Last week I attended the 44th ACM SIGPLAN Symposium on Principles of Programming Languages (POPL 2017). The event was hosted at Paris 6 which is part of the Sorbonne, University of Paris. It was one of the best POPLs I can remember. The papers are both interesting and informative (you can read them all, for free, from the Open TOC), and the talks I attended were generally of very high quality. (Videos of the talks will be available soon—I will add links to this post.) Also, the attendance hit an all-time high: more than 720 people registered for POPL and/or one of its co-located events.

In this blog post I will highlight a few of my favorite papers at this POPL, as well as the co-located N40AI event, which celebrated 40 years of abstract interpretation. Disclaimer: I do not have time to describe all of the great stuff I saw, and I could only see a fraction of the whole event. So just because I don’t mention something here doesn’t mean it isn’t equally great. ^{1}

## Fast Polyhedra Abstract Domain

A significant portion of the POPL program covered the area of static program analysis. My favorite paper in this category was

Fast Polyhedra Abstract Domain, written by Gagandeep Singh, Markus Puschel, and Martin Vechev, from ETH Zurich. The paper presents a new implementation of the convex polyhedra abstract domain used in numeric static program analysis. Such an analysis considers the possible numeric values that variables could take on during a program’s execution. The analysis might do this to, for example, check whether indices of an array are always within bounds to thus ensure the absence of buffer overflow vulnerabilities.

Several numeric abstract domains have been designed, and several implementations of each are available (e.g., in the popular APRON and PPL libraries). The most precise domain, but the most expensive, is *Convex Polyhedra* (CP). In existing implementations, CP is significantly slower than the least expensive domain, *Intervals*. As such, many analyses have tended to avoid using CP.

Singh et al’s paper presents a new implementation of CP that performs significantly better than existing implementations. Quoting the abstract:

Our experimental results demonstrate massive gains in both space and

time: we show end-to-endspeedups of two to five orders of magnitude

compared to state-of-the-art Polyhedra implementations as well as

significant memory gains, on all larger benchmarks. In fact, in many

cases our analysis terminates in seconds where prior code runs out

of memory or times out after 4 hours.

(Emphasis was mine.) There are two key elements of their approach. First, the authors use a *hybrid representation* of the underlying domain, rather than a single representation. Second, they maintain a separate polyhedron for each set of related variables, rather than a single polyhedron for all variables.

Elaborating on the first idea: There are two implementations of convex polyhedra that are popular, the “constraint” approach, and the “generator” approach. These two approaches differ in the running time of different operations. Notably, operations that are exponential in one implementation are polynomial in the other. As such, we might implement the domain to switch from using one representation when an exponential operation is polynomial in the other. I understand that APRON does this. But converting between representations is itself an exponential operation, so it’s not a panacea.

The second idea is where the rubber hits the road, apparently. Many times, variables used in a program have independent values; e.g., because the program does not compare them to one another directly or transitively. When too many variables are all put in a single polyhedron, however, the result is bad performance. The paper proposes to separately maintain so-called *permissable* decompositions of variables so that a separate polyhedron is maintained for each set of related variables, at no loss to precision. Having three small polyhedra rather than one big one helps overall costs. Moreover, it apparently also underpins more efficient conversions between representations.

The dramatic performance improvements are exciting. An algorithmic revolution in SAT solving ultimately resulted in orders-of-magnitude performance improvements, which prompted a renaissance in program analysis and program synthesis tools that target SAT. Perhaps a similar revolution is in store for numeric static analysis.

## LightDP

Since it was introduced by Dwork, McSherry, Nissim, and Smith in 2006, the concept of differential privacy (DP) has underpinned a substantial amount of research on data privacy. The motivation of DP is to ensure that computations over a collection of individuals’ data do not reveal too much about any single individual. Both Apple and Google have started using DP in some of their data-driven products (in RAPPOR, and various end-user predictive software, respectively).

Several research projects have considered programming languages or systems that ensure DP by construction, e.g., the Fuzz programming language and the Airavat framework for MapReduce computations. However, these systems cannot express sophisticated DP algorithms that boast both private and precise outcomes. For example, neither system could express Dwork and Roth’s Sparse Vector analysis algorithm.

