Completely rewrite domset, idom, and domtree implementation. Now it is based

on the algorithm for directly computing immediate dominators presented in this
paper:

  A Fast Algorithm for Finding Dominators in a Flowgraph
  T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.

This _substantially_ speeds up construction of all dominator related information.
Post-dominators to follow.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10301 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 266 insertions, 159 deletions

diff --git a/lib/VMCore/Dominators.cpp b/lib/VMCore/Dominators.cpp
index dbdb409730..b2d4d942d4 100644
--- a/lib/VMCore/Dominators.cpp
+++ b/lib/VMCore/Dominators.cpp
@@ -22,11 +22,218 @@
 using namespace llvm;
 
 //===----------------------------------------------------------------------===//
+//  ImmediateDominators Implementation
+//===----------------------------------------------------------------------===//
+//
+// Immediate Dominators construction - This pass constructs immediate dominator
+// information for a flow-graph based on the algorithm described in this
+// document:
+//
+//   A Fast Algorithm for Finding Dominators in a Flowgraph
+//   T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.
+//
+// This implements both the O(n*ack(n)) and the O(n*log(n)) versions of EVAL and
+// LINK, but it turns out that the theoretically slower O(n*log(n))
+// implementation is actually faster than the "efficient" algorithm (even for
+// large CFGs) because the constant overheads are substantially smaller.  The
+// lower-complexity version can be enabled with the following #define:
+//
+#define BALANCE_IDOM_TREE 0
+//
+//===----------------------------------------------------------------------===//
+
+static RegisterAnalysis<ImmediateDominators>
+C("idom", "Immediate Dominators Construction", true);
+
+unsigned ImmediateDominators::DFSPass(BasicBlock *V, InfoRec &VInfo,
+                                      unsigned N) {
+  VInfo.Semi = ++N;
+  VInfo.Label = V;
+
+  Vertex.push_back(V);        // Vertex[n] = V;
+  //Info[V].Ancestor = 0;     // Ancestor[n] = 0
+  //Child[V] = 0;             // Child[v] = 0
+  VInfo.Size = 1;             // Size[v] = 1
+
+  for (succ_iterator SI = succ_begin(V), E = succ_end(V); SI != E; ++SI) {
+    InfoRec &SuccVInfo = Info[*SI];
+    if (SuccVInfo.Semi == 0) {
+      SuccVInfo.Parent = V;
+      N = DFSPass(*SI, SuccVInfo, N);
+    }
+  }
+  return N;
+}
+
+void ImmediateDominators::Compress(BasicBlock *V, InfoRec &VInfo) {
+  BasicBlock *VAncestor = VInfo.Ancestor;
+  InfoRec &VAInfo = Info[VAncestor];
+  if (VAInfo.Ancestor == 0)
+    return;
+
+  Compress(VAncestor, VAInfo);
+
+  BasicBlock *VAncestorLabel = VAInfo.Label; 
+  BasicBlock *VLabel = VInfo.Label;
+  if (Info[VAncestorLabel].Semi < Info[VLabel].Semi)
+    VInfo.Label = VAncestorLabel;
+
+  VInfo.Ancestor = VAInfo.Ancestor;
+}
+
+BasicBlock *ImmediateDominators::Eval(BasicBlock *V) {
+  InfoRec &VInfo = Info[V];
+#if !BALANCE_IDOM_TREE
+  // Higher-complexity but faster implementation
+  if (VInfo.Ancestor == 0)
+    return V;
+  Compress(V, VInfo);
+  return VInfo.Label;
+#else
+  // Lower-complexity but slower implementation
+  if (VInfo.Ancestor == 0)
+    return VInfo.Label;
+  Compress(V, VInfo);
+  BasicBlock *VLabel = VInfo.Label;
+
+  BasicBlock *VAncestorLabel = Info[VInfo.Ancestor].Label;
+  if (Info[VAncestorLabel].Semi >= Info[VLabel].Semi)
+    return VLabel;
+  else
+    return VAncestorLabel;
+#endif
+}
+
+void ImmediateDominators::Link(BasicBlock *V, BasicBlock *W, InfoRec &WInfo){
+#if !BALANCE_IDOM_TREE
+  // Higher-complexity but faster implementation
+  WInfo.Ancestor = V;
+#else
