/**
* @return an integer telling which predecessor of Y in CFG is X. The jth operand of a
* phi-function in Y corresponds to the jth predecessor of Y from the listing of the in edges
* of Y
*/
private int whichPred(BasicBlock X, BasicBlock Y) {
int j = 0;
for (BasicBlock P : cfg.getPredecessors(X)) {
if (P.equals(Y)) {
return j;
}
j++;
}
throw new IllegalArgumentException("X is not a predecessor of Y");
}
/** Inserts PHI functions at the beginning of basic blocks. */
private void insertPhiFunctions() {
// See Figure 11
// http://www.cs.utexas.edu/~pingali/CS380C/2010/papers/ssaCytron.pdf
int iterCount = 0;
Map<BasicBlock, Integer> hasAlready = Maps.newHashMap();
Map<BasicBlock, Integer> work = Maps.newHashMap();
for (BasicBlock X : cfg.getLiveBasicBlocks()) {
hasAlready.put(X, 0);
work.put(X, 0);
}
Queue<BasicBlock> W = Lists.newLinkedList();
for (Variable V : cfg.variables()) {
iterCount = iterCount + 1;
for (BasicBlock X :
Iterables.filter(cfg.getLiveBasicBlocks(), CfgPredicates.containsAssignmentTo(V))) {
work.put(X, iterCount);
W.add(X);
}
while (!W.isEmpty()) {
BasicBlock X = W.poll();
for (BasicBlock Y : dtree.getFrontier(X)) {
if (X != cfg.getExit()) {
if (hasAlready.get(Y) < iterCount) {
Y.insertPhiFunction(V, cfg.getPredecessors(Y).size());
// place (V <- phi(V,..., V)) at Y
hasAlready.put(Y, iterCount);
if (work.get(Y) < iterCount) {
work.put(Y, iterCount);
W.add(Y);
}
}
}
}
}
}
}
private void search(BasicBlock X) {
for (Statement stmt : X.getStatements()) {
Expression rhs = stmt.getRHS();
if (!(rhs instanceof PhiFunction)) {
for (Variable V : rhs.variables()) {
int i = Top(V);
rhs = rhs.replaceVariable(V, new SsaVariable(V, i));
}
stmt = X.replaceStatement(stmt, stmt.withRHS(rhs));
}
if (stmt instanceof Assignment) {
Assignment assignment = (Assignment) stmt;
if (assignment.getLHS() instanceof Variable) {
Variable V = (Variable) assignment.getLHS();
int i = C.get(V);
X.replaceStatement(assignment, assignment.withLHS(new SsaVariable(V, i)));
S.get(V).push(i);
C.put(V, i + 1);
}
}
}
for (BasicBlock Y : cfg.getSuccessors(X)) {
int j = whichPred(Y, X);
for (Assignment A : Lists.newArrayList(Y.phiAssignments())) {
PhiFunction rhs = (PhiFunction) A.getRHS();
Variable V = rhs.getArgument(j);
int i = Top(V);
rhs = rhs.replaceVariable(j, i);
Y.replaceStatement(A, new Assignment(A.getLHS(), rhs));
// replace the j-th operand V in RHS(F) by V_i where i = Top(S(V))
}
}
for (BasicBlock Y : dtree.getChildren(X)) {
search(Y);
}
for (Assignment A : X.assignments()) {
if (A.getLHS() instanceof SsaVariable) {
SsaVariable lhs = (SsaVariable) A.getLHS();
S.get(lhs.getInner()).pop();
}
}
}