On Single-Valuedness in Textually Aligned SPMD Programs

Abstract

Single-valuedness is a property of an expression occurring in a SPMD program and states that concomitant evaluations of this expression lead to the same value at all processes. Although widely used, this property still lacks a formal definition, which is necessary to tackle the subtleties of the notion of concomitance. First, we propose such a definition in which the states of all processes can be compared when they reach textually aligned program points. These program points, of which all processes execute exactly the same textual instances, act as logical global synchronization barriers. Single-valuedness is then defined at these program points. Secondly, we show how textual alignment and single-valuedness can be used to ensure proper use of Direct Remote Memory Access (DRMA) in BSP programs.