1 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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/Analysis/ProfileInfo.h"
25 #include "llvm/Function.h"
26 #include "llvm/Instructions.h"
27 #include "llvm/Type.h"
28 #include "llvm/Support/CFG.h"
29 #include "llvm/Support/ErrorHandling.h"
30 #include "llvm/ADT/SmallVector.h"
31 #include "llvm/ADT/Statistic.h"
34 STATISTIC(NumBroken, "Number of blocks inserted");
37 struct BreakCriticalEdges : public FunctionPass {
38 static char ID; // Pass identification, replacement for typeid
39 BreakCriticalEdges() : FunctionPass(&ID) {}
41 virtual bool runOnFunction(Function &F);
43 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
44 AU.addPreserved<DominatorTree>();
45 AU.addPreserved<DominanceFrontier>();
46 AU.addPreserved<LoopInfo>();
47 AU.addPreserved<ProfileInfo>();
49 // No loop canonicalization guarantees are broken by this pass.
50 AU.addPreservedID(LoopSimplifyID);
55 char BreakCriticalEdges::ID = 0;
56 static RegisterPass<BreakCriticalEdges>
57 X("break-crit-edges", "Break critical edges in CFG");
59 // Publically exposed interface to pass...
60 const PassInfo *const llvm::BreakCriticalEdgesID = &X;
61 FunctionPass *llvm::createBreakCriticalEdgesPass() {
62 return new BreakCriticalEdges();
65 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
66 // edges as they are found.
68 bool BreakCriticalEdges::runOnFunction(Function &F) {
70 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
71 TerminatorInst *TI = I->getTerminator();
72 if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
73 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
74 if (SplitCriticalEdge(TI, i, this)) {
83 //===----------------------------------------------------------------------===//
84 // Implementation of the external critical edge manipulation functions
85 //===----------------------------------------------------------------------===//
87 // isCriticalEdge - Return true if the specified edge is a critical edge.
88 // Critical edges are edges from a block with multiple successors to a block
89 // with multiple predecessors.
91 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
92 bool AllowIdenticalEdges) {
93 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
94 if (TI->getNumSuccessors() == 1) return false;
96 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
97 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
99 // If there is more than one predecessor, this is a critical edge...
100 assert(I != E && "No preds, but we have an edge to the block?");
101 const BasicBlock *FirstPred = *I;
102 ++I; // Skip one edge due to the incoming arc from TI.
103 if (!AllowIdenticalEdges)
106 // If AllowIdenticalEdges is true, then we allow this edge to be considered
107 // non-critical iff all preds come from TI's block.
111 // Note: leave this as is until no one ever compiles with either gcc 4.0.1
112 // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
119 /// CreatePHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
120 /// may require new PHIs in the new exit block. This function inserts the
121 /// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
122 /// is the new loop exit block, and DestBB is the old loop exit, now the
123 /// successor of SplitBB.
124 static void CreatePHIsForSplitLoopExit(SmallVectorImpl<BasicBlock *> &Preds,
126 BasicBlock *DestBB) {
127 // SplitBB shouldn't have anything non-trivial in it yet.
128 assert(SplitBB->getFirstNonPHI() == SplitBB->getTerminator() &&
129 "SplitBB has non-PHI nodes!");
131 // For each PHI in the destination block...
132 for (BasicBlock::iterator I = DestBB->begin();
133 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
134 unsigned Idx = PN->getBasicBlockIndex(SplitBB);
135 Value *V = PN->getIncomingValue(Idx);
136 // If the input is a PHI which already satisfies LCSSA, don't create
138 if (const PHINode *VP = dyn_cast<PHINode>(V))
139 if (VP->getParent() == SplitBB)
141 // Otherwise a new PHI is needed. Create one and populate it.
142 PHINode *NewPN = PHINode::Create(PN->getType(), "split",
143 SplitBB->getTerminator());
144 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
145 NewPN->addIncoming(V, Preds[i]);
146 // Update the original PHI.
147 PN->setIncomingValue(Idx, NewPN);
151 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
152 /// split the critical edge. This will update DominatorTree and
153 /// DominatorFrontier information if it is available, thus calling this pass
154 /// will not invalidate either of them. This returns the new block if the edge
155 /// was split, null otherwise.
157 /// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the
158 /// specified successor will be merged into the same critical edge block.
