Restrictification of function arguments

Abstract

Pointer aliasing still hinders compiler optimizations, in spite of years of research on pointer disambiguation. Because the automatic disambiguation of pointers is a difficult endeavor, several programming languages offer programmers mechanisms to distinguish memory references, such as the “restrict” keyword in C. However, the use of such mechanisms is prone to human mistakes. In this paper we present a suite of automatic techniques that mitigate this problem. We have designed, implemented and tested three different ways to disambiguate pointers passed as arguments of functions. Our techniques combine static analyses to infer symbolic bounds of memory regions and code versioning. We generate a clone for each function whose arguments we can disambiguate and optimize it assuming the absence of aliasing among formal parameters. At runtime, we use the results of the symbolic interval tests to decide which version of a function we should call: the original one or the “restricted” clone, whenever we can prove that no aliasing can occur at runtime. An implementation of our restrictification methods in LLVM shows that we can vectorize up to 63% more operations than what could be accomplished using the -O3 optimization level of said compiler. When applying the optimization on OpenCV benchmarks, we have observed speedups as great as 40%.