Zhang and Kifer’s LightDP fills this gap. The approach’s first key element is a type system that captures the connection between two runs of the program, written in a fairly standard imperative language. ^{2} These runs are over “adjacent” inputs, e.g., those that differ because one contains a particular individual’s data while the other does not. LightDP’s second key element is a program transformation that takes a correctly typed program and extracts calculations of its impact on the overall *privacy budget.* This is, roughly, the difference in probability of a particular outcome of the program depending on which adjacent input is used. The privacy budget calculations (which account for the program’s use of noise to obscure the outcome) can be solved by an off-the-shelf model checker. The paper shows that LightDP can be used to prove that the Sparse Vector Method, and several other sophisticated algorithms, can be expressed (and verified) with LightDP with minimal programmer effort.

The use of relational typing to drive a program transformation that exposes the impact on privacy budget is really intriguing to me. I’m excited to explore how the approach might be extended to prove leakage bounds on cryptographic algorithms. I am particularly interested in those for oblivious data structures that also rely on randomness to make secret-dependent outcomes hard to distinguish.

## Type Soundness Proofs with Definitional Interpreters

*Small-step* operational semantics express individual steps of a program’s execution, while *big-step* semantics express entire executions. Type soundness proofs on big-step semantics tend to be simpler to construct, but conventional wisdom has it that such proofs do not cover non-terminating executions (at least not without a co-inductive interpretation of the operational rules). But, as pointed out in Amin and Rompf’s POPL paper, Type Soundness Proofs with Definitional Interpreters, Jeremy Siek previously showed that it can be done for the simply-typed lambda calculus (STLC) with three easy lemmas. And these lemmas can be proved with standard structural induction—no coinduction required. The paper title’s mention of a definitional interpreter comes from the fact that big-step semantics can be viewed as a (definitional) interpreter, expressed as a recursive function.

The key idea of Siek’s technique is to extend the semantics with a *fuel* argument. Each recursive call to the interpreter deducts one from the fuel. If the fuel hits zero, there is a “timeout.” If a property (such as type soundness) is proved for all executions of fuel *n*, for all *n*, then it must hold for all finite executions. Specifically, infinite executions are by definition not “stuck,” which is what type soundness is ultimately trying to show. Amin and Rompf’s contribution is to show that this idea scales beyond simple languages like STLC to far more sophisticated languages involving subtyping, polymorphism, and mutable references. In particular, they use this technique to prove correct (with a mechanization in Coq) the Dependent Object Type (DOT) calculus which underpins the type system of Scala.

I had seen the idea of indexing an interpreter function with a “fuel” argument in Pierce’s Software Foundations chapter extension on the IMP imperative language. In that case the index was needed to prove the semantics function terminates, as required by Coq. The use of fuel also resembles step-indexing a logical relation (e.g., per Amed) and indexing a semantics with costs (e.g., per Hoffman’s “potentials” for resource aware ML). Despite knowing of these past works, I had not made the leap that one could similarly use fuel to simplify type soundness proofs, and I had not seen the technique in other papers I’ve read.

Now I am hoping to retrofit some ongoing work to use big-step rather than small-step semantics, and to thereby simplify the currently delicate and complicated proofs. One thing I’ve found I don’t like, so far, is that the semantics is expressed as a Coq function in the paper, while the type system is expressed with inference rules; there is a disconnect in notation between the two. I am experimenting with ways of bringing them more in sync.

In general, I’d be pleased to see more papers (or monographs, blog posts, essays, pearls, etc.) on best practices for mechanizing mathematics. Such practices can ultimately be transformative in helping the community move forward faster, just as Wright and Felleisen’s original paper on syntactic type soundness has done over the last 20+ years.

## 40 years of AI

My final day in Paris was spent at the celebration of 40 years of Abstract Interpretation (counted since the publication of Cousot and Cousot’s seminal paper introducing abstract interpretation at POPL’77). Overall the talks were really interesting; the organizers (listed on the linked page) did a fantastic job! My two favorite talks of the day were Francesco Logozzo‘s talk about the use of abstract interpretation at Facebook, and Patrick Cousot‘s retrospective on the origins and development of abstract interpretation, and what might be in store.

