Doubling the number of registers on ARM processors

Abstract

It is critical that more architectural registers are available to the compiler and programmer, as a small number of architectural registers might hinder the compiler and programmer from producing efficient code. Although modern chip manufacturing processes could easily put many registers on an ARM processor, only 16 general registers are accessible by the compiler and programmer. Doubling the number of architectural registers requires adding another bit to each register field of instructions, and hence increases code size. One approach has been developed to use the underutilized condition bits to double the number of architectural registers from 16 to 32 but maintain the width of ARM instructions. Although performance gain can generally be achieved by most of benchmark programs on a 32-register ARM, this approach would still hurt performance for programs with high ratios of instructions that can be conditionalized. Therefore, a better approach would to keep the conditional execution feature even when the 4-bit condition field is used to encode the extra registers. This paper proposes to borrow the IT instruction from Thumb II ISA to represent the predicates of conditionalized instructions on the 32-register ARM. GCC and SimpleScalar/ARM have been modified to handle the new instruction format and the revised IT instruction, and experimental results have shown that performance can be improved by 10.4% on average for MediaBench II benchmarks on the 32-register ARM with conditional execution.

Addressing Mode and Bit Extensions to the Thumb-2 Instruction Set Architecture

“…Increasing the number of physical registers would improve performance, but this requires additional bits to encode the register number. To maintain the width of 32-bit ARM instructions, references [11]- [12] observe that the conditional field is underutilized, and thus, trade the conditional field for the register field in the instruction, and use the 4-bit conditional field to encode the extra registers, which allows the number of physical registers to be doubled from 16 to 32 without increasing code size.…”