Software vector chaining
M. Anton Ertl · 2018
Providing vectors of run-time determined length as opaque value types is a good interface between the machine-level SIMD instructions and portable application-oriented programming languages. Implementing vector operations requires a loop that breaks the vector into SIMD-register-sized chunks. A compiler can fuse the loops of several vector operations together. However, during normal compilation this is only easy if no other control structures are involved. This paper explores an alternative: collect a trace of vector operations at run-time (following the program control flow during this collecting step), and then perform the combined vector loop. This arrangement has a certain run-time overhead, but its implementation is simpler and can happen independently, in a library. Preliminary performance results indicate that the overhead makes this approach beneficial only for long vectors (> 1KB). For shorter vectors, unfused loops should be used in a library setting. Fortunately, this choice can be made at run time, individually for each vector operation.