Motivation
In C and C++ programs, a source of particularly difficult-to-diagnose errors is the nondeterministic behavior that results from reading uninitialized variables; this behavior can vary between platforms, builds, and even from run to run.
There are two common ways to solve this problem. One is to ensure that all locations are written before they are read. Rice's theorem establishes that this problem cannot be solved in general for all programs; however, it is possible to create a conservative (imprecise) analysis that will accept only programs that satisfy this constraint, while rejecting some correct programs, and definite assignment analysis is such an analysis. The Java and C# programming language specifications require that the compiler report a compile-time error if the analysis fails. Both languages require a specific form of the analysis that is spelled out in meticulous detail. In Java, this analysis was formalized by Stärk et al., and some correct programs are rejected and must be altered to introduce explicit unnecessary assignments. In C#, this analysis was formalized by Fruja, and is precise as well as sound, in the sense that all variables assigned along all control flow paths will be considered definitely assigned. The Cyclone language also requires programs to pass a definite assignment analysis, but only on variables with pointer types, to ease porting of C programs.
The second way to solve the problem is to automatically initialize all locations to some fixed, predictable value at the point at which they are defined, but this introduces new assignments that may impede performance. In this case, definite assignment analysis enables a compiler optimization where redundant assignments — assignments followed only by other assignments with no possible intervening reads — can be eliminated. In this case, no programs are rejected, but programs for which the analysis fails to recognize definite assignment may contain redundant initialization. The Common Language Infrastructure relies on this approach.
Read more about this topic: Definite Assignment Analysis
Famous quotes containing the word motivation:
“Self-determination has to mean that the leader is your individual gut, and heart, and mind or we’re talking about power, again, and its rather well-known impurities. Who is really going to care whether you live or die and who is going to know the most intimate motivation for your laughter and your tears is the only person to be trusted to speak for you and to decide what you will or will not do.”
—June Jordan (b. 1939)