Porting of GNU linker for the EVP DSP
João Carlos Cordeiro Pires · Portuguese National Funding Agency for Science, Research and Technology (RCAAP Project by FCT) · 2014
Nowadays, with the ubiquitous presence of mobile devices, modems play a relevant role. The EVP is a vector processor used by Ericsson on its 2G, 3G and 4G modems. Due to size, weight and power constraints, mobile devices have limited power sources, and because of that the hardware must be power efficient. The processors used in this type of devices have multiple power consumption states. If a processor is in a idle state i.e. without code to execute, it can switch off and consume less power. Moreover, if it is known beforehand that the processing takes less time than available, the processor can be set to a lower frequency (and lower voltage level), executing the same amount of work, but consuming considerable less energy. With this dynamic power consumption the autonomy of the devices is extended, because the processor can be in a reduced power consumption state most of the time. One way to maximize the time during which the processor stays in a reduced power consumption state is through the execution of optimized code. This can be achieved through manual code optimization, or by letting the compiler apply optimizations on the code. Link Time Optimization (LTO) is a compilation technique that tries to expand the optimizations scope by joining the input files and compile them as a unique file. Using this technique the compiler has more information during the optimization phase and should be able to do better decisions about the optimizations to apply. The team responsible for the development of Embedded Vector Processor (EVP) wanted to evaluate the benefits of LTO. To be able to implement and evaluate LTO it was necessary to migrate the GNU linker to support EVP and enable LTO compilation on the EVP backend of GNU Compiler Collection (GCC). This dissertation reports this work, including the necessary concepts to understand the problem addressed, how was added EVP support in the GNU linker, and the necessary step to enable LTO compilation on GCC. Also it will be presented and analysed the performance improvements of using LTO. The LTO compilation presents mixed results some of the test cases have a small performance improvements, and other present a small performance degradation. This was kind is expected because of the organization of the test code. It is composed by several files that do not have dependencies between them, in this situation the LTO compilation do not present much benefits.