+  // Lower-complexity but slower implementation
+  BasicBlock *WLabel = WInfo.Label;
+  unsigned WLabelSemi = Info[WLabel].Semi;
+  BasicBlock *S = W;
+  InfoRec *SInfo = &Info[S];
+  
+  BasicBlock *SChild = SInfo->Child;
+  InfoRec *SChildInfo = &Info[SChild];
+  
+  while (WLabelSemi < Info[SChildInfo->Label].Semi) {
+    BasicBlock *SChildChild = SChildInfo->Child;
+    if (SInfo->Size+Info[SChildChild].Size >= 2*SChildInfo->Size) {
+      SChildInfo->Ancestor = S;
+      SInfo->Child = SChild = SChildChild;
+      SChildInfo = &Info[SChild];
+    } else {
+      SChildInfo->Size = SInfo->Size;
+      S = SInfo->Ancestor = SChild;
+      SInfo = SChildInfo;
+      SChild = SChildChild;
+      SChildInfo = &Info[SChild];
+    }
+  }
+  
+  InfoRec &VInfo = Info[V];
+  SInfo->Label = WLabel;
+  
+  assert(V != W && "The optimization here will not work in this case!");
+  unsigned WSize = WInfo.Size;
+  unsigned VSize = (VInfo.Size += WSize);
+  
+  if (VSize < 2*WSize)
+    std::swap(S, VInfo.Child);
+  
+  while (S) {
+    SInfo = &Info[S];
+    SInfo->Ancestor = V;
+    S = SInfo->Child;
+  }
+#endif
+}
+
+
+
+bool ImmediateDominators::runOnFunction(Function &F) {
+  IDoms.clear();     // Reset from the last time we were run...
+  BasicBlock *Root = &F.getEntryBlock();
+  Roots.clear();
+  Roots.push_back(Root);
+
+  Vertex.push_back(0);
+  
+  // Step #1: Number blocks in depth-first order and initialize variables used
+  // in later stages of the algorithm.
+  unsigned N = 0;
+  for (unsigned i = 0, e = Roots.size(); i != e; ++i)
+    N = DFSPass(Roots[i], Info[Roots[i]], 0);
+
+  for (unsigned i = N; i >= 2; --i) {
+    BasicBlock *W = Vertex[i];
+    InfoRec &WInfo = Info[W];
+
+    // Step #2: Calculate the semidominators of all vertices
+    for (pred_iterator PI = pred_begin(W), E = pred_end(W); PI != E; ++PI)
+      if (Info.count(*PI)) {  // Only if this predecessor is reachable!
+        unsigned SemiU = Info[Eval(*PI)].Semi;
+        if (SemiU < WInfo.Semi)
+          WInfo.Semi = SemiU;
+      }
+    
+    Info[Vertex[WInfo.Semi]].Bucket.push_back(W);
+
+    BasicBlock *WParent = WInfo.Parent;
+    Link(WParent, W, WInfo);
+
+    // Step #3: Implicitly define the immediate dominator of vertices
+    std::vector<BasicBlock*> &WParentBucket = Info[WParent].Bucket;
+    while (!WParentBucket.empty()) {
+      BasicBlock *V = WParentBucket.back();
+      WParentBucket.pop_back();
+      BasicBlock *U = Eval(V);
+      IDoms[V] = Info[U].Semi < Info[V].Semi ? U : WParent;
+    }
+  }
+
+  // Step #4: Explicitly define the immediate dominator of each vertex
+  for (unsigned i = 2; i <= N; ++i) {
+    BasicBlock *W = Vertex[i];
+    BasicBlock *&WIDom = IDoms[W];
+    if (WIDom != Vertex[Info[W].Semi])
+      WIDom = IDoms[WIDom];
+  }
+
+  // Free temporary memory used to construct idom's
+  Info.clear();
+  std::vector<BasicBlock*>().swap(Vertex);
+
+  return false;
+}
+
+void ImmediateDominatorsBase::print(std::ostream &o) const {
+  for (const_iterator I = begin(), E = end(); I != E; ++I) {
+    o << "  Immediate Dominator For Basic Block:";
+    if (I->first)
+      WriteAsOperand(o, I->first, false);
+    else
+      o << " <<exit node>>";
+    o << " is:";
+    if (I->second)
+      WriteAsOperand(o, I->second, false);
+    else
+      o << " <<exit node>>";
+    o << "\n";
+  }
+  o << "\n";
+}
+
+
+
+//===----------------------------------------------------------------------===//
 //  DominatorSet Implementation
 //===----------------------------------------------------------------------===//
 
 static RegisterAnalysis<DominatorSet>
-A("domset", "Dominator Set Construction", true);
+B("domset", "Dominator Set Construction", true);
 
