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, tree, and frontier)
17 //===----------------------------------------------------------------------===//
19 #include "llvm/Transforms/Scalar.h"
20 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
21 #include "llvm/Analysis/Dominators.h"
22 #include "llvm/Function.h"
23 #include "llvm/iTerminators.h"
24 #include "llvm/iPHINode.h"
25 #include "llvm/Support/CFG.h"
26 #include "Support/Statistic.h"
31 Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
33 struct BreakCriticalEdges : public FunctionPass {
34 virtual bool runOnFunction(Function &F);
36 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
37 AU.addPreserved<DominatorSet>();
38 AU.addPreserved<ImmediateDominators>();
39 AU.addPreserved<DominatorTree>();
40 AU.addPreserved<DominanceFrontier>();
42 // No loop canonicalization guarantees are broken by this pass.
43 AU.addPreservedID(LoopSimplifyID);
47 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
48 "Break critical edges in CFG");
51 // Publically exposed interface to pass...
52 const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
53 Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
55 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
56 // edges as they are found.
58 bool BreakCriticalEdges::runOnFunction(Function &F) {
60 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
61 TerminatorInst *TI = I->getTerminator();
62 if (TI->getNumSuccessors() > 1)
63 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
64 if (SplitCriticalEdge(TI, i, this)) {
73 //===----------------------------------------------------------------------===//
74 // Implementation of the external critical edge manipulation functions
75 //===----------------------------------------------------------------------===//
77 // isCriticalEdge - Return true if the specified edge is a critical edge.
78 // Critical edges are edges from a block with multiple successors to a block
79 // with multiple predecessors.
81 bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
82 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
83 if (TI->getNumSuccessors() == 1) return false;
85 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
86 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
88 // If there is more than one predecessor, this is a critical edge...
89 assert(I != E && "No preds, but we have an edge to the block?");
90 ++I; // Skip one edge due to the incoming arc from TI.
94 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
95 // split the critical edge. This will update DominatorSet, ImmediateDominator,
96 // DominatorTree, and DominatorFrontier information if it is available, thus
97 // calling this pass will not invalidate either of them. This returns true if
98 // the edge was split, false otherwise.
100 bool SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
101 if (!isCriticalEdge(TI, SuccNum)) return false;
102 BasicBlock *TIBB = TI->getParent();
103 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
105 // Create a new basic block, linking it into the CFG.
106 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
107 DestBB->getName() + "_crit_edge");
108 // Create our unconditional branch...
109 new BranchInst(DestBB, NewBB);
111 // Branch to the new block, breaking the edge...
112 TI->setSuccessor(SuccNum, NewBB);
114 // Insert the block into the function... right after the block TI lives in.
115 Function &F = *TIBB->getParent();
116 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
118 // If there are any PHI nodes in DestBB, we need to update them so that they
119 // merge incoming values from NewBB instead of from TIBB.
121 for (BasicBlock::iterator I = DestBB->begin();
122 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
123 // We no longer enter through TIBB, now we come in through NewBB.
124 PN->replaceUsesOfWith(TIBB, NewBB);
127 // If we don't have a pass object, we can't update anything...
128 if (P == 0) return true;
130 // Now update analysis information. These are the analyses that we are
131 // currently capable of updating...
134 // Should we update DominatorSet information?
135 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
136 // The blocks that dominate the new one are the blocks that dominate TIBB
137 // plus the new block itself.
138 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
139 DomSet.insert(NewBB); // A block always dominates itself.
140 DS->addBasicBlock(NewBB, DomSet);
143 // Should we update ImmediateDominator information?
144 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
145 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
147 ID->addNewBlock(NewBB, TIBB);
150 // Should we update DominatorTree information?
151 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
152 DominatorTree::Node *TINode = DT->getNode(TIBB);
154 // The new block is not the immediate dominator for any other nodes, but
155 // TINode is the immediate dominator for the new node.
157 if (TINode) // Don't break unreachable code!
158 DT->createNewNode(NewBB, TINode);
161 // Should we update DominanceFrontier information?
162 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
163 // Since the new block is dominated by its only predecessor TIBB,
164 // it cannot be in any block's dominance frontier. Its dominance
165 // frontier is {DestBB}.
166 DominanceFrontier::DomSetType NewDFSet;
167 NewDFSet.insert(DestBB);
168 DF->addBasicBlock(NewBB, NewDFSet);
173 } // End llvm namespace