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<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 bool AllowIdenticalEdges) {
89 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
90 if (TI->getNumSuccessors() == 1) return false;
92 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
93 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
95 // If there is more than one predecessor, this is a critical edge...
96 assert(I != E && "No preds, but we have an edge to the block?");
97 const BasicBlock *FirstPred = *I;
98 ++I; // Skip one edge due to the incoming arc from TI.
99 if (!AllowIdenticalEdges)
102 // If AllowIdenticalEdges is true, then we allow this edge to be considered
103 // non-critical iff all preds come from TI's block.
105 if (*I != FirstPred) return true;
109 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
110 // split the critical edge. This will update ETForest, ImmediateDominator,
111 // DominatorTree, and DominatorFrontier information if it is available, thus
112 // calling this pass will not invalidate any of them. This returns true if
113 // the edge was split, false otherwise. This ensures that all edges to that
114 // dest go to one block instead of each going to a different block.
116 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P,
117 bool MergeIdenticalEdges) {
118 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
119 BasicBlock *TIBB = TI->getParent();
120 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
122 // Create a new basic block, linking it into the CFG.
123 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
124 DestBB->getName() + "_crit_edge");
125 // Create our unconditional branch...
126 new BranchInst(DestBB, NewBB);
128 // Branch to the new block, breaking the edge.
129 TI->setSuccessor(SuccNum, NewBB);
131 // Insert the block into the function... right after the block TI lives in.
132 Function &F = *TIBB->getParent();
133 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
135 // If there are any PHI nodes in DestBB, we need to update them so that they
136 // merge incoming values from NewBB instead of from TIBB.
138 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
139 PHINode *PN = cast<PHINode>(I);
140 // We no longer enter through TIBB, now we come in through NewBB. Revector
141 // exactly one entry in the PHI node that used to come from TIBB to come
143 int BBIdx = PN->getBasicBlockIndex(TIBB);
144 PN->setIncomingBlock(BBIdx, NewBB);
147 // If there are any other edges from TIBB to DestBB, update those to go
148 // through the split block, making those edges non-critical as well (and
149 // reducing the number of phi entries in the DestBB if relevant).
150 if (MergeIdenticalEdges) {
151 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
152 if (TI->getSuccessor(i) != DestBB) continue;
154 // Remove an entry for TIBB from DestBB phi nodes.
155 DestBB->removePredecessor(TIBB);
157 // We found another edge to DestBB, go to NewBB instead.
158 TI->setSuccessor(i, NewBB);
164 // If we don't have a pass object, we can't update anything...
165 if (P == 0) return true;
167 // Now update analysis information. Since the only predecessor of NewBB is
168 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
169 // anything, as there are other successors of DestBB. However, if all other
170 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
171 // loop header) then NewBB dominates DestBB.
172 SmallVector<BasicBlock*, 8> OtherPreds;
174 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
176 OtherPreds.push_back(*I);
178 bool NewBBDominatesDestBB = true;
180 // Update the forest?
181 if (ETForest *EF = P->getAnalysisToUpdate<ETForest>()) {
182 // NewBB is dominated by TIBB.
183 EF->addNewBlock(NewBB, TIBB);
185 // If NewBBDominatesDestBB hasn't been computed yet, do so with EF.
186 if (!OtherPreds.empty()) {
187 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
188 NewBBDominatesDestBB = EF->dominates(DestBB, OtherPreds.back());
189 OtherPreds.pop_back();
194 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
195 // doesn't dominate anything.
196 if (NewBBDominatesDestBB)
197 EF->setImmediateDominator(DestBB, NewBB);
200 // Should we update ImmediateDominator information?
201 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
202 // Only do this if TIBB is reachable.
203 if (ID->get(TIBB) || &TIBB->getParent()->getEntryBlock() == TIBB) {
204 // TIBB is the new immediate dominator for NewBB.
205 ID->addNewBlock(NewBB, TIBB);
207 // If NewBBDominatesDestBB hasn't been computed yet, do so with ID.
208 if (!OtherPreds.empty()) {
209 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
210 NewBBDominatesDestBB = ID->dominates(DestBB, OtherPreds.back());
211 OtherPreds.pop_back();
216 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
217 // doesn't dominate anything.
218 if (NewBBDominatesDestBB)
219 ID->setImmediateDominator(DestBB, NewBB);
223 // Should we update DominatorTree information?
224 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
225 DominatorTree::Node *TINode = DT->getNode(TIBB);
227 // The new block is not the immediate dominator for any other nodes, but
228 // TINode is the immediate dominator for the new node.
230 if (TINode) { // Don't break unreachable code!
231 DominatorTree::Node *NewBBNode = DT->createNewNode(NewBB, TINode);
232 DominatorTree::Node *DestBBNode = 0;
234 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
235 if (!OtherPreds.empty()) {
236 DestBBNode = DT->getNode(DestBB);
237 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
238 if (DominatorTree::Node *OPNode = DT->getNode(OtherPreds.back()))
239 NewBBDominatesDestBB = DestBBNode->dominates(OPNode);
240 OtherPreds.pop_back();
245 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
246 // doesn't dominate anything.
247 if (NewBBDominatesDestBB) {
248 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
249 DT->changeImmediateDominator(DestBBNode, NewBBNode);
254 // Should we update DominanceFrontier information?
255 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
256 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
257 if (!OtherPreds.empty()) {
258 // FIXME: IMPLEMENT THIS!
259 assert(0 && "Requiring domfrontiers but not idom/domtree/domset."
260 " not implemented yet!");
263 // Since the new block is dominated by its only predecessor TIBB,
264 // it cannot be in any block's dominance frontier. If NewBB dominates
265 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
267 DominanceFrontier::DomSetType NewDFSet;
268 if (NewBBDominatesDestBB) {
269 DominanceFrontier::iterator I = DF->find(DestBB);
271 DF->addBasicBlock(NewBB, I->second);
273 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
275 DominanceFrontier::DomSetType NewDFSet;
276 NewDFSet.insert(DestBB);
277 DF->addBasicBlock(NewBB, NewDFSet);
281 // Update LoopInfo if it is around.
282 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
283 // If one or the other blocks were not in a loop, the new block is not
284 // either, and thus LI doesn't need to be updated.
285 if (Loop *TIL = LI->getLoopFor(TIBB))
286 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
287 if (TIL == DestLoop) {
288 // Both in the same loop, the NewBB joins loop.
289 DestLoop->addBasicBlockToLoop(NewBB, *LI);
290 } else if (TIL->contains(DestLoop->getHeader())) {
291 // Edge from an outer loop to an inner loop. Add to the outer loop.
292 TIL->addBasicBlockToLoop(NewBB, *LI);
293 } else if (DestLoop->contains(TIL->getHeader())) {
294 // Edge from an inner loop to an outer loop. Add to the outer loop.
295 DestLoop->addBasicBlockToLoop(NewBB, *LI);
297 // Edge from two loops with no containment relation. Because these
298 // are natural loops, we know that the destination block must be the
299 // header of its loop (adding a branch into a loop elsewhere would
300 // create an irreducible loop).
301 assert(DestLoop->getHeader() == DestBB &&
302 "Should not create irreducible loops!");
303 if (Loop *P = DestLoop->getParentLoop())
304 P->addBasicBlockToLoop(NewBB, *LI);