159 /// This is most commonly interesting with switch instructions, which may
160 /// have many edges to any one destination. This ensures that all edges to that
161 /// dest go to one block instead of each going to a different block, but isn't
162 /// the standard definition of a "critical edge".
164 /// It is invalid to call this function on a critical edge that starts at an
165 /// IndirectBrInst. Splitting these edges will almost always create an invalid
166 /// program because the address of the new block won't be the one that is jumped
169 BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
170 Pass *P, bool MergeIdenticalEdges) {
171 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
173 assert(!isa<IndirectBrInst>(TI) &&
174 "Cannot split critical edge from IndirectBrInst");
176 BasicBlock *TIBB = TI->getParent();
177 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
179 // Create a new basic block, linking it into the CFG.
180 BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
181 TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
182 // Create our unconditional branch.
183 BranchInst::Create(DestBB, NewBB);
185 // Branch to the new block, breaking the edge.
186 TI->setSuccessor(SuccNum, NewBB);
188 // Insert the block into the function... right after the block TI lives in.
189 Function &F = *TIBB->getParent();
190 Function::iterator FBBI = TIBB;
191 F.getBasicBlockList().insert(++FBBI, NewBB);
193 // If there are any PHI nodes in DestBB, we need to update them so that they
194 // merge incoming values from NewBB instead of from TIBB.
197 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
198 // We no longer enter through TIBB, now we come in through NewBB. Revector
199 // exactly one entry in the PHI node that used to come from TIBB to come
201 PHINode *PN = cast<PHINode>(I);
203 // Reuse the previous value of BBIdx if it lines up. In cases where we have
204 // multiple phi nodes with *lots* of predecessors, this is a speed win
205 // because we don't have to scan the PHI looking for TIBB. This happens
206 // because the BB list of PHI nodes are usually in the same order.
207 if (PN->getIncomingBlock(BBIdx) != TIBB)
208 BBIdx = PN->getBasicBlockIndex(TIBB);
209 PN->setIncomingBlock(BBIdx, NewBB);
212 // If there are any other edges from TIBB to DestBB, update those to go
213 // through the split block, making those edges non-critical as well (and
214 // reducing the number of phi entries in the DestBB if relevant).
215 if (MergeIdenticalEdges) {
216 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
217 if (TI->getSuccessor(i) != DestBB) continue;
219 // Remove an entry for TIBB from DestBB phi nodes.
220 DestBB->removePredecessor(TIBB);
222 // We found another edge to DestBB, go to NewBB instead.
223 TI->setSuccessor(i, NewBB);
229 // If we don't have a pass object, we can't update anything...
230 if (P == 0) return NewBB;
232 // Now update analysis information. Since the only predecessor of NewBB is
233 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
234 // anything, as there are other successors of DestBB. However, if all other
235 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
236 // loop header) then NewBB dominates DestBB.
237 SmallVector<BasicBlock*, 8> OtherPreds;
239 // If there is a PHI in the block, loop over predecessors with it, which is
240 // faster than iterating pred_begin/end.
241 if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
242 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
243 if (PN->getIncomingBlock(i) != NewBB)
244 OtherPreds.push_back(PN->getIncomingBlock(i));
246 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB);
249 OtherPreds.push_back(*I);
252 bool NewBBDominatesDestBB = true;
254 // Should we update DominatorTree information?
255 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
256 DomTreeNode *TINode = DT->getNode(TIBB);
258 // The new block is not the immediate dominator for any other nodes, but
259 // TINode is the immediate dominator for the new node.
261 if (TINode) { // Don't break unreachable code!
262 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
263 DomTreeNode *DestBBNode = 0;
265 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
266 if (!OtherPreds.empty()) {
267 DestBBNode = DT->getNode(DestBB);
268 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
269 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
270 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
271 OtherPreds.pop_back();
276 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
277 // doesn't dominate anything.
278 if (NewBBDominatesDestBB) {
279 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
280 DT->changeImmediateDominator(DestBBNode, NewBBNode);
285 // Should we update DominanceFrontier information?
286 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
287 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
288 if (!OtherPreds.empty()) {
289 // FIXME: IMPLEMENT THIS!
290 llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset."
