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/Compiler.h"
30 #include "llvm/Support/ErrorHandling.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/ADT/Statistic.h"
35 STATISTIC(NumBroken, "Number of blocks inserted");
38 struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
39 static char ID; // Pass identification, replacement for typeid
40 BreakCriticalEdges() : FunctionPass(&ID) {}
42 virtual bool runOnFunction(Function &F);
44 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
45 AU.addPreserved<DominatorTree>();
46 AU.addPreserved<DominanceFrontier>();
47 AU.addPreserved<LoopInfo>();
48 AU.addPreserved<ProfileInfo>();
50 // No loop canonicalization guarantees are broken by this pass.
51 AU.addPreservedID(LoopSimplifyID);
56 char BreakCriticalEdges::ID = 0;
57 static RegisterPass<BreakCriticalEdges>
58 X("break-crit-edges", "Break critical edges in CFG");
60 // Publically exposed interface to pass...
61 const PassInfo *const llvm::BreakCriticalEdgesID = &X;
62 FunctionPass *llvm::createBreakCriticalEdgesPass() {
63 return new BreakCriticalEdges();
66 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
67 // edges as they are found.
69 bool BreakCriticalEdges::runOnFunction(Function &F) {
71 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
72 TerminatorInst *TI = I->getTerminator();
73 if (TI->getNumSuccessors() > 1)
74 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
75 if (SplitCriticalEdge(TI, i, this)) {
84 //===----------------------------------------------------------------------===//
85 // Implementation of the external critical edge manipulation functions
86 //===----------------------------------------------------------------------===//
88 // isCriticalEdge - Return true if the specified edge is a critical edge.
89 // Critical edges are edges from a block with multiple successors to a block
90 // with multiple predecessors.
92 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
93 bool AllowIdenticalEdges) {
94 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
95 if (TI->getNumSuccessors() == 1) return false;
97 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
98 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
100 // If there is more than one predecessor, this is a critical edge...
101 assert(I != E && "No preds, but we have an edge to the block?");
102 const BasicBlock *FirstPred = *I;
103 ++I; // Skip one edge due to the incoming arc from TI.
104 if (!AllowIdenticalEdges)
107 // If AllowIdenticalEdges is true, then we allow this edge to be considered
108 // non-critical iff all preds come from TI's block.
112 // Note: leave this as is until no one ever compiles with either gcc 4.0.1
113 // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
120 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
121 /// split the critical edge. This will update DominatorTree and
122 /// DominatorFrontier information if it is available, thus calling this pass
123 /// will not invalidate any of them. This returns true if the edge was split,
124 /// false otherwise. This ensures that all edges to that dest go to one block
125 /// instead of each going to a different block.
127 BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
128 Pass *P, bool MergeIdenticalEdges) {
129 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
130 BasicBlock *TIBB = TI->getParent();
131 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
133 // Create a new basic block, linking it into the CFG.
134 BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
135 TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
136 // Create our unconditional branch...
137 BranchInst::Create(DestBB, NewBB);
139 // Branch to the new block, breaking the edge.
140 TI->setSuccessor(SuccNum, NewBB);
142 // Insert the block into the function... right after the block TI lives in.
143 Function &F = *TIBB->getParent();
144 Function::iterator FBBI = TIBB;
145 F.getBasicBlockList().insert(++FBBI, NewBB);
147 // If there are any PHI nodes in DestBB, we need to update them so that they
148 // merge incoming values from NewBB instead of from TIBB.
150 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
151 PHINode *PN = cast<PHINode>(I);
152 // We no longer enter through TIBB, now we come in through NewBB. Revector
153 // exactly one entry in the PHI node that used to come from TIBB to come
155 int BBIdx = PN->getBasicBlockIndex(TIBB);
156 PN->setIncomingBlock(BBIdx, NewBB);
159 // If there are any other edges from TIBB to DestBB, update those to go
160 // through the split block, making those edges non-critical as well (and
161 // reducing the number of phi entries in the DestBB if relevant).
162 if (MergeIdenticalEdges) {
163 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
164 if (TI->getSuccessor(i) != DestBB) continue;
166 // Remove an entry for TIBB from DestBB phi nodes.
167 DestBB->removePredecessor(TIBB);
169 // We found another edge to DestBB, go to NewBB instead.
170 TI->setSuccessor(i, NewBB);
176 // If we don't have a pass object, we can't update anything...
177 if (P == 0) return NewBB;
179 // Now update analysis information. Since the only predecessor of NewBB is
180 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
181 // anything, as there are other successors of DestBB. However, if all other
182 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
183 // loop header) then NewBB dominates DestBB.
184 SmallVector<BasicBlock*, 8> OtherPreds;
186 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
188 OtherPreds.push_back(*I);
190 bool NewBBDominatesDestBB = true;
192 // Should we update DominatorTree information?
193 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
194 DomTreeNode *TINode = DT->getNode(TIBB);
196 // The new block is not the immediate dominator for any other nodes, but
197 // TINode is the immediate dominator for the new node.
199 if (TINode) { // Don't break unreachable code!
