Porting GCC to the IBM S/390 platform

IBM’s mainframe architecture S/390 is the living architecture with the longest heritage, defined in a time when assembler program-ming was predominant and compilers were in their childhood. Hence in porting GCC to S/390 we had to cope with certain architec-ture features that were difficult or impossible to model in GCC’s architecture-independent framework. These include 31-bit addressing mode, instruction-dependent address formats, limited availability of address displacements and immediate literals, and the condition code handling. These problems notwithstanding, the S/390 back end matured over the last couple of years to make GCC a stable and competi-tive compiler for the S/390 platform. In this paper we want to share how we managed to handle most of the mentioned architecture fea-tures. We also want to point out areas that promise room for further improvement in the back end itself and suggest middle-end modifi-cations that would benefit our platform in par-ticular.