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 #define DEBUG_TYPE "break-crit-edges"
20 #include "llvm/Transforms/Scalar.h"
21 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
22 #include "llvm/Analysis/Dominators.h"
23 #include "llvm/Analysis/LoopInfo.h"
24 #include "llvm/Function.h"
25 #include "llvm/Instructions.h"
26 #include "llvm/Type.h"
27 #include "llvm/Support/CFG.h"
28 #include "llvm/Support/Compiler.h"
29 #include "llvm/ADT/SmallVector.h"
30 #include "llvm/ADT/Statistic.h"
33 STATISTIC(NumBroken, "Number of blocks inserted");
36 struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
37 virtual bool runOnFunction(Function &F);
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 AU.addPreserved<ETForest>();
41 AU.addPreserved<DominatorSet>();
42 AU.addPreserved<ImmediateDominators>();
43 AU.addPreserved<DominatorTree>();
44 AU.addPreserved<DominanceFrontier>();
45 AU.addPreserved<LoopInfo>();
47 // No loop canonicalization guarantees are broken by this pass.
48 AU.addPreservedID(LoopSimplifyID);
52 RegisterPass<BreakCriticalEdges> X("break-crit-edges",
53 "Break critical edges in CFG");
56 // Publically exposed interface to pass...
57 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
58 FunctionPass *llvm::createBreakCriticalEdgesPass() {
59 return new BreakCriticalEdges();
62 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
63 // edges as they are found.
65 bool BreakCriticalEdges::runOnFunction(Function &F) {
67 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
68 TerminatorInst *TI = I->getTerminator();
69 if (TI->getNumSuccessors() > 1)
70 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
71 if (SplitCriticalEdge(TI, i, this)) {
80 //===----------------------------------------------------------------------===//
81 // Implementation of the external critical edge manipulation functions
82 //===----------------------------------------------------------------------===//
84 // isCriticalEdge - Return true if the specified edge is a critical edge.
85 // Critical edges are edges from a block with multiple successors to a block
86 // with multiple predecessors.
88 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
89 bool AllowIdenticalEdges) {
90 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
91 if (TI->getNumSuccessors() == 1) return false;
93 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
94 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
96 // If there is more than one predecessor, this is a critical edge...
97 assert(I != E && "No preds, but we have an edge to the block?");
98 const BasicBlock *FirstPred = *I;
99 ++I; // Skip one edge due to the incoming arc from TI.
100 if (!AllowIdenticalEdges)
103 // If AllowIdenticalEdges is true, then we allow this edge to be considered
104 // non-critical iff all preds come from TI's block.
106 if (*I != FirstPred) return true;
110 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
111 // split the critical edge. This will update DominatorSet, ImmediateDominator,
112 // DominatorTree, and DominatorFrontier information if it is available, thus
113 // calling this pass will not invalidate any of them. This returns true if
114 // the edge was split, false otherwise. This ensures that all edges to that
115 // dest go to one block instead of each going to a different block.
117 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P,
118 bool MergeIdenticalEdges) {
119 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
120 BasicBlock *TIBB = TI->getParent();
121 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
123 // Create a new basic block, linking it into the CFG.
124 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
125 DestBB->getName() + "_crit_edge");
126 // Create our unconditional branch...
127 new BranchInst(DestBB, NewBB);
129 // Branch to the new block, breaking the edge.
130 TI->setSuccessor(SuccNum, NewBB);
132 // Insert the block into the function... right after the block TI lives in.
133 Function &F = *TIBB->getParent();
134 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
136 // If there are any PHI nodes in DestBB, we need to update them so that they
137 // merge incoming values from NewBB instead of from TIBB.
139 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
140 PHINode *PN = cast<PHINode>(I);
141 // We no longer enter through TIBB, now we come in through NewBB. Revector
142 // exactly one entry in the PHI node that used to come from TIBB to come
144 int BBIdx = PN->getBasicBlockIndex(TIBB);
145 PN->setIncomingBlock(BBIdx, NewBB);
148 // If there are any other edges from TIBB to DestBB, update those to go
149 // through the split block, making those edges non-critical as well (and
150 // reducing the number of phi entries in the DestBB if relevant).
151 if (MergeIdenticalEdges) {
152 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
153 if (TI->getSuccessor(i) != DestBB) continue;
155 // Remove an entry for TIBB from DestBB phi nodes.
156 DestBB->removePredecessor(TIBB);
158 // We found another edge to DestBB, go to NewBB instead.
159 TI->setSuccessor(i, NewBB);
165 // If we don't have a pass object, we can't update anything...
166 if (P == 0) return true;
168 // Now update analysis information. Since the only predecessor of NewBB is
169 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
170 // anything, as there are other successors of DestBB. However, if all other
171 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
172 // loop header) then NewBB dominates DestBB.
173 SmallVector<BasicBlock*, 8> OtherPreds;
175 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
177 OtherPreds.push_back(*I);
179 // NewBBDominatesDestBB is valid if OtherPreds is empty, otherwise it isn't
181 bool NewBBDominatesDestBB = true;
183 // Should we update DominatorSet information?
184 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
185 DominatorSet::iterator DSI = DS->find(TIBB);
186 if (DSI != DS->end()) { // TIBB is reachable?
