Rework post dominator information so that we do not have to

unify all exit nodes of a function to compute post-dominance information.
This does not work with functions that have both unwind and return nodes,
because we cannot unify these blocks.  The new implementation is better
anyway. :)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8460 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 97 insertions, 88 deletions

diff --git a/lib/Analysis/PostDominators.cpp b/lib/Analysis/PostDominators.cpp
index ddf8d7f..6be8f3d 100644
--- a/lib/Analysis/PostDominators.cpp
+++ b/lib/Analysis/PostDominators.cpp
@@ -5,7 +5,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/PostDominators.h"
-#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
+#include "llvm/iTerminators.h"
 #include "llvm/Support/CFG.h"
 #include "Support/DepthFirstIterator.h"
 #include "Support/SetOperations.h"
@@ -23,75 +23,77 @@ B("postdomset", "Post-Dominator Set Construction", true);
 //
 bool PostDominatorSet::runOnFunction(Function &F) {
   Doms.clear();   // Reset from the last time we were run...
-  // Since we require that the unify all exit nodes pass has been run, we know
-  // that there can be at most one return instruction in the function left.
-  // Get it.
-  //
-  Root = getAnalysis<UnifyFunctionExitNodes>().getExitNode();
 
-  if (Root == 0) {  // No exit node for the function?  Postdomsets are all empty
-    for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
-      Doms[FI] = DomSetType();
-    return false;
+  // 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.
+  Roots.clear();
+  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
+    Doms[I];  // Initialize to empty
+
+    if (isa<ReturnInst>(I->getTerminator()) ||
+        isa<UnwindInst>(I->getTerminator()))
+      Roots.push_back(I);
   }
 
+  // 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)
+    Doms[0].insert(0);
+
   bool Changed;
   do {
     Changed = false;
 
     std::set<const BasicBlock*> Visited;
     DomSetType WorkingSet;
-    idf_iterator<BasicBlock*> It = idf_begin(Root), End = idf_end(Root);
-    for ( ; It != End; ++It) {
-      BasicBlock *BB = *It;
-      succ_iterator PI = succ_begin(BB), PEnd = succ_end(BB);
-      if (PI != PEnd) {                // Is there SOME predecessor?
-	// 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[*PI].size() == 0) ++PI;
-	WorkingSet = Doms[*PI];
-
-	for (++PI; PI != PEnd; ++PI) { // Intersect all of the successor sets
-	  DomSetType &PredSet = Doms[*PI];
-	  if (PredSet.size())
-	    set_intersect(WorkingSet, PredSet);
-	}
-      } else if (BB != Root) {
-        // If this isn't the root basic block and it has no successors, it must
-        // be an non-returning block.  Fib a bit by saying that the root node
-        // postdominates this unreachable node.  This isn't exactly true,
-        // because there is no path from this node to the root node, but it is
-        // sorta true because any paths to the exit node would have to go
-        // through this node.
-        //
-        // This allows for postdominator properties to be built for code that
-        // doesn't return in a reasonable manner.
-        //
-        WorkingSet = Doms[Root];
-      }
+
+    for (unsigned i = 0, e = Roots.size(); i != e; ++i)
+      for (idf_iterator<BasicBlock*> It = idf_begin(Roots[i]),
+             E = idf_end(Roots[i]); 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(0);
+        }
 	
-      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.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
       }
-      WorkingSet.clear();              // Clear out the set for next iteration
-    }
   } while (Changed);
   return false;
 }
 
-// getAnalysisUsage - This obviously provides a post-dominator set, but it also
-// requires the UnifyFunctionExitNodes pass.
-//
-void PostDominatorSet::getAnalysisUsage(AnalysisUsage &AU) const {
-  AU.setPreservesAll();
-  AU.addRequired<UnifyFunctionExitNodes>();
-}
-
 //===----------------------------------------------------------------------===//
 //  ImmediatePostDominators Implementation
 //===----------------------------------------------------------------------===//
@@ -107,17 +109,25 @@ static RegisterAnalysis<PostDominatorTree>
 F("postdomtree", "Post-Dominator Tree Construction", true);
 
 void PostDominatorTree::calculate(const PostDominatorSet &DS) {
-  Nodes[Root] = new Node(Root, 0);   // Add a node for the root...
+  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...
 
-  if (Root) {
-    // Iterate over all nodes in depth first order...
-    for (idf_iterator<BasicBlock*> I = idf_begin(Root), E = idf_end(Root);
-         I != E; ++I) {
+  // 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
@@ -130,28 +140,27 @@ void PostDominatorTree::calculate(const PostDominatorSet &DS) {
       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?");
+        // 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;
-	}
+          // 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;
+        }
       }
     }
-  }
 }
 
 //===----------------------------------------------------------------------===//
@@ -167,14 +176,14 @@ PostDominanceFrontier::calculate(const PostDominatorTree &DT,
   // Loop over CFG successors to calculate DFlocal[Node]
   BasicBlock *BB = Node->getNode();
   DomSetType &S = Frontiers[BB];       // The new set to fill in...
-  if (!Root) return S;
-
-  for (pred_iterator SI = pred_begin(BB), SE = pred_end(BB);
-       SI != SE; ++SI) {
-    // Does Node immediately dominate this predeccessor?
-    if (DT[*SI]->getIDom() != Node)
-      S.insert(*SI);
-  }
+  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 predeccessor?
+      if (DT[*SI]->getIDom() != Node)
+        S.insert(*SI);
 
   // 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