 // dominates - Return true if A dominates B.  This performs the special checks
 // necessary if A and B are in the same basic block.
@@ -44,53 +251,45 @@ bool DominatorSetBase::dominates(Instruction *A, Instruction *B) const {
 }
 
 
-void DominatorSet::calculateDominatorsFromBlock(BasicBlock *RootBB) {
-  bool Changed;
-  Doms[RootBB].insert(RootBB);  // Root always dominates itself...
-  do {
-    Changed = false;
-
-    DomSetType WorkingSet;
-    df_iterator<BasicBlock*> It = df_begin(RootBB), End = df_end(RootBB);
-    for ( ; It != End; ++It) {
-      BasicBlock *BB = *It;
-      pred_iterator PI = pred_begin(BB), PEnd = pred_end(BB);
-      if (PI != PEnd) {                // Is there SOME predecessor?
-	// Loop until we get to a predecessor that has had its 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,
-	// except when there are unreachable blocks.
-	//
-	while (PI != PEnd && Doms[*PI].empty()) ++PI;
-        if (PI != PEnd) {     // Not unreachable code case?
-          WorkingSet = Doms[*PI];
-
-          // Intersect all of the predecessor sets
-          for (++PI; PI != PEnd; ++PI) {
-            DomSetType &PredSet = Doms[*PI];
-            if (PredSet.size())
-              set_intersect(WorkingSet, PredSet);
-          }
+void DominatorSet::recalculate() {
+  ImmediateDominators &ID = getAnalysis<ImmediateDominators>();
+  Doms.clear();
+  if (Roots.empty()) return;
+
+  // Root nodes only dominate themselves.
+  for (unsigned i = 0, e = Roots.size(); i != e; ++i)
+    Doms[Roots[i]].insert(Roots[i]);
+
+  Function *F = Roots.back()->getParent();
+
+  // Loop over all of the blocks in the function, calculating dominator sets for
+  // each function.
+  for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
+    if (BasicBlock *IDom = ID[I]) {   // Get idom if block is reachable
+      DomSetType &DS = Doms[I];
+      assert(DS.empty() && "Domset already filled in for this block?");
+      DS.insert(I);  // Blocks always dominate themselves
+      
+      // Insert all dominators into the set... 
+      while (IDom) {
+        // If we have already computed the dominator sets for our immediate
+        // dominator, just use it instead of walking all the way up to the root.
+        DomSetType &IDS = Doms[IDom];
+        if (!IDS.empty()) {
+          DS.insert(IDS.begin(), IDS.end());
+          break;
+        } else {
+          DS.insert(IDom);
+          IDom = ID[IDom];
         }
-      } else {
-        assert(Roots.size() == 1 && BB == Roots[0] &&
-               "We got into unreachable code somehow!");
-      }
-	
-      WorkingSet.insert(BB);           // A block always dominates itself
-      DomSetType &BBSet = Doms[BB];
-      if (BBSet != WorkingSet) {
-        //assert(WorkingSet.size() > BBSet.size() && "Must only grow sets!");
-	BBSet.swap(WorkingSet);        // Constant time operation!
-	Changed = true;                // The sets changed.
       }
-      WorkingSet.clear();              // Clear out the set for next iteration
+    } else {
+      // Ensure that every basic block has at least an empty set of nodes.  This
+      // is important for the case when there is unreachable blocks.
+      Doms[I];
     }
-  } while (Changed);
 }
 
-
-
 // runOnFunction - This method calculates the forward dominator sets for the
 // specified function.
 //
@@ -104,21 +303,6 @@ bool DominatorSet::runOnFunction(Function &F) {
   return false;
 }
 
