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 identification, 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<DominatorTree>();
44 AU.addPreserved<DominanceFrontier>();
45 AU.addPreserved<LoopInfo>();
47 // No loop canonicalization guarantees are broken by this pass.
48 AU.addPreservedID(LoopSimplifyID);
52 char BreakCriticalEdges::ID = 0;
53 RegisterPass<BreakCriticalEdges> X("break-crit-edges",
54 "Break critical edges in CFG");
57 // Publically exposed interface to pass...
58 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
59 FunctionPass *llvm::createBreakCriticalEdgesPass() {
60 return new BreakCriticalEdges();
63 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
64 // edges as they are found.
66 bool BreakCriticalEdges::runOnFunction(Function &F) {
68 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
69 TerminatorInst *TI = I->getTerminator();
70 if (TI->getNumSuccessors() > 1)
71 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
72 if (SplitCriticalEdge(TI, i, this)) {
81 //===----------------------------------------------------------------------===//
82 // Implementation of the external critical edge manipulation functions
83 //===----------------------------------------------------------------------===//
85 // isCriticalEdge - Return true if the specified edge is a critical edge.
86 // Critical edges are edges from a block with multiple successors to a block
87 // with multiple predecessors.
89 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
90 bool AllowIdenticalEdges) {
91 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
92 if (TI->getNumSuccessors() == 1) return false;
94 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
95 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
97 // If there is more than one predecessor, this is a critical edge...
98 assert(I != E && "No preds, but we have an edge to the block?");
99 const BasicBlock *FirstPred = *I;
100 ++I; // Skip one edge due to the incoming arc from TI.
101 if (!AllowIdenticalEdges)
104 // If AllowIdenticalEdges is true, then we allow this edge to be considered
105 // non-critical iff all preds come from TI's block.
107 if (*I != FirstPred) return true;
111 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
112 // split the critical edge. This will update DominatorTree, and DominatorFrontier
113 // information if it is available, thus calling this pass will not invalidate
114 // any of them. This returns true if the edge was split, false otherwise.
115 // This ensures that all edges to that dest go to one block instead of each
116 // going to a different block.
118 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P,
119 bool MergeIdenticalEdges) {
120 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
121 BasicBlock *TIBB = TI->getParent();
122 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
124 // Create a new basic block, linking it into the CFG.
125 BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
126 DestBB->getName() + "_crit_edge");
127 // Create our unconditional branch...
128 new BranchInst(DestBB, NewBB);
130 // Branch to the new block, breaking the edge.
131 TI->setSuccessor(SuccNum, NewBB);
133 // Insert the block into the function... right after the block TI lives in.
134 Function &F = *TIBB->getParent();
135 Function::iterator FBBI = TIBB;
136 F.getBasicBlockList().insert(++FBBI, NewBB);
138 // If there are any PHI nodes in DestBB, we need to update them so that they
139 // merge incoming values from NewBB instead of from TIBB.
141 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
142 PHINode *PN = cast<PHINode>(I);
143 // We no longer enter through TIBB, now we come in through NewBB. Revector
144 // exactly one entry in the PHI node that used to come from TIBB to come
146 int BBIdx = PN->getBasicBlockIndex(TIBB);
147 PN->setIncomingBlock(BBIdx, NewBB);
150 // If there are any other edges from TIBB to DestBB, update those to go
151 // through the split block, making those edges non-critical as well (and
152 // reducing the number of phi entries in the DestBB if relevant).
153 if (MergeIdenticalEdges) {
154 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
155 if (TI->getSuccessor(i) != DestBB) continue;
157 // Remove an entry for TIBB from DestBB phi nodes.
158 DestBB->removePredecessor(TIBB);
160 // We found another edge to DestBB, go to NewBB instead.
161 TI->setSuccessor(i, NewBB);
167 // If we don't have a pass object, we can't update anything...
168 if (P == 0) return true;
170 // Now update analysis information. Since the only predecessor of NewBB is
171 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
172 // anything, as there are other successors of DestBB. However, if all other
173 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
174 // loop header) then NewBB dominates DestBB.
175 SmallVector<BasicBlock*, 8> OtherPreds;
177 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
179 OtherPreds.push_back(*I);
181 bool NewBBDominatesDestBB = true;
183 // Should we update DominatorTree information?
184 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
185 DomTreeNode *TINode = DT->getNode(TIBB);
187 // The new block is not the immediate dominator for any other nodes, but
188 // TINode is the immediate dominator for the new node.
190 if (TINode) { // Don't break unreachable code!
191 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
192 DomTreeNode *DestBBNode = 0;
194 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
195 if (!OtherPreds.empty()) {
196 DestBBNode = DT->getNode(DestBB);
197 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
198 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
199 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
200 OtherPreds.pop_back();
205 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
206 // doesn't dominate anything.
207 if (NewBBDominatesDestBB) {
208 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
209 DT->changeImmediateDominator(DestBBNode, NewBBNode);
214 // Should we update DominanceFrontier information?
215 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
216 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
217 if (!OtherPreds.empty()) {
218 // FIXME: IMPLEMENT THIS!
219 assert(0 && "Requiring domfrontiers but not idom/domtree/domset."
220 " not implemented yet!");
223 // Since the new block is dominated by its only predecessor TIBB,
224 // it cannot be in any block's dominance frontier. If NewBB dominates
225 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
227 DominanceFrontier::DomSetType NewDFSet;
228 if (NewBBDominatesDestBB) {
229 DominanceFrontier::iterator I = DF->find(DestBB);
230 if (I != DF->end()) {
231 DF->addBasicBlock(NewBB, I->second);
232 // However NewBB's frontier does not include DestBB.
233 DominanceFrontier::iterator NF = DF->find(NewBB);
234 DF->removeFromFrontier(NF, DestBB);
237 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
239 DominanceFrontier::DomSetType NewDFSet;
240 NewDFSet.insert(DestBB);
241 DF->addBasicBlock(NewBB, NewDFSet);
245 // Update LoopInfo if it is around.
246 if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
247 // If one or the other blocks were not in a loop, the new block is not
248 // either, and thus LI doesn't need to be updated.
249 if (Loop *TIL = LI->getLoopFor(TIBB))
250 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
251 if (TIL == DestLoop) {
252 // Both in the same loop, the NewBB joins loop.
253 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
254 } else if (TIL->contains(DestLoop->getHeader())) {
255 // Edge from an outer loop to an inner loop. Add to the outer loop.
256 TIL->addBasicBlockToLoop(NewBB, LI->getBase());
257 } else if (DestLoop->contains(TIL->getHeader())) {
258 // Edge from an inner loop to an outer loop. Add to the outer loop.
259 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
261 // Edge from two loops with no containment relation. Because these
262 // are natural loops, we know that the destination block must be the
263 // header of its loop (adding a branch into a loop elsewhere would
264 // create an irreducible loop).
265 assert(DestLoop->getHeader() == DestBB &&
266 "Should not create irreducible loops!");
267 if (Loop *P = DestLoop->getParentLoop())
268 P->addBasicBlockToLoop(NewBB, LI->getBase());