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//===- PostDominators.cpp - Post-Dominator Calculation --------------------===//
// The LLVM Compiler Infrastructure
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
// This file implements the post-dominator construction algorithms.
#include "llvm/Analysis/PostDominators.h"
#include "llvm/iTerminators.h"
#include "llvm/Support/CFG.h"
#include "Support/DepthFirstIterator.h"
#include "Support/SetOperations.h"
// PostDominatorSet Implementation
static RegisterAnalysis<PostDominatorSet>
B("postdomset", "Post-Dominator Set Construction", true);
// Postdominator set construction. This converts the specified function to only
// have a single exit node (return stmt), then calculates the post dominance
// sets for the function.
bool PostDominatorSet::runOnFunction(Function &F) {
Doms.clear(); // Reset from the last time we were run...
// Scan the function looking for the root nodes of the post-dominance
// relationships. These blocks end with return and unwind instructions.
// While we are iterating over the function, we also initialize all of the
// domsets to empty.
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
Doms[I]; // Initialize to empty
if (isa<ReturnInst>(I->getTerminator()) ||
// If there are no exit nodes for the function, postdomsets are all empty.
// This can happen if the function just contains an infinite loop, for
// example.
if (Roots.empty()) return false;
// If we have more than one root, we insert an artificial "null" exit, which
// has "virtual edges" to each of the real exit nodes.
if (Roots.size() > 1)
bool Changed;
do {
Changed = false;
std::set<BasicBlock*> Visited;
DomSetType WorkingSet;
for (unsigned i = 0, e = Roots.size(); i != e; ++i)
for (idf_ext_iterator<BasicBlock*> It = idf_ext_begin(Roots[i], Visited),
E = idf_ext_end(Roots[i], Visited); It != E; ++It) {
BasicBlock *BB = *It;
succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
if (SI != SE) { // Is there SOME successor?
// Loop until we get to a successor that has had it's dom set filled
// in at least once. We are guaranteed to have this because we are
// traversing the graph in DFO and have handled start nodes specially.
while (Doms[*SI].size() == 0) ++SI;
WorkingSet = Doms[*SI];
for (++SI; SI != SE; ++SI) { // Intersect all of the successor sets
DomSetType &SuccSet = Doms[*SI];
if (SuccSet.size())
set_intersect(WorkingSet, SuccSet);
} else {
// If this node has no successors, it must be one of the root nodes.
// We will already take care of the notion that the node
// post-dominates itself. The only thing we have to add is that if
// there are multiple root nodes, we want to insert a special "null"
// exit node which dominates the roots as well.
if (Roots.size() > 1)
WorkingSet.insert(BB); // A block always dominates itself
DomSetType &BBSet = Doms[BB];
if (BBSet != WorkingSet) {
BBSet.swap(WorkingSet); // Constant time operation!
Changed = true; // The sets changed.
WorkingSet.clear(); // Clear out the set for next iteration
} while (Changed);
return false;
// ImmediatePostDominators Implementation
static RegisterAnalysis<ImmediatePostDominators>
D("postidom", "Immediate Post-Dominators Construction", true);
// PostDominatorTree Implementation
static RegisterAnalysis<PostDominatorTree>
F("postdomtree", "Post-Dominator Tree Construction", true);
void PostDominatorTree::calculate(const PostDominatorSet &DS) {
if (Roots.empty()) return;
BasicBlock *Root = Roots.size() == 1 ? Roots[0] : 0;
Nodes[Root] = RootNode = new Node(Root, 0); // Add a node for the root...
// Iterate over all nodes in depth first order...
for (unsigned i = 0, e = Roots.size(); i != e; ++i)
for (idf_iterator<BasicBlock*> I = idf_begin(Roots[i]),
E = idf_end(Roots[i]); I != E; ++I) {
BasicBlock *BB = *I;
const DominatorSet::DomSetType &Dominators = DS.getDominators(BB);
unsigned DomSetSize = Dominators.size();
if (DomSetSize == 1) continue; // Root node... IDom = null
// If we have already computed the immediate dominator for this node,
// don't revisit. This can happen due to nodes reachable from multiple
// roots, but which the idf_iterator doesn't know about.
if (Nodes.find(BB) != Nodes.end()) continue;
// Loop over all dominators of this node. This corresponds to looping
// over nodes in the dominator chain, looking for a node whose dominator
// set is equal to the current nodes, except that the current node does
// not exist in it. This means that it is one level higher in the dom
// chain than the current node, and it is our idom! We know that we have
// already added a DominatorTree node for our idom, because the idom must
// be a predecessor in the depth first order that we are iterating through
// the function.
DominatorSet::DomSetType::const_iterator I = Dominators.begin();
DominatorSet::DomSetType::const_iterator End = Dominators.end();
for (; I != End; ++I) { // Iterate over dominators...
// All of our dominators should form a chain, where the number
// of elements in the dominator set indicates what level the
// node is at in the chain. We want the node immediately
// above us, so it will have an identical dominator set,
// except that BB will not dominate it... therefore it's
// dominator set size will be one less than BB's...
if (DS.getDominators(*I).size() == DomSetSize - 1) {
// We know that the immediate dominator should already have a node,
// because we are traversing the CFG in depth first order!
Node *IDomNode = Nodes[*I];
assert(IDomNode && "No node for IDOM?");
// Add a new tree node for this BasicBlock, and link it as a child of
// IDomNode
Nodes[BB] = IDomNode->addChild(new Node(BB, IDomNode));
// PostDominanceFrontier Implementation
static RegisterAnalysis<PostDominanceFrontier>
H("postdomfrontier", "Post-Dominance Frontier Construction", true);
const DominanceFrontier::DomSetType &
PostDominanceFrontier::calculate(const PostDominatorTree &DT,
const DominatorTree::Node *Node) {
// Loop over CFG successors to calculate DFlocal[Node]
BasicBlock *BB = Node->getBlock();
DomSetType &S = Frontiers[BB]; // The new set to fill in...
if (getRoots().empty()) return S;
if (BB)
for (pred_iterator SI = pred_begin(BB), SE = pred_end(BB);
SI != SE; ++SI)
// Does Node immediately dominate this predecessor?
if (DT[*SI]->getIDom() != Node)
// At this point, S is DFlocal. Now we union in DFup's of our children...
// Loop through and visit the nodes that Node immediately dominates (Node's
// children in the IDomTree)
for (PostDominatorTree::Node::const_iterator
NI = Node->begin(), NE = Node->end(); NI != NE; ++NI) {
DominatorTree::Node *IDominee = *NI;
const DomSetType &ChildDF = calculate(DT, IDominee);
DomSetType::const_iterator CDFI = ChildDF.begin(), CDFE = ChildDF.end();
for (; CDFI != CDFE; ++CDFI) {
if (!Node->dominates(DT[*CDFI]))
return S;
// stub - a dummy function to make linking work ok.
void PostDominanceFrontier::stub() {