of the function, i.e. find in the function the instructions
845that update the cost variable and establish the number of times they are passed
846through during the flow of execution. In order to do the analysis the plugin
847makes the following assumptions on the programs:
8481) there are no recursive functions;
8492) every loop must be annotated with a variant. The variants of `for' loops are
850automatically inferred.
851
852The plugin proceeds as follows.
853First the call graph of the program is computed.
854Then the computation of the cost of the function is performed by traversing its
855control flow graph. If the function $f$ calls the function $g$ 
856then the function $g$ 
857is processed before the function $f$. The cost at a node is the maximum of the
858costs of the successors.
859In the case of a loop with a body that has a constant cost for every step of the
860loop, the cost is the product of the cost of the body and of the variant taken
861at the start of the loop.
862In the case of a loop with a body whose cost depends on the values of some free
863variables, a fresh logic function $f$ is introduced to represent the cost of the
864loop in the logic assertions. This logic function takes the variant as a first
865parameter. The other parameters of $f$ are the free variables of the body of the
866loop. An axiom is added to account the fact that the cost is accumulated at each
867step of the loop.
868The cost of the function is directly added as post-condition of the function.
869
870The user can influence the annotation by two different means:
8711) by using more precise variants;
8722) by annotating functions with cost specifications. The plugin will use this cost
873for the function instead of computing it.
874\paragraph{C programs with pointers.}
875When it comes to verifying programs involving pointer-based data structures,
876such as linked lists, trees, or graphs, the use of traditional first-order logic
877to specify, and of SMT solvers to verify, shows some limitations. Separation
878logic~\cite{separation} is an elegant alternative. It is a program logic
879with a new notion of conjunction to express spatial heap separation. Bobot has
880recently introduced automatically generated separation
881predicates to simulate separation logic reasoning in the Jessie plugin where the specification language, the verification condition
882generator, and the theorem provers were not designed with separation logic in
883mind. CerCo's plugin can exploit the separation predicates to automatically
884reason on the cost of execution of simple heap manipulation programs such as an
885in-place list reversal.
886
887\subsection{The CerCo compiler}
888In CerCo we have developed a certain number of cost preserving compilers based
889on the labeling approach. Excluding an initial certified compiler for a `while'
890language, all remaining compilers target realistic source languages---a pure
891higher order functional language and a large subset of C with pointers, gotos
892and all data structures---and real world target processors---MIPS and the
893Intel 8051/8052 processor family. Moreover, they achieve a level of optimisation
894that ranges from moderate (comparable to GCC level 1) to intermediate (including
895loop peeling and unrolling, hoisting and late constant propagation). The so
896called \emph{Trusted CerCo Compiler} is the only one that was implemented in the
897interactive theorem prover Matita~\cite{matita} and its costs certified. The code distributed
898is extracted OCaml code from the Matita implementation. In the rest of
899this section we will only focus on the Trusted CerCo Compiler, that only targets
900the C language and the 8051/8052 family, and that does not implement any
901advanced optimisations yet. Its user interface, however, is the same as the one
902of the other versions, in order to trade safety with additional performances. In
903particular, the Frama-C CerCo plugin can work without recompilation with all
904compilers.
905
906The 8051/8052 microprocessor is a very simple one, with constant-cost
907instructions. It was chosen to separate the issue of exact propagation of the
908cost model from the orthogonal problem of the static analysis of object code
909that may require approximations or dependent costs.
910
911The (trusted) CerCo compiler implements the following optimisations: cast
912simplification, constant propagation in expressions, liveness analysis driven
913spilling of registers, dead code elimination, branch displacement, and tunneling.
914The two latter optimisations are performed by our optimising assembler
915\cite{correctness}. The back-end of the compiler works on three address
916instructions, preferred to static single assignment code for the simplicity of
917the formal certification.
918
919The CerCo compiler is loosely based on the CompCert compiler \cite{compcert}, a
920recently developed certified compiler from C to the PowerPC, ARM and x86
921microprocessors. Contrary to CompCert, both the CerCo code and its
922certification are open source. Some data structures and language definitions for
923the front-end are directly taken from CompCert, while the back-end is a redesign
924of a compiler from Pascal to MIPS used by Fran\c{c}ois Pottier for a course at the
925Ecole Polytechnique.
926
927The main peculiarities of the CerCo compiler are the following.
928\begin{enumerate}
929\item All the intermediate languages include label emitting instructions to
930implement the labeling approach, and the compiler preserves execution traces.
931\item Traditionally compilers target an assembly language with additional
932macro-instructions to be expanded before assembly; for CerCo we need to go all
933the way down to object code in order to perform the static analysis. Therefore
934we integrated also an optimising assembler and a static analyser.
935\item In order to avoid implementing a linker and a loader, we do not support separate
936compilation and external calls. Adding them is a transparent
937process to the labeling approach and should create no unknown problem.
938\item We target an 8-bit processor, in contrast to CompCert's 32-bit targets. Targeting an 8-bit processor requires
939several changes and increased code size, but it is not fundamentally more
940complex. The proof of correctness, however, becomes much harder.
941\item We target a microprocessor that has a non uniform memory model, which is
942still often the case for microprocessors used in embedded systems and that is
943becoming common again in multi-core processors. Therefore the compiler has to
944keep track of data and it must move data between memory regions in the proper
945way. Moreover the size of pointers to different regions is not uniform. An
946additional difficulty was that the space available for the stack in internal
947memory in the 8051 is tiny, allowing only a minor number of nested calls. To
948support full recursion in order to test the CerCo tools also on recursive
949programs, the compiler implements a stack in external memory.
