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/ADT/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>();
41 AU.addPreserved<LoopInfo>();
43 // No loop canonicalization guarantees are broken by this pass.
44 AU.addPreservedID(LoopSimplifyID);
48 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
49 "Break critical edges in CFG");
52 // Publically exposed interface to pass...
53 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
54 FunctionPass *llvm::createBreakCriticalEdgesPass() {
55 return new BreakCriticalEdges();
58 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
59 // edges as they are found.
61 bool BreakCriticalEdges::runOnFunction(Function &F) {
63 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
64 TerminatorInst *TI = I->getTerminator();
65 if (TI->getNumSuccessors() > 1)
66 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
67 if (SplitCriticalEdge(TI, i, this)) {
76 //===----------------------------------------------------------------------===//
77 // Implementation of the external critical edge manipulation functions
78 //===----------------------------------------------------------------------===//
80 // isCriticalEdge - Return true if the specified edge is a critical edge.
81 // Critical edges are edges from a block with multiple successors to a block
82 // with multiple predecessors.
84 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
85 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
86 if (TI->getNumSuccessors() == 1) return false;
88 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
89 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
91 // If there is more than one predecessor, this is a critical edge...
92 assert(I != E && "No preds, but we have an edge to the block?");
93 ++I; // Skip one edge due to the incoming arc from TI.
97 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
98 // split the critical edge. This will update DominatorSet, ImmediateDominator,
99 // DominatorTree, and DominatorFrontier information if it is available, thus
100 // calling this pass will not invalidate either of them. This returns true if
101 // the edge was split, false otherwise.
103 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
104 if (!isCriticalEdge(TI, SuccNum)) return false;
105 BasicBlock *TIBB = TI->getParent();
106 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
108 // Create a new basic block, linking it into the CFG.
109 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
110 DestBB->getName() + "_crit_edge");
111 // Create our unconditional branch...
112 new BranchInst(DestBB, NewBB);
114 // Branch to the new block, breaking the edge...
115 TI->setSuccessor(SuccNum, NewBB);
117 // Insert the block into the function... right after the block TI lives in.
118 Function &F = *TIBB->getParent();
119 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
121 // If there are any PHI nodes in DestBB, we need to update them so that they
122 // merge incoming values from NewBB instead of from TIBB.
124 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
125 PHINode *PN = cast<PHINode>(I);
126 // We no longer enter through TIBB, now we come in through NewBB. Revector
127 // exactly one entry in the PHI node that used to come from TIBB to come
129 int BBIdx = PN->getBasicBlockIndex(TIBB);
130 PN->setIncomingBlock(BBIdx, NewBB);
133 // If we don't have a pass object, we can't update anything...
134 if (P == 0) return true;
136 // Now update analysis information. These are the analyses that we are
137 // currently capable of updating...
140 // Should we update DominatorSet information?
141 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
142 // The blocks that dominate the new one are the blocks that dominate TIBB
143 // plus the new block itself.
144 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
145 DomSet.insert(NewBB); // A block always dominates itself.
146 DS->addBasicBlock(NewBB, DomSet);
149 // Should we update ImmediateDominator information?
150 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
151 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
153 ID->addNewBlock(NewBB, TIBB);
156 // Should we update DominatorTree information?
157 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
158 DominatorTree::Node *TINode = DT->getNode(TIBB);
160 // The new block is not the immediate dominator for any other nodes, but
161 // TINode is the immediate dominator for the new node.
163 if (TINode) // Don't break unreachable code!
164 DT->createNewNode(NewBB, TINode);
167 // Should we update DominanceFrontier information?
168 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
169 // Since the new block is dominated by its only predecessor TIBB,
170 // it cannot be in any block's dominance frontier. Its dominance
171 // frontier is {DestBB}.
172 DominanceFrontier::DomSetType NewDFSet;
173 NewDFSet.insert(DestBB);
174 DF->addBasicBlock(NewBB, NewDFSet);
177 // Update LoopInfo if it is around.
178 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
179 // If one or the other blocks were not in a loop, the new block is not
180 // either, and thus LI doesn't need to be updated.
181 if (Loop *TIL = LI->getLoopFor(TIBB))
182 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
183 if (TIL == DestLoop) {
184 // Both in the same loop, the NewBB joins loop.
185 DestLoop->addBasicBlockToLoop(NewBB, *LI);
186 } else if (TIL->contains(DestLoop->getHeader())) {
187 // Edge from an outer loop to an inner loop. Add to the outer lopo.
188 TIL->addBasicBlockToLoop(NewBB, *LI);
189 } else if (DestLoop->contains(TIL->getHeader())) {
190 // Edge from an inner loop to an outer loop. Add to the outer lopo.
191 DestLoop->addBasicBlockToLoop(NewBB, *LI);
193 // Edge from two loops with no containment relation. Because these
194 // are natural loops, we know that the destination block must be the
195 // header of its loop (adding a branch into a loop elsewhere would
196 // create an irreducible loop).
197 assert(DestLoop->getHeader() == DestBB &&
198 "Should not create irreducible loops!");
199 if (Loop *P = DestLoop->getParentLoop())
200 P->addBasicBlockToLoop(NewBB, *LI);