1 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
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.
9 //===----------------------------------------------------------------------===//
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"
20 Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
22 struct BreakCriticalEdges : public FunctionPass {
23 virtual bool runOnFunction(Function &F);
25 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
26 AU.addPreserved<DominatorSet>();
27 AU.addPreserved<ImmediateDominators>();
28 AU.addPreserved<DominatorTree>();
29 AU.addPreserved<DominanceFrontier>();
31 // No loop canonicalization guarantees are broken by this pass.
32 AU.addPreservedID(LoopSimplifyID);
36 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
37 "Break critical edges in CFG");
40 // Publically exposed interface to pass...
41 const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
42 Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
45 // isCriticalEdge - Return true if the specified edge is a critical edge.
46 // Critical edges are edges from a block with multiple successors to a block
47 // with multiple predecessors.
49 bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
50 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
51 if (TI->getNumSuccessors() == 1) return false;
53 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
54 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
56 // If there is more than one predecessor, this is a critical edge...
57 assert(I != E && "No preds, but we have an edge to the block?");
58 ++I; // Skip one edge due to the incoming arc from TI.
62 // SplitCriticalEdge - Insert a new node node to split the critical edge. This
63 // will update DominatorSet, ImmediateDominator and DominatorTree information if
64 // it is available, thus calling this pass will not invalidate either of them.
66 void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
67 assert(isCriticalEdge(TI, SuccNum) &&
68 "Cannot break a critical edge, if it isn't a critical edge");
69 BasicBlock *TIBB = TI->getParent();
70 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
72 // Create a new basic block, linking it into the CFG.
73 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
74 DestBB->getName() + "_crit_edge");
75 // Create our unconditional branch...
76 BranchInst *BI = new BranchInst(DestBB);
77 NewBB->getInstList().push_back(BI);
79 // Branch to the new block, breaking the edge...
80 TI->setSuccessor(SuccNum, NewBB);
82 // Insert the block into the function... right after the block TI lives in.
83 Function &F = *TIBB->getParent();
84 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
86 // If there are any PHI nodes in DestBB, we need to update them so that they
87 // merge incoming values from NewBB instead of from TIBB.
89 for (BasicBlock::iterator I = DestBB->begin();
90 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
91 // We no longer enter through TIBB, now we come in through NewBB.
92 PN->replaceUsesOfWith(TIBB, NewBB);
95 // If we don't have a pass object, we can't update anything...
98 // Now update analysis information. These are the analyses that we are
99 // currently capable of updating...
102 // Should we update DominatorSet information?
103 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
104 // The blocks that dominate the new one are the blocks that dominate TIBB
105 // plus the new block itself.
106 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
107 DomSet.insert(NewBB); // A block always dominates itself.
108 DS->addBasicBlock(NewBB, DomSet);
111 // Should we update ImmediateDominator information?
112 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
113 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
115 ID->addNewBlock(NewBB, TIBB);
118 // Should we update DominatorTree information?
119 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
120 DominatorTree::Node *TINode = DT->getNode(TIBB);
122 // The new block is not the immediate dominator for any other nodes, but
123 // TINode is the immediate dominator for the new node.
125 if (TINode) // Don't break unreachable code!
126 DT->createNewNode(NewBB, TINode);
129 // Should we update DominanceFrontier information?
130 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
131 // Since the new block is dominated by its only predecessor TIBB,
132 // it cannot be in any block's dominance frontier. Its dominance
133 // frontier is {DestBB}.
134 DominanceFrontier::DomSetType NewDFSet;
135 NewDFSet.insert(DestBB);
136 DF->addBasicBlock(NewBB, NewDFSet);
140 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
141 // edges as they are found.
143 bool BreakCriticalEdges::runOnFunction(Function &F) {
144 bool Changed = false;
145 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
146 TerminatorInst *TI = I->getTerminator();
147 if (TI->getNumSuccessors() > 1)
148 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
149 if (isCriticalEdge(TI, i)) {
150 SplitCriticalEdge(TI, i, this);