BreakCriticalEdges.cpp revision a92f696b74a99325026ebbdbffd2a44317e0c10b 
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/Statistic.h"
18
19namespace {
20  Statistic<> NumBroken("break-crit-edges", "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      AU.addPreservedID(LoopPreheadersID);   // No preheaders deleted.
30    }
31  private:
32    void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum);
33  };
34
35  RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
36                                    "Break critical edges in CFG");
37}
38
39// Publically exposed interface to pass...
40const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
41Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
42
43
44// isCriticalEdge - Return true if the specified edge is a critical edge.
45// Critical edges are edges from a block with multiple successors to a block
46// with multiple predecessors.
47//
48static bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
49  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
50  assert (TI->getNumSuccessors() > 1);
51
52  const BasicBlock *Dest = TI->getSuccessor(SuccNum);
53  pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
54
55  // If there is more than one predecessor, this is a critical edge...
56  assert(I != E && "No preds, but we have an edge to the block?");
57  ++I;        // Skip one edge due to the incoming arc from TI.
58  return I != E;
59}
60
61// SplitCriticalEdge - Insert a new node node to split the critical edge.  This
62// will update DominatorSet, ImmediateDominator and DominatorTree information if
63// it is available, thus calling this pass will not invalidate either of them.
64//
65void BreakCriticalEdges::SplitCriticalEdge(TerminatorInst *TI,unsigned SuccNum){
66  assert(isCriticalEdge(TI, SuccNum) &&
67         "Cannot break a critical edge, if it isn't a critical edge");
68  BasicBlock *TIBB = TI->getParent();
69
70  // Create a new basic block, linking it into the CFG.
71  BasicBlock *NewBB = new BasicBlock(TIBB->getName()+"_crit_edge");
72  BasicBlock *DestBB = TI->getSuccessor(SuccNum);
73  // Create our unconditional branch...
74  BranchInst *BI = new BranchInst(DestBB);
75  NewBB->getInstList().push_back(BI);
76
77  // Branch to the new block, breaking the edge...
78  TI->setSuccessor(SuccNum, NewBB);
79
80  // Insert the block into the function... right after the block TI lives in.
81  Function &F = *TIBB->getParent();
82  F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
83
84  // If there are any PHI nodes in DestBB, we need to update them so that they
85  // merge incoming values from NewBB instead of from TIBB.
86  //
87  for (BasicBlock::iterator I = DestBB->begin();
88       PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
89    // We no longer enter through TIBB, now we come in through NewBB.
90    PN->replaceUsesOfWith(TIBB, NewBB);
91  }
92
93  // Now update analysis information.  These are the analyses that we are
94  // currently capable of updating...
95  //
96
97  // Should we update DominatorSet information?
98  if (DominatorSet *DS = getAnalysisToUpdate<DominatorSet>()) {
99    // The blocks that dominate the new one are the blocks that dominate TIBB
100    // plus the new block itself.
101    DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
102    DomSet.insert(NewBB);  // A block always dominates itself.
103    DS->addBasicBlock(NewBB, DomSet);
104  }
105
106  // Should we update ImmdediateDominator information?
107  if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
108    // TIBB is the new immediate dominator for NewBB.  NewBB doesn't dominate
109    // anything.
110    ID->addNewBlock(NewBB, TIBB);
111  }
112
113  // Should we update DominatorTree information?
114  if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
115    DominatorTree::Node *TINode = DT->getNode(TIBB);
116
117    // The new block is not the immediate dominator for any other nodes, but
118    // TINode is the immediate dominator for the new node.
119    //
120    DT->createNewNode(NewBB, TINode);
121  }
122}
123
124// runOnFunction - Loop over all of the edges in the CFG, breaking critical
125// edges as they are found.
126//
127bool BreakCriticalEdges::runOnFunction(Function &F) {
128  bool Changed = false;
129  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
130    TerminatorInst *TI = I->getTerminator();
131    if (TI->getNumSuccessors() > 1)
132      for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
133        if (isCriticalEdge(TI, i)) {
134          SplitCriticalEdge(TI, i);
135          ++NumBroken;
136          Changed = true;
137        }
138  }
139
140  return Changed;
141}
142