Francesco left Microsoft Research a couple of years ago to work on static analysis tools at Facebook. In his talk, he reported on a static taint analysis tool for PHP programs that now regularly runs on millions of lines of Facebook code. Incredibly, the tool returns very precise results in 25 minutes when run on a 48-core machine with 256ย GB. Francesco’s exhortation to the attendees was to focus on what makes his analysis work: modular, parallelizable designs, to emphasize scalability. To make sure you are doing that, he suggested forcing grad students to build their analyses out of 100 Raspberry Pis, rather than using a proper computer! I was excited to hear that Facebook’s taint analysis code may be released open-source, and that Francesco is preparing a paper about the analysis.

Patrick’s talk was really interesting. It’s rare that we get to learn about the origins of a scientific discovery; usually, we just see the final result. The road to the development of abstract interpretation occurred over many years, so that quite a lot of work was covered in that first paper. Patrick highlighted key ideas along the way (several of which were due to the late Radhia Cousot while she was a student), and key moments of encouragement by fellow scientists. The latter half of his talk showed how ideas suggested in that first paper, or in his thesis, eventually came to fruition in papers published 10, 20, or even more years later. It was very inspiring to see how such a progressive research agenda has unfolded over time. One interesting point that came up in the talk was the fact that Patrick has never published a book about abstract interpretation. He now feels this was a mistake; ^{3} I wonder if that means a book is in the offing …

In all, I really enjoyed my time in Paris at POPL. It has galvanized me to do more reading, thinking, and writing! And I suspect the same is true for the many others who attended.

Notes:

- I also attended PLMW just before POPL, and gave a talk. I may discuss that in another blog post. ↩
- In general, reasoning about multiple executions is important for capturing how one execution might reveal more information than another (or none at all, per the noninterference property). ↩
- He said that he felt if he published a book it might stymie new ideas—everyone might focus on his presentation rather than consider new directions. ↩

I don’t usually leave self-promotional replies to blog posts, but since you asked for more papers in your definitional interpreters section, I’ll make an exception.

For CakeML (https://cakeml.org), we have entirely abandoned small-step semantics and moved all of the compiler and type soundness proofs over to the “fuel”-based style. See our ESOP 16 paper “Functional Big-step Semantics” for a description of the techniques on some simple examples: type soundness for Wright and Felleisen’s example polymorphic, impure ML-language, a step-indexed logical relation for untyped lambda calculus, and compiler correctness for a minimal imperative WHILE language that includes non-deterministic features like I/O and undefined evaluation order.

Neat! I’ll check it out, thanks.

To follow on to what Scott said, a fuel-based (or step-indexed) big-step interpreter is an old idea. For instance, I used a variant of this in my lecture notes in the Oregon Summer School years ago. And Matthew Fluet and I used this for a paper more than a decade ago, and even then stole the idea (with credit) from someone else.

Thanks for clueing me in to this old work! I wish I’d known more about it. I wonder: How do you present this in Latex in your paper? Showing the Coq function for the interpreter but the typeset type rules (as Amin and Rompf did) seems ugly to me; I’d like more uniform notation.

Yes, Greg and I used “natural transition semantics” from Volpano and Smith in the monadic regions work (ICFP’04 and JFP’06), which uses transitions between partial derivations of the big-step semantics. In LaTeX, this looks pretty much like normal big-step rules, except that you allow judgments like “e => ?” in addition to “e => v” and a bit more explicit treatment of the derivations themselves. But, and this is a pretty big flaw with respect to modern formalized PL in proof assistants, we rather handwavily define the partial derivations by reference to the normal big-step semantics; in the JFP paper, we “give up” explicitly writing them down after giving one full page of rules, essentially saying to the reader “you get the idea”. In truth, the blow-up factor is really just the same as going to a small-step semantics, but without some serious reflection capabilities, you wouldn’t be able to automatically generate them from a typical big-step semantics inductive definition in Coq.

