Alias-free parameters in C using multibodies

Assaad, Medhat (2001) Alias-free parameters in C using multibodies. Technical Report 01-05, Computer Science, Iowa State University.

Abstract

Aliasing may cause problems for both optimization and reasoning about programs. Since static alias analysis is expensive, and sometimes, even impossible, compilers tend to be conservative. This usually leads to missing some optimization opportunities. Furthermore, when trying to understand code, one must consider all possible aliasing patterns. This can make reasonning about code difficult and non-trivial. We have implemented an approach for having alias-free parameters in C. This approach allows aliasing between the arguments at the call site, but guarantees that there will be no aliasing between arguments and between arguments and globals inside procedure bodies. This is done by having multiple bodies, up to one for each aliasing pattern. Procedure calls will be dispatched to the body that matches the aliasing pattern at the call site. By having alias-free parameters in the bodies, good optimization can be achieved. Furthermore, verification and reasoning about the code is easier when there is no aliasing. The main advantages of our approach are that code to determine the aliasing pattern is automatically generated, and the programmer does not have to code it by hand. We tested our implementation, and found that using this approach can be very practical. It is easy to convert already-existing code to use it. And in some cases, we had run-time execution speedup as much as 29%.