Introduction

Variable tracking is not an optimization pass. It computes information that helps the debug output to be better. It tracks where the variables are stored at each position in instruction stream. It is run as one of the last passes in GCC so that no pass could make this information out of date. The debug info is then generated according to this information.

Intel wrote the pass before us and it worked. But they used the code for writing of dwarf so that it could not be contributed to GCC. Cygnus tried to write similar thing with Live-Range Splitting (splitting a variable into several ones so that the variables that are not used in a loop can be moved to stack) and Stabs. Their solution was done by notes generated before Reload and nonstandard extensions and never came to GCC mainline.

People working on GDB like our solution and they are working on the support for it. But it is not done yet so the only way how to use Variable Tracking is use Intel compiler for i960 and run GDB for i960 modified by Intel in a simulator.

After our patch will be contributed to mainline, it will be possible to add the Live-Range Splitting and Webizer, the Register Renaming will be allowed to run with -O2^9.1 instead of -O3^9.1etc. These optimizations will make the code generated by GCC even faster.