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PostDominators.cpp

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00001 //===- PostDominators.cpp - Post-Dominator Calculation --------------------===//
00002 // 
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 // 
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file implements the post-dominator construction algorithms.
00011 //
00012 //===----------------------------------------------------------------------===//
00013 
00014 #include "llvm/Analysis/PostDominators.h"
00015 #include "llvm/Instructions.h"
00016 #include "llvm/Support/CFG.h"
00017 #include "llvm/ADT/DepthFirstIterator.h"
00018 #include "llvm/ADT/SetOperations.h"
00019 using namespace llvm;
00020 
00021 //===----------------------------------------------------------------------===//
00022 //  PostDominatorSet Implementation
00023 //===----------------------------------------------------------------------===//
00024 
00025 static RegisterAnalysis<PostDominatorSet>
00026 B("postdomset", "Post-Dominator Set Construction", true);
00027 
00028 // Postdominator set construction.  This converts the specified function to only
00029 // have a single exit node (return stmt), then calculates the post dominance
00030 // sets for the function.
00031 //
00032 bool PostDominatorSet::runOnFunction(Function &F) {
00033   Doms.clear();   // Reset from the last time we were run...
00034 
00035   // Scan the function looking for the root nodes of the post-dominance
00036   // relationships.  These blocks end with return and unwind instructions.
00037   // While we are iterating over the function, we also initialize all of the
00038   // domsets to empty.
00039   Roots.clear();
00040   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
00041     Doms[I];  // Initialize to empty
00042 
00043     if (succ_begin(I) == succ_end(I))
00044       Roots.push_back(I);
00045   }
00046 
00047   // If there are no exit nodes for the function, postdomsets are all empty.
00048   // This can happen if the function just contains an infinite loop, for
00049   // example.
00050   if (Roots.empty()) return false;
00051 
00052   // If we have more than one root, we insert an artificial "null" exit, which
00053   // has "virtual edges" to each of the real exit nodes.
00054   if (Roots.size() > 1)
00055     Doms[0].insert(0);
00056 
00057   bool Changed;
00058   do {
00059     Changed = false;
00060 
00061     std::set<BasicBlock*> Visited;
00062     DomSetType WorkingSet;
00063 
00064     for (unsigned i = 0, e = Roots.size(); i != e; ++i)
00065       for (idf_ext_iterator<BasicBlock*> It = idf_ext_begin(Roots[i], Visited),
00066              E = idf_ext_end(Roots[i], Visited); It != E; ++It) {
00067         BasicBlock *BB = *It;
00068         succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
00069         if (SI != SE) {                // Is there SOME successor?
00070           // Loop until we get to a successor that has had it's dom set filled
00071           // in at least once.  We are guaranteed to have this because we are
00072           // traversing the graph in DFO and have handled start nodes specially.
00073           //
00074           while (Doms[*SI].size() == 0) ++SI;
00075           WorkingSet = Doms[*SI];
00076           
00077           for (++SI; SI != SE; ++SI) { // Intersect all of the successor sets
00078             DomSetType &SuccSet = Doms[*SI];
00079             if (SuccSet.size())
00080               set_intersect(WorkingSet, SuccSet);
00081           }
00082         } else {
00083           // If this node has no successors, it must be one of the root nodes.
00084           // We will already take care of the notion that the node
00085           // post-dominates itself.  The only thing we have to add is that if
00086           // there are multiple root nodes, we want to insert a special "null"
00087           // exit node which dominates the roots as well.
00088           if (Roots.size() > 1)
00089             WorkingSet.insert(0);
00090         }
00091   
00092         WorkingSet.insert(BB);           // A block always dominates itself
00093         DomSetType &BBSet = Doms[BB];
00094         if (BBSet != WorkingSet) {
00095           BBSet.swap(WorkingSet);        // Constant time operation!
00096           Changed = true;                // The sets changed.
00097         }
00098         WorkingSet.clear();              // Clear out the set for next iteration
00099       }
00100   } while (Changed);
00101   return false;
00102 }
00103 
00104 //===----------------------------------------------------------------------===//
00105 //  ImmediatePostDominators Implementation
00106 //===----------------------------------------------------------------------===//
00107 
00108 static RegisterAnalysis<ImmediatePostDominators>
00109 D("postidom", "Immediate Post-Dominators Construction", true);
00110 
00111 
00112 // calcIDoms - Calculate the immediate dominator mapping, given a set of
00113 // dominators for every basic block.
00114 void ImmediatePostDominators::calcIDoms(const DominatorSetBase &DS) {
00115   // Loop over all of the nodes that have dominators... figuring out the IDOM
00116   // for each node...
00117   //
00118   for (DominatorSet::const_iterator DI = DS.begin(), DEnd = DS.end(); 
00119        DI != DEnd; ++DI) {
00120     BasicBlock *BB = DI->first;
00121     const DominatorSet::DomSetType &Dominators = DI->second;
00122     unsigned DomSetSize = Dominators.size();
00123     if (DomSetSize == 1) continue;  // Root node... IDom = null
00124 
00125     // Loop over all dominators of this node.  This corresponds to looping over
00126     // nodes in the dominator chain, looking for a node whose dominator set is
00127     // equal to the current nodes, except that the current node does not exist
00128     // in it.  This means that it is one level higher in the dom chain than the
00129     // current node, and it is our idom!
00130     //
00131     DominatorSet::DomSetType::const_iterator I = Dominators.begin();
00132     DominatorSet::DomSetType::const_iterator End = Dominators.end();
00133     for (; I != End; ++I) {   // Iterate over dominators...
