Porting the LHCb Stack from x86 (Intel) to aarch64 (ARM) and ppc64le (PowerPC)

LHCb is undergoing major changes in its data selection and processing chain for the upcoming LHC Run 3 starting in 2021. With this in sight several initiatives have been launched to optimise the software stack. This contribution discusses porting the LHCb Stack from x86_64 architecture to both architectures aarch64 and ppc64le with the goal to evaluate the performance and the cost of the computing infrastructure for the High Level Trigger (HLT). This requires porting a stack with more than five million lines of code and finding working versions of external libraries provided by LCG. Across all software packages the biggest challenge is the growing use of vectorisation - as many vectorisation libraries are specialised on x86 architecture and do not have any support for other architectures. In spite of these challenges we have successfully ported the LHCb High Level Trigger code to aarch64 and ppc64le. This contribution discusses the status and plans for the porting of the software as well as the LHCb approach for tackling code vectorisation in a platform independent way.