From reaching definitions, a use-definition chain, ud-chain, can be constructed. This is a list of variable definitions linked with their uses. Within a block, a definition is matched to its subsequent use. If there is no definition within the block, then all definitions which reach the block are matched with the use.
The ud-chain may be used for constant propagation.
This can be optimized still further with an algebraic substitution and a dead code elimination. The result is shown in Example 14.