950\end{enumerate}
951
952\subsection{Formal certification of the CerCo compiler}
953We implemented the
954CerCo compiler in the interactive theorem prover Matita and have formally
955certified that the cost model induced on the source code correctly and precisely
956predicts the object code behaviour. We actually induce two cost models, one for
957time and one for stack space consumption. We show the correctness of the prediction
958only for those programs that do not exhaust the available machine stack space, a
959property that---thanks to the stack cost model---we can statically analyse on the
960source code using our Frama-C tool. The preservation of functional properties we
961take as an assumption, not itself formally proved in CerCo.  Other projects have
962already certified the preservation of functional semantics in similar compilers,
963and we have not attempted to directly repeat that work. In order to complete the
964proof for non-functional properties, we have introduced a new semantics for
965programming languages based on a new kind of structured observables with the
966relative notions of forward similarity and the study of the intentional
967consequences of forward similarity. We have also introduced a unified
968representation for back-end intermediate languages that was exploited to provide
969a uniform proof of forward similarity.
970
971The details on the proof techniques employed
972and
973the proof sketch can be collected from the CerCo deliverables and papers.
974In this section we will only hint at the correctness statement, which turned
975out to be more complex than what we expected at the beginning.
976
977\paragraph{The statement.}
978Real time programs are often reactive programs that loop forever responding to
979events (inputs) by performing some computation followed by some action (output)
980and the return to the initial state. For looping programs the overall execution
981time does not make sense. The same happens for reactive programs that spend an
982unpredictable amount of time in I/O. What is interesting is the reaction time
983that measure the time spent between I/O events. Moreover, we are interested in
984predicting and ruling out programs that crash running out of space on a certain
985input.
986Therefore we need to look for a statement that talks about sub-runs of a
987program. The most natural statement is that the time predicted on the source
988code and spent on the object code by two corresponding sub-runs are the same.
989The problem to solve to make this statement formal is how to identify the
990corresponding sub-runs and how to single out those that are meaningful.
991The solution we found is based on the notion of measurability. We say that a run
992has a \emph{measurable sub-run} when both the prefix of the sub-run and the
993sub-run do not exhaust the stack space, the number of function calls and returns
994in the sub-run is the same, the sub-run does not perform any I/O and the sub-run
995starts with a label emission statement and ends with a return or another label
996emission statements. The stack usage is estimated using the stack usage model
997that is computed by the compiler.
998
999The statement that we formally proved is: for each C run with a measurable
1000sub-run, there exists an object code run with a sub-run, such that the observables
1001of the pairs of prefixes and sub-runs are the same and the time spent by the
1002object code in the sub-run is the same as the one predicted on the source code
1003using the time cost model generated by the compiler.
1004We briefly discuss the constraints for measurability. Not exhausting the stack
1005space is a clear requirement of meaningfulness of a run, because source programs
1006do not crash for lack of space while object code ones do. The balancing of
1007function calls and returns is a requirement for precision: the labeling approach
1008allows the scope of label emission statements to extend after function calls to
1009minimize the number of labels. Therefore a label pays for all the instructions
1010in a block, excluding those executed in nested function calls. If the number of
1011calls/returns is unbalanced, it means that there is a call we have not returned
1012to that could be followed by additional instructions whose cost has already been
1013taken in account. To make the statement true (but less precise) in this
1014situation, we could only say that the cost predicted on the source code is a
1015safe bound, not that it is exact. The last condition on the entry/exit points of
1016a run is used to identify sub-runs whose code correspond to a sequence of blocks
1017that we can measure precisely. Any other choice would start or end the run in the
1018middle of a block and we would be forced again to weaken the statement taking as
1019a bound the cost obtained counting in all the instructions that precede the
1020starting one in the block, or follow the final one in the block.
1021I/O operations can be performed in the prefix of the run, but not in the
1022measurable sub-run. Therefore we prove that we can predict reaction times, but
1023not I/O times, as it should be.
1024
1025\section{Conclusions and future work}
1026
1027All the CerCo software and deliverables can be found on the CerCo homepage at~\url{http://cerco.cs.unibo.it}.
1028
1029The results obtained so far are encouraging and provide evidence that
1030it is possible to perform static time and space analysis at the source level
1031without losing accuracy, reducing the trusted code base and reconciling the
1032study of functional and non-functional properties of programs. The
1033techniques introduced seem to be scalable, cover both imperative and
1034functional languages and are compatible with every compiler optimisation
1035considered by us so far.
1036
1037To prove that compilers can keep track of optimisations
1038and induce a precise cost model on the source code, we targeted a simple
1039architecture that admits a cost model that is execution history independent.
1040The most important future work is dealing with hardware architectures
1041characterized by history dependent stateful components, like caches and
1042pipelines. The main issue consists in assigning a parametric, dependent cost
1043to basic blocks that can be later transferred by the labeling approach to
1044the source code and represented in a meaningful way to the user. The dependent
1045labeling approach that we have studied seems a promising tool to achieve
1046this goal, but the cost model generated for a realistic processor could be too
1047large and complex to be exposed in the source code. Further study is required
1048to evaluate the technique on a realistic processor and to introduce early
1049approximations of the cost model to make the technique feasible.
1050
1051Examples of further future work consist in improving the cost invariant
1052generator algorithms and the coverage of compiler optimizations, in combining
1053the labeling approach with the type and effect discipline of~\cite{typeffects}
1054to handle languages with implicit memory management, and in experimenting with
1055our tools in early development phases. Some larger case study is also necessary
1056to evaluate the CerCo's prototype on realistic, industrial-scale programs.
1057
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1142
1143\end{document}
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