Next use information

. for register and temporary allocation

. remove variables from registers if not used

. statement X = Y op Z defines X and uses Y and Z

. scan each basic blocks backwards

. assume all temporaries are dead on exit and all user variables are live on exit

The use of a name in a three-address statement is defined as follows. Suppose three-address statement i assigns a value to x. If statement j has x as an operand, and control can flow from statement i to j along a path that has no intervening assignments to x, then we say statement j uses the value of x computed at i. We wish to determine for each three-address statement x := y op z what the next uses of x, y and z are. We collect next-use information about names in basic blocks. If the name in a register is no longer needed, then the register can be assigned to some other name. This idea of keeping a name in storage only if it will be used subsequently can be applied in a number of contexts. It is used to assign space for attribute values. The simple code generator applies it to register assignment. Our algorithm is to determine next uses makes a backward pass over each basic block, recording (in the symbol table) for each name x whether x has a next use in the block and if not, whether it is live on exit from that block. We can assume that all non-temporary variables are live on exit and all temporary variables are dead on exit.