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/Analysis/LoopInfo.h"
23 #include "llvm/Function.h"
24 #include "llvm/Instructions.h"
25 #include "llvm/Type.h"
26 #include "llvm/Support/CFG.h"
27 #include "llvm/Support/Compiler.h"
28 #include "llvm/ADT/Statistic.h"
32 Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
34 struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
35 virtual bool runOnFunction(Function &F);
37 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
38 AU.addPreserved<ETForest>();
39 AU.addPreserved<DominatorSet>();
40 AU.addPreserved<ImmediateDominators>();
41 AU.addPreserved<DominatorTree>();
42 AU.addPreserved<DominanceFrontier>();
43 AU.addPreserved<LoopInfo>();
45 // No loop canonicalization guarantees are broken by this pass.
46 AU.addPreservedID(LoopSimplifyID);
50 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
51 "Break critical edges in CFG");
54 // Publically exposed interface to pass...
55 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
56 FunctionPass *llvm::createBreakCriticalEdgesPass() {
57 return new BreakCriticalEdges();
60 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
61 // edges as they are found.
63 bool BreakCriticalEdges::runOnFunction(Function &F) {
65 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
66 TerminatorInst *TI = I->getTerminator();
67 if (TI->getNumSuccessors() > 1)
68 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
69 if (SplitCriticalEdge(TI, i, this)) {
78 //===----------------------------------------------------------------------===//
79 // Implementation of the external critical edge manipulation functions
80 //===----------------------------------------------------------------------===//
82 // isCriticalEdge - Return true if the specified edge is a critical edge.
83 // Critical edges are edges from a block with multiple successors to a block
84 // with multiple predecessors.
86 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
87 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
88 if (TI->getNumSuccessors() == 1) return false;
90 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
91 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
93 // If there is more than one predecessor, this is a critical edge...
94 assert(I != E && "No preds, but we have an edge to the block?");
95 ++I; // Skip one edge due to the incoming arc from TI.
99 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
100 // split the critical edge. This will update DominatorSet, ImmediateDominator,
101 // DominatorTree, and DominatorFrontier information if it is available, thus
102 // calling this pass will not invalidate either of them. This returns true if
103 // the edge was split, false otherwise.
105 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
106 if (!isCriticalEdge(TI, SuccNum)) return false;
107 BasicBlock *TIBB = TI->getParent();
108 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
110 // Create a new basic block, linking it into the CFG.
111 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
112 DestBB->getName() + "_crit_edge");
113 // Create our unconditional branch...
114 new BranchInst(DestBB, NewBB);
116 // Branch to the new block, breaking the edge...
117 TI->setSuccessor(SuccNum, NewBB);
119 // Insert the block into the function... right after the block TI lives in.
120 Function &F = *TIBB->getParent();
121 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
123 // If there are any PHI nodes in DestBB, we need to update them so that they
124 // merge incoming values from NewBB instead of from TIBB.
126 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
127 PHINode *PN = cast<PHINode>(I);
128 // We no longer enter through TIBB, now we come in through NewBB. Revector
129 // exactly one entry in the PHI node that used to come from TIBB to come
131 int BBIdx = PN->getBasicBlockIndex(TIBB);
132 PN->setIncomingBlock(BBIdx, NewBB);
135 // If we don't have a pass object, we can't update anything...
136 if (P == 0) return true;
138 // Now update analysis information. These are the analyses that we are
139 // currently capable of updating...
142 // Should we update DominatorSet information?
143 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
144 // The blocks that dominate the new one are the blocks that dominate TIBB
145 // plus the new block itself.
146 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
147 DomSet.insert(NewBB); // A block always dominates itself.
148 DS->addBasicBlock(NewBB, DomSet);
151 // Should we update ImmediateDominator information?
152 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
153 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
155 ID->addNewBlock(NewBB, TIBB);
158 // Update the forest?
159 if (ETForest *EF = P->getAnalysisToUpdate<ETForest>())
160 EF->addNewBlock(NewBB, TIBB);
162 // Should we update DominatorTree information?
163 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
164 DominatorTree::Node *TINode = DT->getNode(TIBB);
166 // The new block is not the immediate dominator for any other nodes, but
167 // TINode is the immediate dominator for the new node.
169 if (TINode) // Don't break unreachable code!
170 DT->createNewNode(NewBB, TINode);
173 // Should we update DominanceFrontier information?
174 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
175 // Since the new block is dominated by its only predecessor TIBB,
176 // it cannot be in any block's dominance frontier. Its dominance
177 // frontier is {DestBB}.
178 DominanceFrontier::DomSetType NewDFSet;
179 NewDFSet.insert(DestBB);
180 DF->addBasicBlock(NewBB, NewDFSet);
183 // Update LoopInfo if it is around.
184 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
185 // If one or the other blocks were not in a loop, the new block is not
186 // either, and thus LI doesn't need to be updated.
187 if (Loop *TIL = LI->getLoopFor(TIBB))
188 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
189 if (TIL == DestLoop) {
190 // Both in the same loop, the NewBB joins loop.
191 DestLoop->addBasicBlockToLoop(NewBB, *LI);
192 } else if (TIL->contains(DestLoop->getHeader())) {
193 // Edge from an outer loop to an inner loop. Add to the outer lopo.
194 TIL->addBasicBlockToLoop(NewBB, *LI);
195 } else if (DestLoop->contains(TIL->getHeader())) {
196 // Edge from an inner loop to an outer loop. Add to the outer lopo.
197 DestLoop->addBasicBlockToLoop(NewBB, *LI);
199 // Edge from two loops with no containment relation. Because these
200 // are natural loops, we know that the destination block must be the
201 // header of its loop (adding a branch into a loop elsewhere would
202 // create an irreducible loop).
203 assert(DestLoop->getHeader() == DestBB &&
204 "Should not create irreducible loops!");
205 if (Loop *P = DestLoop->getParentLoop())
206 P->addBasicBlockToLoop(NewBB, *LI);