03/01/1992 09:00 AM
Computer Science
Optimizing compilers produce code that impedes source-level debugging. Examples are given in which optimization changes the behavior of a program even when the optimizer is correct, showing that in some circumstances it is not possible to completely debug an unoptimized version of a program. Source-level debuggers designed for unoptimized code may mislead the debugger user when invoked on optimized code. One situation that can mislead the user is a mismatch between where the user expects a breakpoint to be located and the breakpoint\'s actual location. This mismatch may occur due to statement reordering and discontiguous code generated from a statement. This paper describes a mapping between statements and breakpoint locations that ameliorates this problem. The mapping enables debugger behavior on optimized code that approximates debugger behavior on unoptimized code closely enough that the user need not make severe changes in debugging strategies. Another situation that can mislead the user is when optimization has caused the value of a variable to be noncurrent -- - to differ from the value that would be predicted by a close reading of the source code. This paper gives and proves a method of determining when this has occurred, and shows how a debugger can describe the relevant effects of optimization. The determination method is more general than previously published methods. The information a compiler must make available to the debugger for this task is also described.
