Optimal Stack Slot Assignment in GCC Naveen Sharma Sanjiv Kumar Gupta System Software Group HCL Technologies Ltd

Several microprocessors, used in digital signal processing and embedded devices, have lim-ited displacement (4-6 bit) in “register + off-set ” addressing mode. In some cases, only auto increment/decrement addressing modes are available. Hence, while accessing data on local frame, there are number of explicit in-structions whose sole purpose is to reach the desired data. This paper describes the impact of layout of local variables on performance and code size for these architectures. It also de-scribes the techniques for optimal assignments of stack offsets such that instructions for ad-dress arithmetic for access of local variables are minimized. The implementation of the techniques in GCC is also discussed. Results indicate an improvement of 2%-7 % in code size and 5-9 % improvement in execution tim-ings for several benchmarks.