1 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // BreakCriticalEdges pass - Break all of the critical edges in the CFG by
11 // inserting a dummy basic block. This pass may be "required" by passes that
12 // cannot deal with critical edges. For this usage, the structure type is
13 // forward declared. This pass obviously invalidates the CFG, but can update
14 // forward dominator (set, immediate dominators, and tree) information.
16 //===----------------------------------------------------------------------===//
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/Analysis/Dominators.h"
20 #include "llvm/Function.h"
21 #include "llvm/iTerminators.h"
22 #include "llvm/iPHINode.h"
23 #include "llvm/Support/CFG.h"
24 #include "Support/Statistic.h"
27 Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
29 struct BreakCriticalEdges : public FunctionPass {
30 virtual bool runOnFunction(Function &F);
32 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
33 AU.addPreserved<DominatorSet>();
34 AU.addPreserved<ImmediateDominators>();
35 AU.addPreserved<DominatorTree>();
36 AU.addPreserved<DominanceFrontier>();
38 // No loop canonicalization guarantees are broken by this pass.
39 AU.addPreservedID(LoopSimplifyID);
43 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
44 "Break critical edges in CFG");
47 // Publically exposed interface to pass...
48 const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
49 Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
52 // isCriticalEdge - Return true if the specified edge is a critical edge.
53 // Critical edges are edges from a block with multiple successors to a block
54 // with multiple predecessors.
56 bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
57 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
58 if (TI->getNumSuccessors() == 1) return false;
60 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
61 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
63 // If there is more than one predecessor, this is a critical edge...
64 assert(I != E && "No preds, but we have an edge to the block?");
65 ++I; // Skip one edge due to the incoming arc from TI.
69 // SplitCriticalEdge - Insert a new node node to split the critical edge. This
70 // will update DominatorSet, ImmediateDominator and DominatorTree information if
71 // it is available, thus calling this pass will not invalidate either of them.
73 void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
74 assert(isCriticalEdge(TI, SuccNum) &&
75 "Cannot break a critical edge, if it isn't a critical edge");
76 BasicBlock *TIBB = TI->getParent();
77 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
79 // Create a new basic block, linking it into the CFG.
80 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
81 DestBB->getName() + "_crit_edge");
82 // Create our unconditional branch...
83 BranchInst *BI = new BranchInst(DestBB);
84 NewBB->getInstList().push_back(BI);
86 // Branch to the new block, breaking the edge...
87 TI->setSuccessor(SuccNum, NewBB);
89 // Insert the block into the function... right after the block TI lives in.
90 Function &F = *TIBB->getParent();
91 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
93 // If there are any PHI nodes in DestBB, we need to update them so that they
94 // merge incoming values from NewBB instead of from TIBB.
96 for (BasicBlock::iterator I = DestBB->begin();
97 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
98 // We no longer enter through TIBB, now we come in through NewBB.
99 PN->replaceUsesOfWith(TIBB, NewBB);
102 // If we don't have a pass object, we can't update anything...
105 // Now update analysis information. These are the analyses that we are
106 // currently capable of updating...
109 // Should we update DominatorSet information?
110 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
111 // The blocks that dominate the new one are the blocks that dominate TIBB
112 // plus the new block itself.
113 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
114 DomSet.insert(NewBB); // A block always dominates itself.
115 DS->addBasicBlock(NewBB, DomSet);
118 // Should we update ImmediateDominator information?
119 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
120 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
122 ID->addNewBlock(NewBB, TIBB);
125 // Should we update DominatorTree information?
126 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
127 DominatorTree::Node *TINode = DT->getNode(TIBB);
129 // The new block is not the immediate dominator for any other nodes, but
130 // TINode is the immediate dominator for the new node.
132 if (TINode) // Don't break unreachable code!
133 DT->createNewNode(NewBB, TINode);
136 // Should we update DominanceFrontier information?
137 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
138 // Since the new block is dominated by its only predecessor TIBB,
139 // it cannot be in any block's dominance frontier. Its dominance
140 // frontier is {DestBB}.
141 DominanceFrontier::DomSetType NewDFSet;
142 NewDFSet.insert(DestBB);
143 DF->addBasicBlock(NewBB, NewDFSet);
147 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
148 // edges as they are found.
150 bool BreakCriticalEdges::runOnFunction(Function &F) {
151 bool Changed = false;
152 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
153 TerminatorInst *TI = I->getTerminator();
154 if (TI->getNumSuccessors() > 1)
155 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
156 if (isCriticalEdge(TI, i)) {
157 SplitCriticalEdge(TI, i, this);