Today we continue to read the WRL research report on the Swift Java Compiler. We saw that the Global Common SubExpression elimination and global code motion have significant optimization  benefits In addition there was one modification made to the partitioning algorithm to deal with values that cause exceptions. Exception causing values cannot be moved from their original block so code motion can not be applied to here to ensure that the values dominate their use after CSE is applied. If we take the case of two identical exceptions causing values, the first value can only replace the second if the first values block dominates the second values block. Also, the second values' block is redundant only if the first block did not throw an exception. Here Swift requires that the second values block is dominated by the non-exception successor of the first values block.
These conditions are satisfied if the CSE algorithm is modified as follows. First is the rule that the dominating value of the partition of exception causing values is kept as the first element of the partition. At the end of each run, the value in each partition is dominated by the the exception causing values otherwise the partition is split by making the exception causing value as the first element of a split into those dominated by this exception causing value and otherwise. The smaller of the two partitions is added to the worklist for another run. This is repeated until all the partitions have their value dominating the rest.
Global CSE is effective in eliminating runtime checks automatically and for compressing the control flow graph. Both the CSE and Code motion is executed once after all the inter procedural analysis and during machine independent processing. Swift does another round of it after the conversion of machine-dependent IR, in order to eliminate the common sub expressions that appear in the lower level form.