200 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
201 DomTreeNode *DestBBNode = 0;
203 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
204 if (!OtherPreds.empty()) {
205 DestBBNode = DT->getNode(DestBB);
206 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
207 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
208 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
209 OtherPreds.pop_back();
214 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
215 // doesn't dominate anything.
216 if (NewBBDominatesDestBB) {
217 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
218 DT->changeImmediateDominator(DestBBNode, NewBBNode);
223 // Should we update DominanceFrontier information?
224 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
225 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
226 if (!OtherPreds.empty()) {
227 // FIXME: IMPLEMENT THIS!
228 llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset."
229 " not implemented yet!");
232 // Since the new block is dominated by its only predecessor TIBB,
233 // it cannot be in any block's dominance frontier. If NewBB dominates
234 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
236 DominanceFrontier::DomSetType NewDFSet;
237 if (NewBBDominatesDestBB) {
238 DominanceFrontier::iterator I = DF->find(DestBB);
239 if (I != DF->end()) {
240 DF->addBasicBlock(NewBB, I->second);
242 if (I->second.count(DestBB)) {
243 // However NewBB's frontier does not include DestBB.
244 DominanceFrontier::iterator NF = DF->find(NewBB);
245 DF->removeFromFrontier(NF, DestBB);
249 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
251 DominanceFrontier::DomSetType NewDFSet;
252 NewDFSet.insert(DestBB);
253 DF->addBasicBlock(NewBB, NewDFSet);
257 // Update LoopInfo if it is around.
258 if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
259 if (Loop *TIL = LI->getLoopFor(TIBB)) {
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 *DestLoop = LI->getLoopFor(DestBB)) {
263 if (TIL == DestLoop) {
264 // Both in the same loop, the NewBB joins loop.
265 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
266 } else if (TIL->contains(DestLoop->getHeader())) {
267 // Edge from an outer loop to an inner loop. Add to the outer loop.
268 TIL->addBasicBlockToLoop(NewBB, LI->getBase());
269 } else if (DestLoop->contains(TIL->getHeader())) {
270 // Edge from an inner loop to an outer loop. Add to the outer loop.
271 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
273 // Edge from two loops with no containment relation. Because these
274 // are natural loops, we know that the destination block must be the
275 // header of its loop (adding a branch into a loop elsewhere would
276 // create an irreducible loop).
277 assert(DestLoop->getHeader() == DestBB &&
278 "Should not create irreducible loops!");
279 if (Loop *P = DestLoop->getParentLoop())
280 P->addBasicBlockToLoop(NewBB, LI->getBase());
283 // If TIBB is in a loop and DestBB is outside of that loop, split the
284 // other exit blocks of the loop that also have predecessors outside
285 // the loop, to maintain a LoopSimplify guarantee.
286 if (!TIL->contains(DestBB) &&
287 P->mustPreserveAnalysisID(LoopSimplifyID)) {
288 // For each unique exit block...
289 SmallVector<BasicBlock *, 4> ExitBlocks;
290 TIL->getExitBlocks(ExitBlocks);
291 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
292 // Collect all the preds that are inside the loop, and note
293 // whether there are any preds outside the loop.
294 SmallVector<BasicBlock *, 4> Preds;
295 bool AllPredsInLoop = false;
296 BasicBlock *Exit = ExitBlocks[i];
297 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
299 if (TIL->contains(*I))
302 AllPredsInLoop = true;
303 // If there are any preds not in the loop, we'll need to split
304 // the edges. The Preds.empty() check is needed because a block
305 // may appear multiple times in the list. We can't use
306 // getUniqueExitBlocks above because that depends on LoopSimplify
307 // form, which we're in the process of restoring!
308 if (Preds.empty() || !AllPredsInLoop) continue;
309 BasicBlock *NewBB = SplitBlockPredecessors(Exit,
310 Preds.data(), Preds.size(),
312 // Update LCSSA form by creating new PHIs in the new exit blocks
314 if (P->mustPreserveAnalysisID(LCSSAID))
315 for (BasicBlock::iterator I = Exit->begin();
316 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
317 unsigned Idx = PN->getBasicBlockIndex(NewBB);
318 Value *V = PN->getIncomingValue(Idx);
319 // If the PHI is already suitable, don't create a new one.
320 if (PHINode *VP = dyn_cast<PHINode>(V))
321 if (VP->getParent() == NewBB)
323 // A new PHI is needed. Create one and populate it.
325 PHINode::Create(PN->getType(), "split", NewBB->getTerminator());
326 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
327 NewPN->addIncoming(V, Preds[i]);
328 PN->setIncomingValue(Idx, NewPN);
332 // LCSSA form was updated above for the case where LoopSimplify is
333 // available, which means that all predecessors of loop exit blocks
334 // are within the loop. Without LoopSimplify form, it would be
335 // necessary to insert a new phi.
336 assert((!P->mustPreserveAnalysisID(LCSSAID) ||
337 P->mustPreserveAnalysisID(LoopSimplifyID)) &&
338 "SplitCriticalEdge doesn't know how to update LCCSA form "
339 "without LoopSimplify!");
343 // Update ProfileInfo if it is around.
344 if (ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>()) {
345 PI->splitEdge(TIBB,DestBB,NewBB,MergeIdenticalEdges);