187 // The blocks that dominate the new one are the blocks that dominate TIBB
188 // plus the new block itself.
189 DominatorSet::DomSetType DomSet = DSI->second; // Copy domset.
190 DomSet.insert(NewBB); // A block always dominates itself.
191 DS->addBasicBlock(NewBB, DomSet);
193 // If NewBBDominatesDestBB hasn't been computed yet, do so with DS.
194 if (!OtherPreds.empty()) {
195 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
196 NewBBDominatesDestBB = DS->dominates(DestBB, OtherPreds.back());
197 OtherPreds.pop_back();
202 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
203 // doesn't dominate anything. If NewBB does dominates DestBB, then it
204 // dominates everything that DestBB dominates.
205 if (NewBBDominatesDestBB) {
206 for (DominatorSet::iterator I = DS->begin(), E = DS->end(); I != E; ++I)
207 if (I->second.count(DestBB))
208 I->second.insert(NewBB);
213 // Should we update ImmediateDominator information?
214 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
215 if (ID->get(TIBB)) { // Only do this if TIBB is reachable.
216 // TIBB is the new immediate dominator for NewBB.
217 ID->addNewBlock(NewBB, TIBB);
219 // If NewBBDominatesDestBB hasn't been computed yet, do so with ID.
220 if (!OtherPreds.empty()) {
221 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
222 NewBBDominatesDestBB = ID->dominates(DestBB, OtherPreds.back());
223 OtherPreds.pop_back();
228 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
229 // doesn't dominate anything.
230 if (NewBBDominatesDestBB)
231 ID->setImmediateDominator(DestBB, NewBB);
235 // Update the forest?
236 if (ETForest *EF = P->getAnalysisToUpdate<ETForest>()) {
237 // NewBB is dominated by TIBB.
238 EF->addNewBlock(NewBB, TIBB);
240 // If NewBBDominatesDestBB hasn't been computed yet, do so with EF.
241 if (!OtherPreds.empty()) {
242 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
243 NewBBDominatesDestBB = EF->dominates(DestBB, OtherPreds.back());
244 OtherPreds.pop_back();
249 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
250 // doesn't dominate anything.
251 if (NewBBDominatesDestBB)
252 EF->setImmediateDominator(DestBB, NewBB);
255 // Should we update DominatorTree information?
256 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
257 DominatorTree::Node *TINode = DT->getNode(TIBB);
259 // The new block is not the immediate dominator for any other nodes, but
260 // TINode is the immediate dominator for the new node.
262 if (TINode) { // Don't break unreachable code!
263 DominatorTree::Node *NewBBNode = DT->createNewNode(NewBB, TINode);
264 DominatorTree::Node *DestBBNode = 0;
266 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
267 if (!OtherPreds.empty()) {
268 DestBBNode = DT->getNode(DestBB);
269 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
270 if (DominatorTree::Node *OPNode = DT->getNode(OtherPreds.back()))
271 NewBBDominatesDestBB = DestBBNode->dominates(OPNode);
272 OtherPreds.pop_back();
277 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
278 // doesn't dominate anything.
279 if (NewBBDominatesDestBB) {
280 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
281 DT->changeImmediateDominator(DestBBNode, NewBBNode);
286 // Should we update DominanceFrontier information?
287 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
288 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
289 if (!OtherPreds.empty()) {
290 // FIXME: IMPLEMENT THIS!
291 assert(0 && "Requiring domfrontiers but not idom/domtree/domset."
292 " not implemented yet!");
295 // Since the new block is dominated by its only predecessor TIBB,
296 // it cannot be in any block's dominance frontier. If NewBB dominates
297 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
299 DominanceFrontier::DomSetType NewDFSet;
300 if (NewBBDominatesDestBB) {
301 DominanceFrontier::iterator I = DF->find(DestBB);
303 DF->addBasicBlock(NewBB, I->second);
305 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
307 DominanceFrontier::DomSetType NewDFSet;
308 NewDFSet.insert(DestBB);
309 DF->addBasicBlock(NewBB, NewDFSet);
313 // Update LoopInfo if it is around.
314 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
315 // If one or the other blocks were not in a loop, the new block is not
316 // either, and thus LI doesn't need to be updated.
317 if (Loop *TIL = LI->getLoopFor(TIBB))
318 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
319 if (TIL == DestLoop) {
320 // Both in the same loop, the NewBB joins loop.
321 DestLoop->addBasicBlockToLoop(NewBB, *LI);
322 } else if (TIL->contains(DestLoop->getHeader())) {
323 // Edge from an outer loop to an inner loop. Add to the outer loop.
324 TIL->addBasicBlockToLoop(NewBB, *LI);
325 } else if (DestLoop->contains(TIL->getHeader())) {
326 // Edge from an inner loop to an outer loop. Add to the outer loop.
327 DestLoop->addBasicBlockToLoop(NewBB, *LI);
329 // Edge from two loops with no containment relation. Because these
330 // are natural loops, we know that the destination block must be the
331 // header of its loop (adding a branch into a loop elsewhere would
332 // create an irreducible loop).
333 assert(DestLoop->getHeader() == DestBB &&
334 "Should not create irreducible loops!");
335 if (Loop *P = DestLoop->getParentLoop())
336 P->addBasicBlockToLoop(NewBB, *LI);