Custom Data Mapping for Composable Data Management

Early experiences with OpenMP 4.0, as well as other directive-based offload models, have shown that deep copy is a key challenge to porting complex applications to offload directives. Without a flexible deep-copy mechanism, pointer-based data structures are at best difficult to manage, particularly when shared memory between the host and device cannot be assumed. Despite the importance of the issue, and the considerable effort expended by vendors, standards bodies and users, no solution has emerged as the clear choice. We propose an approach that combines a restricted compiler-assisted (sometimes called "true") deep copy with a mechanism for users to register their own custom mapping implementations that we call packers. This combination offers the flexibility to address complex cases when necessary while keeping the complexity out of the directives, a balance that serves all cases.