Taming Undefined Behavior in LLVM
https://www.microsoft.com/en-us/research/publication/taming-undefined-behavior-llvm/# [www.microsoft.com]
2017-06-25 00:51
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.