Indeed, I learned about it from the POPL 2006 paper by Ernst, Ostermann, and Cook. I cite that work along with some others in the following blog post (which is linked from the three-easy lemmas post in the place where I introduce the counter).

http://siek.blogspot.com/2012/07/big-step-diverging-or-stuck.html

The use of a counter in an interpreter for other purposes (than proving type safety) goes even further back. For example, William Young’s 1988 Ph.D. thesis on a verifying compiler for the gypsy language.

Plotkin used the counter idea to encode process fairness back in 1982 and probably earlier:

Gordon D. Plotkin: A Powerdomain for Countable Non-Determinism (Extended Abstract). ICALP 1982: 418-428 (see “Generative Semantics”)

Thanks. I glanced at Gordon’s paper, but I didn’t see how “fuel” is being used in a way similar to Rompf and Amin’s paper (or the other ones mentioned so far in the comments). That is, in these works, the fuel permits a “big step” semantics to express executions of a certain length, and that length can then be universally quantified in the proof to capture all finite executions. For one difference, I see that Gordon’s paper defines the semantics as small-step, not big-step. Later, the operational semantics is connected to a denotational one. Perhaps you can elaborate on where you see the connection?

Wow… my mind is blown. If I could go back in time when I was working on the FPCC stuff. This would have simplified the whole project. An interesting extension is to make the “fuel” a first class concept and make the monad explicit. This would be interesting to reason about real time deadlines, and execution cost models.

e : int(10) means the expression evaluates to an int *value* with a fuel of 10 without timing out. The interpreter bakes in an execution cost model. Then we have \exists n. e:int(n). to represent expressions that terminate, and \forall n. e: M(int(n)) which just says it’s type safe.. (here M means the interpreter may time out or return a value for any n)

These seem simple extensions, that you could apply to a high-level typed lambda calculus, and then compile down to typed assembly via CPS.

I’m thinking how this can be applied to a sub structural logic. If we can extend this to sub structural logic. I’d be tempted to quit my day job and build a verified compiler for a real time Rust like language. I know folks would kill for this in our day job…

Hi Dan. What you describe about running times resembles Hoffman’s “potentials” technique in RAML, IIUC. And it uses affine reasoning to estimate the worst case.

I took a look a RAML which I was not familiar with before. I don’t think it solves the particular resource issues seen in real time embedded systems, but is more aligned with traditional algorithmic analysis. In embedded systems you only care about worst case, and all inputs are bounded. RAML handles non bounded inputs. Most embedded systems are glorified FSMs written in C. They are buried in the anti-lock breaking system of your car and other safety critical places… because they are glorified FSMs completely simple manual and tedious techniques are often sufficient.

See chapter 14 of http://koopman.us/ which describes best practices from computing these bounds by hand! (crazy, when a computer could do it better.) The book is a good read in general.

Also, I think the general technique using an indexed evaluation function is not limited to big steps, but can be applied to small step with environment semantics. The “wow” factor for me is the syntactic approach to type-safety without meta-theory.

Looking carefully and the induction principles of a small step proof, the only need meta-theory is when you apply progress and preservation to transitive step relation derivation and the substitution principle.

It seems you can index this proof, and prove type soundness for all e –n–> e’ (where e steps to e’ in less than n steps..) you prove that by induction on n. The substitution principle has to be replaced with environment semantics.

I need to allocate some time to fire up Coq and confirm you can use this technique with small-steps environment semantics. This avoids the hand waving around non-terminating evaluations. To me the innovation is syntactic approaches to soundness without metatheory!

I have clearly been doing too much networking. Can someone explain the “problem” with small step semantics that is solved by using big step semantics?

Proofs are simpler in big-step.

For what it’s worth:

In the sub-area of PL for incremental computing, I always gravitate towards the big-step formulation of a semantics (over a small-step one) because I want it to produce *execution traces* that are shaped exactly like the big-step derivation is shaped. In other words, I want a trace structure that resembles a formal notion of a dynamic “call graph”, and not the linearized structure that one would get by “tracing” the small-step semantics.

Further, in the context of an incremental program (that is rerun repeatedly), termination issues aren’t a central concern: We don’t typically try to rerun programs that do not terminate!

BTW: I found the thread above really interesting; I’ll have to look deeper into these “fuel”-based formulations, especially if and when I ever start dealing with programs that don’t terminate. ๐