Using DDL to understand and modify SimpleScalar

Abstract

Many legacy systems are built on the technology of dynamic link libraries. Software engineering tools have not exploited this framework very well. Because the linking is delayed until runtime, it seems natural to provide tools access to the linking process, in order to track it, and even to modify it. There is no reason the dynamic linker must resolve a symbol to the intended function. To this end we have created DDL (the Dynamic Dynamic Linker) by modifying the Gnu dynamic linker to provide an API for developers to use to customize the linking process. We believe that DDL will be useful for a wide variety of software engineering tasks, not the least of which is the reverse engineering of legacy systems, and their maintenance and modification. We present a case study of one such task. This task is the understanding and modification of the SimpleScalar CPU/architectural simulator. We were able to use DDL to understand the behavior of SimpleScalar at a function-call level, and then used DDL to modify the run-time behavior (without modifying the functional source code) to implement the switching between detailed and functional simulation modes (an important extension to the community).