This is another project on the list of open projects from a few months ago. The goal is to improve 6502 assembly code used in the firmware of a graphing calculator project. The first step is to manage how local memory is used, since the current options are not that great. One option is stack-based addressing in zero page, which monopolizes the X register. This is one cycle slower than hard coding a static address in zero page and mostly prevents the use of the X register for anything else useful, leading to less efficient code. The other option is assigning each local variable to its own address in zero page, which is very wasteful. With only 256 bytes, you run out of room pretty fast. This project assigns zero page automatically by determining which functions call each other (call graph pictured above) and assigning memory so that functions which never overlap share the same memory. In this test code, 38 bytes of local variables fit into only 19 bytes of zero page. Here is a graph where each row represents one byte of zero page and each block one byte of local memory in a function:
Depending on the shape of the call graph, it might be possible to fit a large number of variables in a small amount of zero page.