A NEW APPROACH TO DEBUGGING OPTIMIZED CODE

Debugging optimized code is a desirable capability not provided by most current debuggers. Users are forced to debug the unoptimized code when a bug occurs in the optimized version. Current research offers partial solutions for a small class of optimizations, but not a unified approach that handles a wide range of optimizations, such as the sophisticated optimizations performed by supercomputer compilers. The trend with current research is to make the effects of optimization transparent, i.e., provide the same behavior as that of the unoptimized program. We contend that this approach is neither totally feasible nor entirely desirable. Instead, we propose a new approach based on the premise that one should be able to debug the optimized code. This implies mapping the current state of execution back to the original source, tracking the location of variables, and mapping compiler-synthesized variables back to user-defined induction variables. To aid the user in understanding program behavior, various visual means are provided, e.g., different forms of highlighting and annotating of the source/assembly code. While this unavoidably requires the user to have a basic understanding of the optimizations performed, it permits the user to see what is actually happening, infer the optimizations performed, and detect bugs. An example illustrates the effectiveness of visual feedback. To support conventional debugger functionality for optimized code, the compiler must generate additional information. Current compiler-debugger interfaces (CDIs) were neither designed to handle this new information nor are they extensible in a straight forward manner. Therefore, a new CDI was designed that supports providing visual feedback and the debugging of optimized code. This paper specifies the details of a new CDI and relates each feature back to the debugger functionality it supports.