BreakCriticalEdges.cpp revision eb0456c8fd60e6c2ef844d8696baa39d5d55f082 
1//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
2//
3// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
4// inserting a dummy basic block.  This pass may be "required" by passes that
5// cannot deal with critical edges.  For this usage, the structure type is
6// forward declared.  This pass obviously invalidates the CFG, but can update
7// forward dominator (set, immediate dominators, and tree) information.
8//
9//===----------------------------------------------------------------------===//
10
11#include "llvm/Transforms/Scalar.h"
12#include "llvm/Analysis/Dominators.h"
13#include "llvm/Function.h"
14#include "llvm/iTerminators.h"
15#include "llvm/iPHINode.h"
16#include "llvm/Support/CFG.h"
17#include "Support/StatisticReporter.h"
18
19namespace {
20  Statistic<> NumBroken("break-crit-edges\t- Number of blocks inserted");
21
22  struct BreakCriticalEdges : public FunctionPass {
23    virtual bool runOnFunction(Function &F);
24
25    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
26      AU.addPreserved<DominatorSet>();
27      AU.addPreserved<ImmediateDominators>();
28      AU.addPreserved<DominatorTree>();
29    }
30  };
31
32  RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
33                                    "Break critical edges in CFG");
34}
35
36// Publically exposed interface to pass...
37const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
38Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
39
40
41// isCriticalEdge - Return true if the specified edge is a critical edge.
42// Critical edges are edges from a block with multiple successors to a block
43// with multiple predecessors.
44//
45static bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
46  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
47  assert (TI->getNumSuccessors() > 1);
48
49  const BasicBlock *Dest = TI->getSuccessor(SuccNum);
50  pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
51
52  // If there is more than one predecessor, this is a critical edge...
53  assert(I != E && "No preds, but we have an edge to the block?");
54  ++I;        // Skip one edge due to the incoming arc from TI.
55  return I != E;
56}
57
58// SplitCriticalEdge - Insert a new node node to split the critical edge.  This
59// will update DominatorSet, ImmediateDominator and DominatorTree information if
60// it is available, thus calling this pass will not invalidate either of them.
61//
62static void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
63  assert(isCriticalEdge(TI, SuccNum) &&
64         "Cannot break a critical edge, if it isn't a critical edge");
65  BasicBlock *TIBB = TI->getParent();
66
67  // Create a new basic block, linking it into the CFG.
68  BasicBlock *NewBB = new BasicBlock(TIBB->getName()+"_crit_edge");
69  BasicBlock *DestBB = TI->getSuccessor(SuccNum);
70  // Create our unconditional branch...
71  BranchInst *BI = new BranchInst(DestBB);
72  NewBB->getInstList().push_back(BI);
73
74  // Branch to the new block, breaking the edge...
75  TI->setSuccessor(SuccNum, NewBB);
76
77  // Insert the block into the function... right after the block TI lives in.
78  Function &F = *TIBB->getParent();
79  F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
80
81  // If there are any PHI nodes in DestBB, we need to update them so that they
82  // merge incoming values from NewBB instead of from TIBB.
83  //
84  for (BasicBlock::iterator I = DestBB->begin();
85       PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
86    // We no longer enter through TIBB, now we come in through NewBB.
87    PN->replaceUsesOfWith(TIBB, NewBB);
88  }
89
90  // Now if we have a pass object, update analysis information.  Currently we
91  // update DominatorSet and DominatorTree information if it's available.
92  //
93  if (P) {
94    // Should we update DominatorSet information?
95    if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
96      // The blocks that dominate the new one are the blocks that dominate TIBB
97      // plus the new block itself.
98      DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
99      DomSet.insert(NewBB);  // A block always dominates itself.
100      DS->addBasicBlock(NewBB, DomSet);
101    }
102
103    // Should we update ImmdediateDominator information?
104    if (ImmediateDominators *ID =
105        P->getAnalysisToUpdate<ImmediateDominators>()) {
106      // TIBB is the new immediate dominator for NewBB.  NewBB doesn't dominate
107      // anything.
108      ID->addNewBlock(NewBB, TIBB);
109    }
110
111    // Should we update DominatorTree information?
112    if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
113      DominatorTree::Node *TINode = DT->getNode(TIBB);
114
115      // The new block is not the immediate dominator for any other nodes, but
116      // TINode is the immediate dominator for the new node.
117      //
118      DT->createNewNode(NewBB, TINode);
119    }
120  }
121}
122
123// runOnFunction - Loop over all of the edges in the CFG, breaking critical
124// edges as they are found.
125//
126bool BreakCriticalEdges::runOnFunction(Function &F) {
127  bool Changed = false;
128  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
129    TerminatorInst *TI = I->getTerminator();
130    if (TI->getNumSuccessors() > 1)
131      for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
132        if (isCriticalEdge(TI, i)) {
133          SplitCriticalEdge(TI, i, this);
134          ++NumBroken;
135          Changed = true;
136        }
137  }
138
139  return Changed;
140}
141