00134       // All of our dominators should form a chain, where the number of elements
00135       // in the dominator set indicates what level the node is at in the chain.
00136       // We want the node immediately above us, so it will have an identical 
00137       // dominator set, except that BB will not dominate it... therefore it's
00138       // dominator set size will be one less than BB's...
00139       //
00140       if (DS.getDominators(*I).size() == DomSetSize - 1) {
00141   IDoms[BB] = *I;
00142   break;
00143       }
00144     }
00145   }
00146 }
00147 
00148 //===----------------------------------------------------------------------===//
00149 //  PostDominatorTree Implementation
00150 //===----------------------------------------------------------------------===//
00151 
00152 static RegisterAnalysis<PostDominatorTree>
00153 F("postdomtree", "Post-Dominator Tree Construction", true);
00154 
00155 void PostDominatorTree::calculate(const PostDominatorSet &DS) {
00156   if (Roots.empty()) return;
00157   BasicBlock *Root = Roots.size() == 1 ? Roots[0] : 0;
00158 
00159   Nodes[Root] = RootNode = new Node(Root, 0);   // Add a node for the root...
00160 
00161   // Iterate over all nodes in depth first order...
00162   for (unsigned i = 0, e = Roots.size(); i != e; ++i)
00163     for (idf_iterator<BasicBlock*> I = idf_begin(Roots[i]),
00164            E = idf_end(Roots[i]); I != E; ++I) {
00165       BasicBlock *BB = *I;
00166       const DominatorSet::DomSetType &Dominators = DS.getDominators(BB);
00167       unsigned DomSetSize = Dominators.size();
00168       if (DomSetSize == 1) continue;  // Root node... IDom = null
00169 
00170       // If we have already computed the immediate dominator for this node,
00171       // don't revisit.  This can happen due to nodes reachable from multiple
00172       // roots, but which the idf_iterator doesn't know about.
00173       if (Nodes.find(BB) != Nodes.end()) continue;
00174 
00175       // Loop over all dominators of this node.  This corresponds to looping
00176       // over nodes in the dominator chain, looking for a node whose dominator
00177       // set is equal to the current nodes, except that the current node does
00178       // not exist in it.  This means that it is one level higher in the dom
00179       // chain than the current node, and it is our idom!  We know that we have
00180       // already added a DominatorTree node for our idom, because the idom must
00181       // be a predecessor in the depth first order that we are iterating through
00182       // the function.
00183       //
00184       for (DominatorSet::DomSetType::const_iterator I = Dominators.begin(),
00185            E = Dominators.end(); I != E; ++I) {  // Iterate over dominators.
00186         // All of our dominators should form a chain, where the number
00187         // of elements in the dominator set indicates what level the
00188         // node is at in the chain.  We want the node immediately
00189         // above us, so it will have an identical dominator set,
00190         // except that BB will not dominate it... therefore it's
00191         // dominator set size will be one less than BB's...
00192         //
00193         if (DS.getDominators(*I).size() == DomSetSize - 1) {
00194           // We know that the immediate dominator should already have a node, 
00195           // because we are traversing the CFG in depth first order!
00196           //
00197           Node *IDomNode = Nodes[*I];
00198           assert(IDomNode && "No node for IDOM?");
00199     
00200           // Add a new tree node for this BasicBlock, and link it as a child of
00201           // IDomNode
00202           Nodes[BB] = IDomNode->addChild(new Node(BB, IDomNode));
00203           break;
00204         }
00205       }
00206     }
00207 }
00208 
00209 //===----------------------------------------------------------------------===//
00210 //  PostDominanceFrontier Implementation
00211 //===----------------------------------------------------------------------===//
00212 
00213 static RegisterAnalysis<PostDominanceFrontier>
00214 H("postdomfrontier", "Post-Dominance Frontier Construction", true);
00215 
00216 const DominanceFrontier::DomSetType &
00217 PostDominanceFrontier::calculate(const PostDominatorTree &DT, 
00218                                  const DominatorTree::Node *Node) {
00219   // Loop over CFG successors to calculate DFlocal[Node]
00220   BasicBlock *BB = Node->getBlock();
00221   DomSetType &S = Frontiers[BB];       // The new set to fill in...
00222   if (getRoots().empty()) return S;
00223 
00224   if (BB)
00225     for (pred_iterator SI = pred_begin(BB), SE = pred_end(BB);
00226          SI != SE; ++SI)
00227       // Does Node immediately dominate this predecessor?
00228       if (DT[*SI]->getIDom() != Node)
00229         S.insert(*SI);
00230 
00231   // At this point, S is DFlocal.  Now we union in DFup's of our children...
00232   // Loop through and visit the nodes that Node immediately dominates (Node's
00233   // children in the IDomTree)
00234   //
00235   for (PostDominatorTree::Node::const_iterator
00236          NI = Node->begin(), NE = Node->end(); NI != NE; ++NI) {
00237     DominatorTree::Node *IDominee = *NI;
00238     const DomSetType &ChildDF = calculate(DT, IDominee);
00239 
00240     DomSetType::const_iterator CDFI = ChildDF.begin(), CDFE = ChildDF.end();
00241     for (; CDFI != CDFE; ++CDFI) {
00242       if (!Node->dominates(DT[*CDFI]))
00243   S.insert(*CDFI);
00244     }
00245   }
00246 
00247   return S;
00248 }
00249 
00250 // stub - a dummy function to make linking work ok.
00251 void PostDominanceFrontier::stub() {
00252 }
00253