VSL 2014: VIENNA SUMMER OF LOGIC 2014
ACL2 ON SUNDAY, JULY 13TH, 2014
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09:00-10:15 Session 23E: Non-standard Analysis
Location: FH, Hörsaal 7
09:00
Formal Verification of Medina's Sequence of Polynomials for Approximating Arctangent
SPEAKER: unknown

ABSTRACT. The verification of many algorithms for calculating transcendental functions is based on polynomial approximations to these functions, often Taylor series approximations. However, computing and verifying approximations to the arctangent function are very challenging problems, in large part because the Taylor series converges very slowly to arctangent---a 57th-degree polynomial is needed to get three decimal places for arctan(0.95). Medina proposed a series of polynomials that approximate arctangent with far faster convergence---a 7th-degree polynomial is all that is needed to get three decimal places for arctan(0.95). We present in this paper a proof in ACL2(r) of the correctness and convergence rate of this sequence of polynomials. The proof is particularly beautiful, in that it uses many results from real analysis. Some of these necessary results were proven in prior work, but some were proven as part of this effort.

09:30
Equivalence of the Traditional and Non-Standard Definitions of Concepts from Real Analysis
SPEAKER: unknown

ABSTRACT. ACL2(r) is a variant of ACL2 that supports the irrational real and complex numbers. Its logical foundation is based on internal set theory (IST), an axiomatic formalization of non-standard analysis (NSA). Familiar ideas from analysis, such as continuity, differentiability, and integrability, are defined quite differently in NSA---some would argue the NSA definitions are more intuitive. In previous work, we have adopted the NSA definitions in ACL2(r), and simply taken as granted that these are equivalent to the traditional analysis notions, e.g., to the familiar $\epsilon$-$\delta$ definitions. However, we argue in this paper that there are circumstances when the more traditional definitions are advantageous in the setting of ACL2(r), precisely because the traditional notions are classical, so they are unencumbered by IST limitations on inference rules such as induction or the use of pseudo-lambda terms in functional instantiation. To address this concern, we describe a formal proof in ACL2(r) of the equivalence of the traditional and non-standards definitions of these notions.

10:15-10:45Coffee Break
10:45-13:00 Session 26R: Verification
Location: FH, Hörsaal 7
10:45
Modeling Algorithms in SystemC and ACL2
SPEAKER: John O'Leary

ABSTRACT. We describe the formal language MASC, based on a subset of SystemC and intended for modeling algorithms to be implemented in hardware. By means of a special-purpose parser, an algorithm coded in SystemC is converted to a MASC model for the purpose of documentation, which in turn is translated to ACL2 for formal verification. The parser also generates a SystemC variant that is suitable as input to a high-level synthesis tool. As an illustration of this methodology, we describe a proof of correctness of a simple 32-bit radix-4 multiplier.

11:15
Development of a Translator from LLVM to ACL2
SPEAKER: unknown

ABSTRACT. In our current work a library of formally verified software components is to be created, and assembled, using the Low-Level Virtual Machine (LLVM) intermediate form, into subsystems whose top-level assurance relies on the assurance of the individual components. We have thus undertaken a project to build a translator from LLVM to the applicative subset of Common Lisp accepted by the ACL2 theorem prover. Our translator produces executable ACL2 formal models, allowing us to both prove theorems about the translated models as well as validate those models by testing. The resulting models can be translated and certified without user intervention, even for code with loops, thanks to the use of the \texttt{def::ung} macro which allows us to defer the question of termination. Initial measurements of concrete execution for translated LLVM functions indicate that performance is nearly 2.4 million LLVM instructions per second on a typical laptop computer. In this paper we overview the translation process and illustrate the translator's capabilities by way of a concrete example, including both a functional correctness theorem as well as a validation test for that example.

11:45
An ACL2 Mechanization of an Axiomatic Framework for Weak Memory

ABSTRACT. Proving the correctness of programs written for multiple processors is a challenging proglem, due in no small part to the weaker memory guarantees afforded by most modern architectures. In particular, the existence of store buffers means that the programmer can no longer assume that writes to different locations become visible to all processors in the same order. However, all practical architectures do provide a collection of weaker guarantees about memory consistency across processors, which enable the programmer to write provably correct

12:15
Using ACL2 to Verify Loop Pipelining in Behavioral Synthesis
SPEAKER: unknown

ABSTRACT. Behavioral synthesis involves compiling an Electronic System-Level (ESL) design into its Register-Transfer Level (RTL) implementation. Loop pipelining is one of the most critical and complex transformations employed in behavioral synthesis. Certifying the loop pipelining algorithm is challenging because there is a huge semantic gap between the input sequential design and the output pipelined implementation making it infeasible to verify their equivalence with automated sequential equivalence checking techniques. We discuss our ongoing effort using ACL2 to certify loop pipelining transformation. The completion of the proof is work in progress. However, some of the insights developed so far may already be of value to the ACL2 community. In particular, we discuss the key invariant we formalized, which is very different from that used in most pipeline proofs. We discuss the needs for this invariant, its formalization in ACL2, and our envisioned proof using the invariant. We also discuss some trade-offs, challenges, and insights developed in course of the project.

13:00-14:30Lunch Break
14:30-16:00 Session 31N: Keynote Magnus Myreen
Location: FH, Hörsaal 7
14:30
Machine-code verification: experience of tackling medium-sized case studies using 'decompilation into logic’
SPEAKER: Magnus Myreen

ABSTRACT. All software executes in the form of machine code. Ideally, software verification should reach down to precise statements about the concrete machine code that runs.

During my PhD, I developed infrastructure in the HOL4 theorem prover for verification of programs at the level of machine code. The infrastructure includes a programming logic and a proof-producing decompiler and compiler.

In the subsequent five years, this infrastructure has been used in two medium-sized case studies: construction of a verified implementation of read-eval-print loop for Lisp, which can run Jared Davis' Milawa prover; and verification of the accuracy of the GCC compiler's output for the C implementation of the seL4 microkernel. These case studies resulted in HOL4 theorems about thousands of lines of concrete x86-64 and ARM machine code, respectively.

In this talk, I will explain the existing infrastructure for machine-code verification; describe how the infrastructure had to adapt to the case studies; and reflect on future research directions.

This talk describes work done in collaboration with Anthony Fox (ARM ISA specification), Jared Davis (Milawa theorem prover), Thomas Sewell and Gerwin Klein (seL4 microkernel).

16:00-16:30Coffee Break