-void DominatorSet::recalculate() {
-  assert(Roots.size() == 1 && "DominatorSet should have single root block!");
-  Doms.clear();   // Reset from the last time we were run...
-
-  // Calculate dominator sets for the reachable basic blocks...
-  calculateDominatorsFromBlock(Roots[0]);
-
-
-  // Loop through the function, ensuring that every basic block has at least an
-  // empty set of nodes.  This is important for the case when there is
-  // unreachable blocks.
-  Function *F = Roots[0]->getParent();
-  for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) Doms[I];
-}
-
 namespace llvm {
 static std::ostream &operator<<(std::ostream &o,
                                 const std::set<BasicBlock*> &BBs) {
@@ -144,67 +328,6 @@ void DominatorSetBase::print(std::ostream &o) const {
 }
 
 //===----------------------------------------------------------------------===//
-//  ImmediateDominators Implementation
-//===----------------------------------------------------------------------===//
-
-static RegisterAnalysis<ImmediateDominators>
-C("idom", "Immediate Dominators Construction", true);
-
-// calcIDoms - Calculate the immediate dominator mapping, given a set of
-// dominators for every basic block.
-void ImmediateDominatorsBase::calcIDoms(const DominatorSetBase &DS) {
-  // Loop over all of the nodes that have dominators... figuring out the IDOM
-  // for each node...
-  //
-  for (DominatorSet::const_iterator DI = DS.begin(), DEnd = DS.end(); 
-       DI != DEnd; ++DI) {
-    BasicBlock *BB = DI->first;
-    const DominatorSet::DomSetType &Dominators = DI->second;
-    unsigned DomSetSize = Dominators.size();
-    if (DomSetSize == 1) continue;  // Root node... IDom = null
-
-    // 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!
-    //
-    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) {
-	IDoms[BB] = *I;
-	break;
-      }
-    }
-  }
-}
-
-void ImmediateDominatorsBase::print(std::ostream &o) const {
-  for (const_iterator I = begin(), E = end(); I != E; ++I) {
-    o << "  Immediate Dominator For Basic Block:";
-    if (I->first)
-      WriteAsOperand(o, I->first, false);
-    else
-      o << " <<exit node>>";
-    o << " is:";
-    if (I->second)
-      WriteAsOperand(o, I->second, false);
-    else
-      o << " <<exit node>>";
-    o << "\n";
-  }
-  o << "\n";
-}
-
-
-//===----------------------------------------------------------------------===//
 //  DominatorTree Implementation
 //===----------------------------------------------------------------------===//
 
@@ -236,56 +359,40 @@ void DominatorTreeBase::Node::setIDom(Node *NewIDom) {
   }
 }
 
+DominatorTreeBase::Node *DominatorTree::getNodeForBlock(BasicBlock *BB) {
+  Node *&BBNode = Nodes[BB];
+  if (BBNode) return BBNode;
+
+  // Haven't calculated this node yet?  Get or calculate the node for the
+  // immediate dominator.
+  BasicBlock *IDom = getAnalysis<ImmediateDominators>()[BB];
+  Node *IDomNode = getNodeForBlock(IDom);
+    
+  // Add a new tree node for this BasicBlock, and link it as a child of
+  // IDomNode
+  return BBNode = IDomNode->addChild(new Node(BB, IDomNode));
+}
 
-
-void DominatorTree::calculate(const DominatorSet &DS) {
+void DominatorTree::calculate(const ImmediateDominators &ID) {
   assert(Roots.size() == 1 && "DominatorTree should have 1 root block!");
   BasicBlock *Root = Roots[0];
   Nodes[Root] = RootNode = new Node(Root, 0); // Add a node for the root...
 
-  // Iterate over all nodes in depth first order...
-  for (df_iterator<BasicBlock*> I = df_begin(Root), E = df_end(Root);
+  // Loop over all of the reachable blocks in the function...
+  for (ImmediateDominators::const_iterator I = ID.begin(), E = ID.end();
        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
-      
-    // 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));
-        break;
-      }
+    Node *&BBNode = Nodes[I->first];
+    if (!BBNode) {  // Haven't calculated this node yet?
+      // Get or calculate the node for the immediate dominator
+      Node *IDomNode = getNodeForBlock(I->second);
+
+      // Add a new tree node for this BasicBlock, and link it as a child of
+      // IDomNode
+      BBNode = IDomNode->addChild(new Node(I->first, IDomNode));
     }
   }
 }
 
-
 static std::ostream &operator<<(std::ostream &o,
                                 const DominatorTreeBase::Node *Node) {
   if (Node->getBlock())