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/SmallVector.h"
29 #include "llvm/ADT/Statistic.h"
33 Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
35 struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
36 virtual bool runOnFunction(Function &F);
38 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
39 AU.addPreserved<ETForest>();
40 AU.addPreserved<DominatorSet>();
41 AU.addPreserved<ImmediateDominators>();
42 AU.addPreserved<DominatorTree>();
43 AU.addPreserved<DominanceFrontier>();
44 AU.addPreserved<LoopInfo>();
46 // No loop canonicalization guarantees are broken by this pass.
47 AU.addPreservedID(LoopSimplifyID);
51 RegisterPass<BreakCriticalEdges> X("break-crit-edges",
52 "Break critical edges in CFG");
55 // Publically exposed interface to pass...
56 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
57 FunctionPass *llvm::createBreakCriticalEdgesPass() {
58 return new BreakCriticalEdges();
61 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
62 // edges as they are found.
64 bool BreakCriticalEdges::runOnFunction(Function &F) {
66 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
67 TerminatorInst *TI = I->getTerminator();
68 if (TI->getNumSuccessors() > 1)
69 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
70 if (SplitCriticalEdge(TI, i, this)) {
79 //===----------------------------------------------------------------------===//
80 // Implementation of the external critical edge manipulation functions
81 //===----------------------------------------------------------------------===//
83 // isCriticalEdge - Return true if the specified edge is a critical edge.
84 // Critical edges are edges from a block with multiple successors to a block
85 // with multiple predecessors.
87 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
88 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
89 if (TI->getNumSuccessors() == 1) return false;
91 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
92 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
94 // If there is more than one predecessor, this is a critical edge...
95 assert(I != E && "No preds, but we have an edge to the block?");
96 ++I; // Skip one edge due to the incoming arc from TI.
100 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
101 // split the critical edge. This will update DominatorSet, ImmediateDominator,
102 // DominatorTree, and DominatorFrontier information if it is available, thus
103 // calling this pass will not invalidate either of them. This returns true if
104 // the edge was split, false otherwise.
106 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
107 if (!isCriticalEdge(TI, SuccNum)) return false;
108 BasicBlock *TIBB = TI->getParent();
109 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
111 // Create a new basic block, linking it into the CFG.
112 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
113 DestBB->getName() + "_crit_edge");
114 // Create our unconditional branch...
115 new BranchInst(DestBB, NewBB);
117 // Branch to the new block, breaking the edge...
118 TI->setSuccessor(SuccNum, NewBB);
120 // Insert the block into the function... right after the block TI lives in.
121 Function &F = *TIBB->getParent();
122 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
124 // If there are any PHI nodes in DestBB, we need to update them so that they
125 // merge incoming values from NewBB instead of from TIBB.
127 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
128 PHINode *PN = cast<PHINode>(I);
129 // We no longer enter through TIBB, now we come in through NewBB. Revector
130 // exactly one entry in the PHI node that used to come from TIBB to come
132 int BBIdx = PN->getBasicBlockIndex(TIBB);
133 PN->setIncomingBlock(BBIdx, NewBB);
136 // If we don't have a pass object, we can't update anything...
137 if (P == 0) return true;
139 // Now update analysis information. Since the only predecessor of NewBB is
140 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
141 // anything, as there are other successors of DestBB. However, if all other
142 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
143 // loop header) then NewBB dominates DestBB.
144 SmallVector<BasicBlock*, 8> OtherPreds;
146 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
148 OtherPreds.push_back(*I);
150 // NewBBDominatesDestBB is valid if OtherPreds is empty, otherwise it isn't
152 bool NewBBDominatesDestBB = true;
154 // Should we update DominatorSet information?
155 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
156 // The blocks that dominate the new one are the blocks that dominate TIBB
157 // plus the new block itself.
158 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
159 DomSet.insert(NewBB); // A block always dominates itself.
160 DS->addBasicBlock(NewBB, DomSet);
162 // If NewBBDominatesDestBB hasn't been computed yet, do so with DS.
163 if (!OtherPreds.empty()) {
164 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
165 NewBBDominatesDestBB = DS->dominates(DestBB, OtherPreds.back());
166 OtherPreds.pop_back();
171 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
172 // doesn't dominate anything. If NewBB does dominates DestBB, then it
173 // dominates everything that DestBB dominates.
