A.4 Data Dependence Relations

For any two statements, S1 and S2 , one of the three types of data dependence relations may be true, or the statements may be data independent.

If some item X is in OUT(S1) and X is in IN(S2) and S2 is to use the value of X computed in S1, S2 is flow dependent on S1 , as in example 1 in Section A.2.

If some item X is in IN(S1) and X is in OUT(S2) , but S1 is to use the value of X before it is changed by S2 , S2 is antidependent on S1 , as in example 2 of Section A.2.

If some item X is in OUT(S1) and X is in OUT(S2) and the value computed by S2 is to be stored after the value computed by S1 is stored, S2 is output dependent on S1 , as in example 3 of Section A.2.

Antidependence and output dependence relations are sometimes inadvertently caused by programmers' coding practices. These dependencies can often be removed by more careful coding.


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