291 " not implemented yet!");
294 // Since the new block is dominated by its only predecessor TIBB,
295 // it cannot be in any block's dominance frontier. If NewBB dominates
296 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
298 DominanceFrontier::DomSetType NewDFSet;
299 if (NewBBDominatesDestBB) {
300 DominanceFrontier::iterator I = DF->find(DestBB);
301 if (I != DF->end()) {
302 DF->addBasicBlock(NewBB, I->second);
304 if (I->second.count(DestBB)) {
305 // However NewBB's frontier does not include DestBB.
306 DominanceFrontier::iterator NF = DF->find(NewBB);
307 DF->removeFromFrontier(NF, DestBB);
311 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
313 DominanceFrontier::DomSetType NewDFSet;
314 NewDFSet.insert(DestBB);
315 DF->addBasicBlock(NewBB, NewDFSet);
319 // Update LoopInfo if it is around.
320 if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
321 if (Loop *TIL = LI->getLoopFor(TIBB)) {
322 // If one or the other blocks were not in a loop, the new block is not
323 // either, and thus LI doesn't need to be updated.
324 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
325 if (TIL == DestLoop) {
326 // Both in the same loop, the NewBB joins loop.
327 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
328 } else if (TIL->contains(DestLoop)) {
329 // Edge from an outer loop to an inner loop. Add to the outer loop.
330 TIL->addBasicBlockToLoop(NewBB, LI->getBase());
331 } else if (DestLoop->contains(TIL)) {
332 // Edge from an inner loop to an outer loop. Add to the outer loop.
333 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
335 // Edge from two loops with no containment relation. Because these
336 // are natural loops, we know that the destination block must be the
337 // header of its loop (adding a branch into a loop elsewhere would
338 // create an irreducible loop).
339 assert(DestLoop->getHeader() == DestBB &&
340 "Should not create irreducible loops!");
341 if (Loop *P = DestLoop->getParentLoop())
342 P->addBasicBlockToLoop(NewBB, LI->getBase());
345 // If TIBB is in a loop and DestBB is outside of that loop, split the
346 // other exit blocks of the loop that also have predecessors outside
347 // the loop, to maintain a LoopSimplify guarantee.
348 if (!TIL->contains(DestBB) &&
349 P->mustPreserveAnalysisID(LoopSimplifyID)) {
350 assert(!TIL->contains(NewBB) &&
351 "Split point for loop exit is contained in loop!");
353 // Update LCSSA form in the newly created exit block.
354 if (P->mustPreserveAnalysisID(LCSSAID)) {
355 SmallVector<BasicBlock *, 1> OrigPred;
356 OrigPred.push_back(TIBB);
357 CreatePHIsForSplitLoopExit(OrigPred, NewBB, DestBB);
360 // For each unique exit block...
361 SmallVector<BasicBlock *, 4> ExitBlocks;
362 TIL->getExitBlocks(ExitBlocks);
363 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
364 // Collect all the preds that are inside the loop, and note
365 // whether there are any preds outside the loop.
366 SmallVector<BasicBlock *, 4> Preds;
367 bool HasPredOutsideOfLoop = false;
368 BasicBlock *Exit = ExitBlocks[i];
369 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
371 if (TIL->contains(*I))
374 HasPredOutsideOfLoop = true;
375 // If there are any preds not in the loop, we'll need to split
376 // the edges. The Preds.empty() check is needed because a block
377 // may appear multiple times in the list. We can't use
378 // getUniqueExitBlocks above because that depends on LoopSimplify
379 // form, which we're in the process of restoring!
380 if (!Preds.empty() && HasPredOutsideOfLoop) {
381 BasicBlock *NewExitBB =
382 SplitBlockPredecessors(Exit, Preds.data(), Preds.size(),
384 if (P->mustPreserveAnalysisID(LCSSAID))
385 CreatePHIsForSplitLoopExit(Preds, NewExitBB, Exit);
389 // LCSSA form was updated above for the case where LoopSimplify is
390 // available, which means that all predecessors of loop exit blocks
391 // are within the loop. Without LoopSimplify form, it would be
392 // necessary to insert a new phi.
393 assert((!P->mustPreserveAnalysisID(LCSSAID) ||
394 P->mustPreserveAnalysisID(LoopSimplifyID)) &&
395 "SplitCriticalEdge doesn't know how to update LCCSA form "
396 "without LoopSimplify!");
400 // Update ProfileInfo if it is around.
401 if (ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>())
402 PI->splitEdge(TIBB, DestBB, NewBB, MergeIdenticalEdges);