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<DominatorTree>();
42 AU.addPreserved<DominanceFrontier>();
43 AU.addPreserved<LoopInfo>();
45 // No loop canonicalization guarantees are broken by this pass.
46 AU.addPreservedID(LoopSimplifyID);
50 RegisterPass<BreakCriticalEdges> X("break-crit-edges",
51 "Break critical edges in CFG");
54 // Publically exposed interface to pass...
55 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
56 FunctionPass *llvm::createBreakCriticalEdgesPass() {
57 return new BreakCriticalEdges();
60 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
61 // edges as they are found.
63 bool BreakCriticalEdges::runOnFunction(Function &F) {
65 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
66 TerminatorInst *TI = I->getTerminator();
67 if (TI->getNumSuccessors() > 1)
68 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
69 if (SplitCriticalEdge(TI, i, this)) {
78 //===----------------------------------------------------------------------===//
79 // Implementation of the external critical edge manipulation functions
80 //===----------------------------------------------------------------------===//
82 // isCriticalEdge - Return true if the specified edge is a critical edge.
83 // Critical edges are edges from a block with multiple successors to a block
84 // with multiple predecessors.
86 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
87 bool AllowIdenticalEdges) {
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 const BasicBlock *FirstPred = *I;
97 ++I; // Skip one edge due to the incoming arc from TI.
98 if (!AllowIdenticalEdges)
101 // If AllowIdenticalEdges is true, then we allow this edge to be considered
102 // non-critical iff all preds come from TI's block.
104 if (*I != FirstPred) return true;
108 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
109 // split the critical edge. This will update ETForest, ImmediateDominator,
110 // DominatorTree, and DominatorFrontier information if it is available, thus
111 // calling this pass will not invalidate any of them. This returns true if
112 // the edge was split, false otherwise. This ensures that all edges to that
113 // dest go to one block instead of each going to a different block.
115 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P,
116 bool MergeIdenticalEdges) {
117 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
118 BasicBlock *TIBB = TI->getParent();
119 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
121 // Create a new basic block, linking it into the CFG.
122 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
123 DestBB->getName() + "_crit_edge");
124 // Create our unconditional branch...
125 new BranchInst(DestBB, NewBB);
127 // Branch to the new block, breaking the edge.
128 TI->setSuccessor(SuccNum, NewBB);
130 // Insert the block into the function... right after the block TI lives in.
131 Function &F = *TIBB->getParent();
132 Function::iterator FBBI = TIBB;
133 F.getBasicBlockList().insert(++FBBI, 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 DominatorTree information?
201 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
202 DominatorTree::Node *TINode = DT->getNode(TIBB);
204 // The new block is not the immediate dominator for any other nodes, but
205 // TINode is the immediate dominator for the new node.
207 if (TINode) { // Don't break unreachable code!
208 DominatorTree::Node *NewBBNode = DT->createNewNode(NewBB, TINode);
209 DominatorTree::Node *DestBBNode = 0;
211 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
212 if (!OtherPreds.empty()) {
213 DestBBNode = DT->getNode(DestBB);
214 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
215 if (DominatorTree::Node *OPNode = DT->getNode(OtherPreds.back()))
216 NewBBDominatesDestBB = DestBBNode->dominates(OPNode);
217 OtherPreds.pop_back();
222 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
223 // doesn't dominate anything.
224 if (NewBBDominatesDestBB) {
225 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
226 DT->changeImmediateDominator(DestBBNode, NewBBNode);
231 // Should we update DominanceFrontier information?
232 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
233 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
234 if (!OtherPreds.empty()) {
235 // FIXME: IMPLEMENT THIS!
236 assert(0 && "Requiring domfrontiers but not idom/domtree/domset."
237 " not implemented yet!");
240 // Since the new block is dominated by its only predecessor TIBB,
241 // it cannot be in any block's dominance frontier. If NewBB dominates
242 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
244 DominanceFrontier::DomSetType NewDFSet;
245 if (NewBBDominatesDestBB) {
246 DominanceFrontier::iterator I = DF->find(DestBB);
248 DF->addBasicBlock(NewBB, I->second);
250 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
252 DominanceFrontier::DomSetType NewDFSet;
253 NewDFSet.insert(DestBB);
254 DF->addBasicBlock(NewBB, NewDFSet);
258 // Update LoopInfo if it is around.
259 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
260 // If one or the other blocks were not in a loop, the new block is not
261 // either, and thus LI doesn't need to be updated.
262 if (Loop *TIL = LI->getLoopFor(TIBB))
263 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
264 if (TIL == DestLoop) {
265 // Both in the same loop, the NewBB joins loop.
266 DestLoop->addBasicBlockToLoop(NewBB, *LI);
267 } else if (TIL->contains(DestLoop->getHeader())) {
268 // Edge from an outer loop to an inner loop. Add to the outer loop.
269 TIL->addBasicBlockToLoop(NewBB, *LI);
270 } else if (DestLoop->contains(TIL->getHeader())) {
271 // Edge from an inner loop to an outer loop. Add to the outer loop.
272 DestLoop->addBasicBlockToLoop(NewBB, *LI);
274 // Edge from two loops with no containment relation. Because these
275 // are natural loops, we know that the destination block must be the
276 // header of its loop (adding a branch into a loop elsewhere would
277 // create an irreducible loop).
278 assert(DestLoop->getHeader() == DestBB &&
279 "Should not create irreducible loops!");
280 if (Loop *P = DestLoop->getParentLoop())
281 P->addBasicBlockToLoop(NewBB, *LI);