Alias Analysis in the DEC C and DIGITAL C++ Compilers

Vol. 10 No. 1 1998 When two or more address expressions reference the same memory location, these address expressions are aliases for each other. A compiler performs alias analysis to detect which address expressions do not reference the same memory locations. Good alias analysis is essential to the generation of efficient code. Code motion out of loops, common subexpression elimination, allocation of variables to registers, and detection of uninitialized variables all depend upon the compiler knowing which objects a load or a store operation could reference. Address expressions may be symbol expressions or pointer expressions. In the C and C++ languages, a compiler always knows what object a symbol expression references. The same is not true with pointer expressions. Determining which objects a pointer expression may reference is an ongoing topic of research. Most of the research in this area focuses on the use of techniques that track which object a pointer expression might point to. When these techniques cannot make this determination, they assume that the pointer expression points to any object whose address has been taken. These techniques generally ignore the type information available to the source program. The best techniques perform interprocedural analysis to improve their accuracy. Although effective, the cost of analyzing a complete program can make this analysis impractical. In contrast, the DEC C and DIGITAL C++ compilers use high-level type information as they perform alias analysis on a routine-by-routine basis. Limiting alias analysis to within a routine reduces its cost, albeit at the cost of reducing its effectiveness. The use of this type information results in slight improvements in the performance of some standardconforming C and C++ programs. These improvements come at little expense in terms of compilation time. There is, however, a risk that the use of this type information on nonstandard-conforming C or C++ programs may result in the compiler producing code that exhibits unexpected behavior. Alias Analysis in the DEC C and DIGITAL C++ Compilers August G. Reinig