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Scalar Symbolic Analysis.

Parallelizing compilers perform data dependence analysis on arrays to check for loop-carried dependences on individual array elements. Array data dependence analysis is most effective when all subscript expressions are affine functions of loop indices and loop invariants. Within this domain, data dependence analysis has been shown to be equivalent to integer programming. While integer programming is potentially expensive, the data dependence problems found in practice are simple, and efficient algorithms have been developed that usually solve these problems exactly [7,20]. For this reason, parallelizing compilers incorporate a host of scalar symbolic analyses to put array indices in affine form, including constant propagation, value numbering, and induction variable recognition. These analyses provide integer coefficients for subscript variables and derive affine equality relationships among variables. Some systems also propagate inequality relations and other relational constraints on integer variables imposed by surrounding code constructs ( IFs and loops) to their uses in array subscripts [12,15].

Relations on Non-Linear Variables. Propagating only affine relations among scalars is not sufficient to parallelize some loops. For example, scientific codes often linearize accesses to (conceptually) multidimensional arrays, resulting in subscript expressions that cannot be expressed as an affine function of the enclosing loop indices. The loop nest in Figure 1(a), from the Perfect benchmark trfd, illustrates such a situation. Figure 1(b) shows information that can be determined by a symbolic analysis of the loop. The access to array XRSIJ does not induce a dependence between iterations of the outer loop, since the index MRSIJ never has the same value on two different iterations.

  
Figure 1: Non-linear analysis example.



Saman Amarasinghe
Fri Sep 15 09:15:06 PDT 1995