Removing Useless Remappings { Report A{275{cri

This report describes the optimization implemented in hpfc, our prototype hpf compiler, to remove useless remappings. This optimization is performed on the remapping graph. The construction of this graph from the control ow graph is rst presented. Then the optimization is formalized as a may data-ow problem on this graph. Finally the results are discussed. 1 Remapping Graph Useless remappings may appear naturally in hpf programs. First, the change of both alignment and distribution of an array requires a realign and a redistribute, hence resulting in two remappings if no special care is taken. Second, the redistribution of a template 1 induces the remapping of all aligned arrays, even if they are not all refer-enced afterwards. Third, at an interprocedural level, two consecutive subroutine calls may require the same remapping for a given array, resulting in a useless remapping on return from the rst subroutine and on entry in the second. If two diierent mappings are required, it may also be interesting to remap data directly rather than using the intermediate original mapping. Such examples do not arise from badly written programs , but from a normal use of hpf features. They demonstrate the need for compile time optimizations to avoid useless costly remappings at runtime. Let us consider example remaps in Figure 1. The loop nest involving two remap-pings is typical of ADI computations. Template T is redistributed at 1, inducing B 1 Even when no templates are used 3] array redistributions generate the problem