Provably correct peephole optimizations with alive

Abstract

Compilers should not miscompile. Our work addresses problems in developing peephole optimizations that perform local rewriting to improve the efficiency of LLVM code. These optimizations are individually difficult to get right, particularly in the presence of undefined behavior; taken together they represent a persistent source of bugs. This paper presents Alive, a domain-specific language for writing optimizations and for automatically either proving them correct or else generating counterexamples. Furthermore, Alive can be automatically translated into C++ code that is suitable for inclusion in an LLVM optimization pass. Alive is based on an attempt to balance usability and formal methods; for example, it captures---but largely hides---the detailed semantics of three different kinds of undefined behavior in LLVM. We have translated more than 300 LLVM optimizations into Alive and, in the process, found that eight of them were wrong.

Towards Bit-Width-Independent Proofs in SMT Solvers

“…We apply the techniques from Section 3 to three case studies: (i) verification of invertibility conditions from Niemetz et al [19]; (ii) verification of compiler optimizations as generated by Alive [17]; and (iii) verification of rewrite rules that are used in SMT solvers. For these case studies, we consider a set of verification conditions that originally use fixed-size bit-vectors, and exclude formulas involving multiple bit-widths.…”

“…Lopes et al [17] introduces Alive, a tool for proving the correctness of compiler peephole optimizations. Alive has a high-level language for specifying optimizations.…”

“…As a second case study, we examine the bit-width independent verification of compiler optimizations in LLVM. For that, we use the Alive tool [17], which generates verification conditions for such optimizations in the theory of fixed-size bit-vectors. Proving the correctness of these optimizations for arbitrary bit-widths would ensure their correctness for any language and underlying architecture rather than specific ones.…”

Alive-Infer: data-driven precondition inference for peephole optimizations in LLVM

“…In LLVM, peephole optimizations find code fragments in an input program that match a pattern, and replace them with an equivalent set of instructions. They are also a persistent source of LLVM bugs [25,30,51].…”

“…We have addressed the problem of peephole optimization bugs with Alive, a domain-specific language for specifying and verifying peephole optimizations in LLVM [30].…”