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/Transforms/Utils/BasicBlockUtils.h"
20 #include "llvm/Analysis/Dominators.h"
21 #include "llvm/Function.h"
22 #include "llvm/iTerminators.h"
23 #include "llvm/iPHINode.h"
24 #include "llvm/Support/CFG.h"
25 #include "Support/Statistic.h"
28 Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
30 struct BreakCriticalEdges : public FunctionPass {
31 virtual bool runOnFunction(Function &F);
33 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
34 AU.addPreserved<DominatorSet>();
35 AU.addPreserved<ImmediateDominators>();
36 AU.addPreserved<DominatorTree>();
37 AU.addPreserved<DominanceFrontier>();
39 // No loop canonicalization guarantees are broken by this pass.
40 AU.addPreservedID(LoopSimplifyID);
44 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
45 "Break critical edges in CFG");
48 // Publically exposed interface to pass...
49 const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
50 Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
53 // isCriticalEdge - Return true if the specified edge is a critical edge.
54 // Critical edges are edges from a block with multiple successors to a block
55 // with multiple predecessors.
57 bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
58 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
59 if (TI->getNumSuccessors() == 1) return false;
61 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
62 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
64 // If there is more than one predecessor, this is a critical edge...
65 assert(I != E && "No preds, but we have an edge to the block?");
66 ++I; // Skip one edge due to the incoming arc from TI.
70 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
71 // split the critical edge. This will update DominatorSet, ImmediateDominator,
72 // DominatorTree, and DominatorFrontier information if it is available, thus
73 // calling this pass will not invalidate either of them. This returns true if
74 // the edge was split, false otherwise.
76 bool SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
77 if (!isCriticalEdge(TI, SuccNum)) return false;
78 BasicBlock *TIBB = TI->getParent();
79 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
81 // Create a new basic block, linking it into the CFG.
82 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
83 DestBB->getName() + "_crit_edge");
84 // Create our unconditional branch...
85 BranchInst *BI = new BranchInst(DestBB);
86 NewBB->getInstList().push_back(BI);
88 // Branch to the new block, breaking the edge...
89 TI->setSuccessor(SuccNum, NewBB);
91 // Insert the block into the function... right after the block TI lives in.
92 Function &F = *TIBB->getParent();
93 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
95 // If there are any PHI nodes in DestBB, we need to update them so that they
96 // merge incoming values from NewBB instead of from TIBB.
98 for (BasicBlock::iterator I = DestBB->begin();
99 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
100 // We no longer enter through TIBB, now we come in through NewBB.
101 PN->replaceUsesOfWith(TIBB, NewBB);
104 // If we don't have a pass object, we can't update anything...
105 if (P == 0) return true;
107 // Now update analysis information. These are the analyses that we are
108 // currently capable of updating...
111 // Should we update DominatorSet information?
112 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
113 // The blocks that dominate the new one are the blocks that dominate TIBB
114 // plus the new block itself.
115 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
116 DomSet.insert(NewBB); // A block always dominates itself.
117 DS->addBasicBlock(NewBB, DomSet);
120 // Should we update ImmediateDominator information?
121 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
122 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
124 ID->addNewBlock(NewBB, TIBB);
127 // Should we update DominatorTree information?
128 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
129 DominatorTree::Node *TINode = DT->getNode(TIBB);
131 // The new block is not the immediate dominator for any other nodes, but
132 // TINode is the immediate dominator for the new node.
134 if (TINode) // Don't break unreachable code!
135 DT->createNewNode(NewBB, TINode);
138 // Should we update DominanceFrontier information?
139 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
140 // Since the new block is dominated by its only predecessor TIBB,
141 // it cannot be in any block's dominance frontier. Its dominance
142 // frontier is {DestBB}.
143 DominanceFrontier::DomSetType NewDFSet;
144 NewDFSet.insert(DestBB);
145 DF->addBasicBlock(NewBB, NewDFSet);
150 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
151 // edges as they are found.
153 bool BreakCriticalEdges::runOnFunction(Function &F) {
154 bool Changed = false;
155 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
156 TerminatorInst *TI = I->getTerminator();
157 if (TI->getNumSuccessors() > 1)
158 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
159 if (SplitCriticalEdge(TI, i, this)) {