174 if (NewBBDominatesDestBB) {
175 for (DominatorSet::iterator I = DS->begin(), E = DS->end(); I != E; ++I)
176 if (I->second.count(DestBB))
177 I->second.insert(NewBB);
181 // Should we update ImmediateDominator information?
182 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
183 // TIBB is the new immediate dominator for NewBB.
184 ID->addNewBlock(NewBB, TIBB);
186 // If NewBBDominatesDestBB hasn't been computed yet, do so with ID.
187 if (!OtherPreds.empty()) {
188 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
189 NewBBDominatesDestBB = ID->dominates(DestBB, OtherPreds.back());
190 OtherPreds.pop_back();
195 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
196 // doesn't dominate anything.
197 if (NewBBDominatesDestBB)
198 ID->setImmediateDominator(DestBB, NewBB);
201 // Update the forest?
202 if (ETForest *EF = P->getAnalysisToUpdate<ETForest>()) {
203 // NewBB is dominated by TIBB.
204 EF->addNewBlock(NewBB, TIBB);
206 // If NewBBDominatesDestBB hasn't been computed yet, do so with EF.
207 if (!OtherPreds.empty()) {
208 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
209 NewBBDominatesDestBB = EF->dominates(DestBB, OtherPreds.back());
210 OtherPreds.pop_back();
215 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
216 // doesn't dominate anything.
217 if (NewBBDominatesDestBB)
218 EF->setImmediateDominator(DestBB, NewBB);
221 // Should we update DominatorTree information?
222 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
223 DominatorTree::Node *TINode = DT->getNode(TIBB);
225 // The new block is not the immediate dominator for any other nodes, but
226 // TINode is the immediate dominator for the new node.
228 if (TINode) { // Don't break unreachable code!
229 DominatorTree::Node *NewBBNode = DT->createNewNode(NewBB, TINode);
230 DominatorTree::Node *DestBBNode = 0;
232 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
233 if (!OtherPreds.empty()) {
234 DestBBNode = DT->getNode(DestBB);
235 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
236 if (DominatorTree::Node *OPNode = DT->getNode(OtherPreds.back()))
237 NewBBDominatesDestBB = DestBBNode->dominates(OPNode);
238 OtherPreds.pop_back();
243 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
244 // doesn't dominate anything.
245 if (NewBBDominatesDestBB) {
246 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
247 DT->changeImmediateDominator(DestBBNode, NewBBNode);
252 // Should we update DominanceFrontier information?
253 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
254 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
255 if (!OtherPreds.empty()) {
256 // FIXME: IMPLEMENT THIS!
257 assert(0 && "Requiring domfrontiers but not idom/domtree/domset."
258 " not implemented yet!");
261 // Since the new block is dominated by its only predecessor TIBB,
262 // it cannot be in any block's dominance frontier. If NewBB dominates
263 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
265 DominanceFrontier::DomSetType NewDFSet;
266 if (NewBBDominatesDestBB) {
267 DominanceFrontier::iterator I = DF->find(DestBB);
269 DF->addBasicBlock(NewBB, I->second);
271 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
273 DominanceFrontier::DomSetType NewDFSet;
274 NewDFSet.insert(DestBB);
275 DF->addBasicBlock(NewBB, NewDFSet);
279 // Update LoopInfo if it is around.
280 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
281 // If one or the other blocks were not in a loop, the new block is not
282 // either, and thus LI doesn't need to be updated.
283 if (Loop *TIL = LI->getLoopFor(TIBB))
284 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
285 if (TIL == DestLoop) {
286 // Both in the same loop, the NewBB joins loop.
287 DestLoop->addBasicBlockToLoop(NewBB, *LI);
288 } else if (TIL->contains(DestLoop->getHeader())) {
289 // Edge from an outer loop to an inner loop. Add to the outer loop.
290 TIL->addBasicBlockToLoop(NewBB, *LI);
291 } else if (DestLoop->contains(TIL->getHeader())) {
292 // Edge from an inner loop to an outer loop. Add to the outer loop.
293 DestLoop->addBasicBlockToLoop(NewBB, *LI);
295 // Edge from two loops with no containment relation. Because these
296 // are natural loops, we know that the destination block must be the
297 // header of its loop (adding a branch into a loop elsewhere would
298 // create an irreducible loop).
299 assert(DestLoop->getHeader() == DestBB &&
300 "Should not create irreducible loops!");
301 if (Loop *P = DestLoop->getParentLoop())
302 P->addBasicBlockToLoop(NewBB, *LI);