We continue our discussion on the WRL research report on the Swift Java compiler.  We were listing the various analysis and optimizations by the Swift compiler. We now go through each of these one by one. Let us first take a look at building the IR.
Like other compilers, Swift generates the IR from a method's byte code. First the byte code is scanned to determine the number of basic blocks and the edges between the basic blocks.  Then the variables that require phi nodes in the basic blocks are determined. This logic is called the phi node placement algorithm. The byte codes of each of the basic blocks are executed via an abstract interpretation. This allows us to maintain an association between the JVM local variables and the SSA values. It also allows us to determine the appropriate inputs for the new values and build the SSA graph.
During the creation of the SSA graph itself, various simple optimizations are performed. These include It replaces the array length operation with the allocated size of the array. It can eliminate bounds check if the index and length are constant in the current method. Profile information generated from earlier runs can also be used to annotate the edges of the CFG to indicate the execution frequency.  This is helpful for code layout and choice of traces by the trace scheduler. Without profile information, Swift has to guess these values from detecting loops.
Swift checks and maintains the invariant that there are no critical edges in the CFG graph. A critical edge is one with multiple successors and whose destination is a block with multiple predecessors. If these are not eliminated, they hinder the register allocations.  Moreover, it allows global code motion to the greatest freedom in placing values so as to minimize unnecessary computation in the final code.  An empty block is placed between the source and the destination to remove a critical edge.
#codingexercise
Double GetAlternateEvenNumberRangeSumEighthPowerRootSquare (Double [] A)
{
if (A == null) return 0;
Return A.AlternateEvenNumberRangeSumEighthPowerRootSquare();
}