Debugging Optimized Code Without Being Misled

ly, this would-be-predicted value is the value that would be given to the variable if the program were running on a machine whose instruction set is the source language. In unoptimized code, at each breakpoint the expected value of every variable is identical to its actual value. In optimized code, as we have seen, the actual value of a variable at some point may differ from its expected value at that point. Hennessy [Hen82] introduced the terms current, noncurrent, and endangered to describe the relationship between a variable's actual value and its expected value at a valid breakpoint. This relationship is described on the basis of a static analysis, one that inherently cannot use information about how the breakpoint was reached. Informally, a variable V is current at a breakpoint B if its actual value at B is guaranteed to be the same as its expected value at B no matter what path was taken to B. Examples of current variables are given in Figures 3.1 and 3.2. All examples use prog..