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 static char ID; // Pass identifcation, replacement for typeid
38 BreakCriticalEdges() : FunctionPass((intptr_t)&ID) {}
40 virtual bool runOnFunction(Function &F);
42 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43 AU.addPreserved<ETForest>();
44 AU.addPreserved<DominatorTree>();
45 AU.addPreserved<DominanceFrontier>();
46 AU.addPreserved<LoopInfo>();
48 // No loop canonicalization guarantees are broken by this pass.
49 AU.addPreservedID(LoopSimplifyID);
53 char BreakCriticalEdges::ID = 0;
54 RegisterPass<BreakCriticalEdges> X("break-crit-edges",
55 "Break critical edges in CFG");
58 // Publically exposed interface to pass...
59 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
60 FunctionPass *llvm::createBreakCriticalEdgesPass() {
61 return new BreakCriticalEdges();
64 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
65 // edges as they are found.
67 bool BreakCriticalEdges::runOnFunction(Function &F) {
69 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
70 TerminatorInst *TI = I->getTerminator();
71 if (TI->getNumSuccessors() > 1)
72 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
73 if (SplitCriticalEdge(TI, i, this)) {
82 //===----------------------------------------------------------------------===//
83 // Implementation of the external critical edge manipulation functions
84 //===----------------------------------------------------------------------===//
86 // isCriticalEdge - Return true if the specified edge is a critical edge.
87 // Critical edges are edges from a block with multiple successors to a block
88 // with multiple predecessors.
90 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
91 bool AllowIdenticalEdges) {
92 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
93 if (TI->getNumSuccessors() == 1) return false;
95 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
96 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
98 // If there is more than one predecessor, this is a critical edge...
99 assert(I != E && "No preds, but we have an edge to the block?");
100 const BasicBlock *FirstPred = *I;
101 ++I; // Skip one edge due to the incoming arc from TI.
102 if (!AllowIdenticalEdges)
105 // If AllowIdenticalEdges is true, then we allow this edge to be considered
106 // non-critical iff all preds come from TI's block.
108 if (*I != FirstPred) return true;
112 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
113 // split the critical edge. This will update ETForest, ImmediateDominator,
114 // DominatorTree, and DominatorFrontier information if it is available, thus
115 // calling this pass will not invalidate any of them. This returns true if
116 // the edge was split, false otherwise. This ensures that all edges to that
117 // dest go to one block instead of each going to a different block.
119 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P,
120 bool MergeIdenticalEdges) {
121 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
122 BasicBlock *TIBB = TI->getParent();
123 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
125 // Create a new basic block, linking it into the CFG.
126 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
127 DestBB->getName() + "_crit_edge");
128 // Create our unconditional branch...
129 new BranchInst(DestBB, NewBB);
131 // Branch to the new block, breaking the edge.
132 TI->setSuccessor(SuccNum, NewBB);
134 // Insert the block into the function... right after the block TI lives in.
135 Function &F = *TIBB->getParent();
136 Function::iterator FBBI = TIBB;
137 F.getBasicBlockList().insert(++FBBI, NewBB);
139 // If there are any PHI nodes in DestBB, we need to update them so that they
140 // merge incoming values from NewBB instead of from TIBB.
142 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
143 PHINode *PN = cast<PHINode>(I);
144 // We no longer enter through TIBB, now we come in through NewBB. Revector
145 // exactly one entry in the PHI node that used to come from TIBB to come
147 int BBIdx = PN->getBasicBlockIndex(TIBB);
148 PN->setIncomingBlock(BBIdx, NewBB);
151 // If there are any other edges from TIBB to DestBB, update those to go
152 // through the split block, making those edges non-critical as well (and
153 // reducing the number of phi entries in the DestBB if relevant).
154 if (MergeIdenticalEdges) {
155 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
156 if (TI->getSuccessor(i) != DestBB) continue;
158 // Remove an entry for TIBB from DestBB phi nodes.
159 DestBB->removePredecessor(TIBB);
161 // We found another edge to DestBB, go to NewBB instead.
162 TI->setSuccessor(i, NewBB);
168 // If we don't have a pass object, we can't update anything...
