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>();
30 AU.addPreservedID(LoopPreheadersID); // No preheaders deleted.
34 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
35 "Break critical edges in CFG");
38 // Publically exposed interface to pass...
39 const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
40 Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
43 // isCriticalEdge - Return true if the specified edge is a critical edge.
44 // Critical edges are edges from a block with multiple successors to a block
45 // with multiple predecessors.
47 bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
48 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
49 if (TI->getNumSuccessors() == 1) return false;
51 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
52 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
54 // If there is more than one predecessor, this is a critical edge...
55 assert(I != E && "No preds, but we have an edge to the block?");
56 ++I; // Skip one edge due to the incoming arc from TI.
60 // SplitCriticalEdge - Insert a new node node to split the critical edge. This
61 // will update DominatorSet, ImmediateDominator and DominatorTree information if
62 // it is available, thus calling this pass will not invalidate either of them.
64 void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
65 assert(isCriticalEdge(TI, SuccNum) &&
66 "Cannot break a critical edge, if it isn't a critical edge");
67 BasicBlock *TIBB = TI->getParent();
68 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
70 // Create a new basic block, linking it into the CFG.
71 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
72 DestBB->getName() + "_crit_edge");
73 // Create our unconditional branch...
74 BranchInst *BI = new BranchInst(DestBB);
75 NewBB->getInstList().push_back(BI);
77 // Branch to the new block, breaking the edge...
78 TI->setSuccessor(SuccNum, NewBB);
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);
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.
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);
93 // If we don't have a pass object, we can't update anything...
96 // Now update analysis information. These are the analyses that we are
97 // currently capable of updating...
100 // Should we update DominatorSet information?
101 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
102 // The blocks that dominate the new one are the blocks that dominate TIBB
103 // plus the new block itself.
104 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
105 DomSet.insert(NewBB); // A block always dominates itself.
106 DS->addBasicBlock(NewBB, DomSet);
109 // Should we update ImmdediateDominator information?
110 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
111 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
113 ID->addNewBlock(NewBB, TIBB);
116 // Should we update DominatorTree information?
117 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
118 DominatorTree::Node *TINode = DT->getNode(TIBB);
120 // The new block is not the immediate dominator for any other nodes, but
121 // TINode is the immediate dominator for the new node.
123 if (TINode) // Don't break unreachable code!
124 DT->createNewNode(NewBB, TINode);
127 // Should we update DominanceFrontier information?
128 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
129 // Since the new block is dominated by its only predecessor TIBB,
130 // it cannot be in any block's dominance frontier. Its dominance
131 // frontier is {DestBB}.
132 DominanceFrontier::DomSetType NewDFSet;
133 NewDFSet.insert(DestBB);
134 DF->addBasicBlock(NewBB, NewDFSet);
138 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
139 // edges as they are found.
141 bool BreakCriticalEdges::runOnFunction(Function &F) {
142 bool Changed = false;
143 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
144 TerminatorInst *TI = I->getTerminator();
145 if (TI->getNumSuccessors() > 1)
146 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
147 if (isCriticalEdge(TI, i)) {
148 SplitCriticalEdge(TI, i, this);