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. We now reviewed Global Code Motion. We now look at branch removal. Swift has a phase that uses two methods to attempt to remove branches. The first method removes branches that are used to compute a value that is either 0 or 1. Instead it wants to assign them directly. However in JVM there are no byte codes that produce a boolean value directly from a comparison. A branch is therefore involved however Swift tries to optimize it. It simply searches through all the phi nodes in the method whose two inputs are constants of 0 and 1. If it translates to an If-then-else, then Swift can replace the use of the phi nodes with the controlling boolean of the If node. When the phi node is optimized away, the if-then-else control flow structure is also removed. Swift also tries to remove branches by converting logical and/or expression to bitwise and/or expression.  For example, the code for a conditional that has two conditions with a logical 'and', requires two branches since the second condition is not evaluated if the first one fails. However, if the conditions are fairly simple and have no side effects, then they can be combined with a bitwise AND and then do a conditional branch. This reduction in the number of branches is important because extra branches can reduce the rate of instruction fetching and may even result in a  speculation down the wrong path.
This optimization is fairly easy to implement in the Swift IR. Swift simply looks through all the blocks in CFG for a block that is controlled by an IF node and one of whose successors is also an IF node. If such a block is found, Swift checks the other criteria namely no side effects. There must be no memory write operation, method call or other exception causing operations. If these are satisfied then the conditions can be combined and the branches can be eliminated.