Paper Info
Title
Taming undefined behavior in LLVM.
Abstract
A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. The IR for every optimizing compiler we have looked at, including GCC, LLVM, Intel's, and Microsoft's, supports one or more forms of undefined behavior (UB), not only to reflect the semantics of UB-heavy programming languages such as C and C++, but also to model inherently unsafe low-level operations such as memory stores and to avoid over-constraining IR semantics to the point that desirable transformations become illegal. The current semantics of LLVM's IR fails to justify some cases of loop unswitching, global value numbering, and other important \"textbook\" optimizations, causing long-standing bugs. We present solutions to the problems we have identified in LLVM's IR and show that most optimizations currently in LLVM remain sound, and that some desirable new transformations become permissible. Our solutions do not degrade compile time or performance of generated code.
Year
DOI
Venue
2017
10.1145/3062341.3062343
PLDI
Keywords
Field
DocType
compilers,undefined behavior,intermediate representations
Global value numbering,Programming language,Computer science,Compile time,Loop unswitching,Parallel computing,Static analysis,Undefined behavior,Real-time computing,Optimizing compiler,Compiler,Semantics
Conference
Volume
Issue
ISSN
52
6
0362-1340
Citations 
PageRank 
References 
9
0.54
19
Authors
8
Name
Order
Citations
PageRank
Juneyoung Lee1111.92
Yoonseung Kim2191.35
Youngju Song3111.24
Chung-Kil Hur425417.07
Sanjoy Das522639.18
David Majnemer690.54
John Regehr7111.25
Nuno Lopes8152.01