169 if (P == 0) return true;
171 // Now update analysis information. Since the only predecessor of NewBB is
172 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
173 // anything, as there are other successors of DestBB. However, if all other
174 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
175 // loop header) then NewBB dominates DestBB.
176 SmallVector<BasicBlock*, 8> OtherPreds;
178 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
180 OtherPreds.push_back(*I);
182 bool NewBBDominatesDestBB = true;
184 // Update the forest?
185 if (ETForest *EF = P->getAnalysisToUpdate<ETForest>()) {
186 // NewBB is dominated by TIBB.
187 EF->addNewBlock(NewBB, TIBB);
189 // If NewBBDominatesDestBB hasn't been computed yet, do so with EF.
190 if (!OtherPreds.empty()) {
191 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
192 NewBBDominatesDestBB = EF->dominates(DestBB, OtherPreds.back());
193 OtherPreds.pop_back();
198 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
199 // doesn't dominate anything.
200 if (NewBBDominatesDestBB)
201 EF->setImmediateDominator(DestBB, NewBB);
204 // Should we update DominatorTree information?
205 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
206 DominatorTree::Node *TINode = DT->getNode(TIBB);
208 // The new block is not the immediate dominator for any other nodes, but
209 // TINode is the immediate dominator for the new node.
211 if (TINode) { // Don't break unreachable code!
212 DominatorTree::Node *NewBBNode = DT->createNewNode(NewBB, TINode);
213 DominatorTree::Node *DestBBNode = 0;
215 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
216 if (!OtherPreds.empty()) {
217 DestBBNode = DT->getNode(DestBB);
218 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
219 if (DominatorTree::Node *OPNode = DT->getNode(OtherPreds.back()))
220 NewBBDominatesDestBB = DestBBNode->dominates(OPNode);
221 OtherPreds.pop_back();
226 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
227 // doesn't dominate anything.
228 if (NewBBDominatesDestBB) {
229 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
230 DT->changeImmediateDominator(DestBBNode, NewBBNode);
235 // Should we update DominanceFrontier information?
236 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
237 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
238 if (!OtherPreds.empty()) {
239 // FIXME: IMPLEMENT THIS!
240 assert(0 && "Requiring domfrontiers but not idom/domtree/domset."
241 " not implemented yet!");
244 // Since the new block is dominated by its only predecessor TIBB,
245 // it cannot be in any block's dominance frontier. If NewBB dominates
246 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
248 DominanceFrontier::DomSetType NewDFSet;
249 if (NewBBDominatesDestBB) {
250 DominanceFrontier::iterator I = DF->find(DestBB);
252 DF->addBasicBlock(NewBB, I->second);
254 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
256 DominanceFrontier::DomSetType NewDFSet;
257 NewDFSet.insert(DestBB);
258 DF->addBasicBlock(NewBB, NewDFSet);
262 // Update LoopInfo if it is around.
263 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
264 // If one or the other blocks were not in a loop, the new block is not
265 // either, and thus LI doesn't need to be updated.
266 if (Loop *TIL = LI->getLoopFor(TIBB))
267 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
268 if (TIL == DestLoop) {
269 // Both in the same loop, the NewBB joins loop.
270 DestLoop->addBasicBlockToLoop(NewBB, *LI);
271 } else if (TIL->contains(DestLoop->getHeader())) {
272 // Edge from an outer loop to an inner loop. Add to the outer loop.
273 TIL->addBasicBlockToLoop(NewBB, *LI);
274 } else if (DestLoop->contains(TIL->getHeader())) {
275 // Edge from an inner loop to an outer loop. Add to the outer loop.
276 DestLoop->addBasicBlockToLoop(NewBB, *LI);
278 // Edge from two loops with no containment relation. Because these
279 // are natural loops, we know that the destination block must be the
280 // header of its loop (adding a branch into a loop elsewhere would
281 // create an irreducible loop).
282 assert(DestLoop->getHeader() == DestBB &&
283 "Should not create irreducible loops!");
284 if (Loop *P = DestLoop->getParentLoop())
285 P->addBasicBlockToLoop(NewBB, *LI);