1//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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)
15// information.
16//
17//===----------------------------------------------------------------------===//
18
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"
32using namespace llvm;
33
34STATISTIC(NumBroken, "Number of blocks inserted");
35
36namespace {
37 struct BreakCriticalEdges : public FunctionPass {
38 static char ID; // Pass identification, replacement for typeid
39 BreakCriticalEdges() : FunctionPass(&ID) {}
40
41 virtual bool runOnFunction(Function &F);
42
43 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
44 AU.addPreserved<DominatorTree>();
45 AU.addPreserved<DominanceFrontier>();
46 AU.addPreserved<LoopInfo>();
47 AU.addPreserved<ProfileInfo>();
48
49 // No loop canonicalization guarantees are broken by this pass.
50 AU.addPreservedID(LoopSimplifyID);
51 }
52 };
53}
54
55char BreakCriticalEdges::ID = 0;
56static RegisterPass<BreakCriticalEdges>
57X("break-crit-edges", "Break critical edges in CFG");
58
59// Publically exposed interface to pass...
60const PassInfo *const llvm::BreakCriticalEdgesID = &X;
61FunctionPass *llvm::createBreakCriticalEdgesPass() {
62 return new BreakCriticalEdges();
63}
64
65// runOnFunction - Loop over all of the edges in the CFG, breaking critical
66// edges as they are found.
67//
68bool BreakCriticalEdges::runOnFunction(Function &F) {
69 bool Changed = false;
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)) {
75 ++NumBroken;
76 Changed = true;
77 }
78 }
79
80 return Changed;
81}
82
83//===----------------------------------------------------------------------===//
84// Implementation of the external critical edge manipulation functions
85//===----------------------------------------------------------------------===//
86
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.
90//
91bool 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;
95
96 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
97 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
98
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)
104 return I != E;
105
106 // If AllowIdenticalEdges is true, then we allow this edge to be considered
107 // non-critical iff all preds come from TI's block.
108 while (I != E) {
109 if (*I != FirstPred)
110 return true;
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
113 E = pred_end(*I);
114 ++I;
115 }
116 return false;
117}
118
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.
124static void CreatePHIsForSplitLoopExit(SmallVectorImpl<BasicBlock *> &Preds,
125 BasicBlock *SplitBB,
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!");
130
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
137 // a new one.
138 if (const PHINode *VP = dyn_cast<PHINode>(V))
139 if (VP->getParent() == SplitBB)
140 continue;
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);
148 }
149}
150
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.
156///
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".
163///
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
167/// to.
168///
169BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
170 Pass *P, bool MergeIdenticalEdges) {
171 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
172
173 assert(!isa<IndirectBrInst>(TI) &&
174 "Cannot split critical edge from IndirectBrInst");
175
176 BasicBlock *TIBB = TI->getParent();
177 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
178
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);
184
185 // Branch to the new block, breaking the edge.
186 TI->setSuccessor(SuccNum, NewBB);
187
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);
192
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.
195 //
196 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
197 PHINode *PN = cast<PHINode>(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
200 // from NewBB.
201 int BBIdx = PN->getBasicBlockIndex(TIBB);
202 PN->setIncomingBlock(BBIdx, NewBB);
203 }
204
205 // If there are any other edges from TIBB to DestBB, update those to go
206 // through the split block, making those edges non-critical as well (and
207 // reducing the number of phi entries in the DestBB if relevant).
208 if (MergeIdenticalEdges) {
209 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
210 if (TI->getSuccessor(i) != DestBB) continue;
211
212 // Remove an entry for TIBB from DestBB phi nodes.
213 DestBB->removePredecessor(TIBB);
214
215 // We found another edge to DestBB, go to NewBB instead.
216 TI->setSuccessor(i, NewBB);
217 }
218 }
219
220
221
222 // If we don't have a pass object, we can't update anything...
223 if (P == 0) return NewBB;
224
225 // Now update analysis information. Since the only predecessor of NewBB is
226 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
227 // anything, as there are other successors of DestBB. However, if all other
228 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
229 // loop header) then NewBB dominates DestBB.
230 SmallVector<BasicBlock*, 8> OtherPreds;
231
232 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
233 if (*I != NewBB)
234 OtherPreds.push_back(*I);
235
236 bool NewBBDominatesDestBB = true;
237
238 // Should we update DominatorTree information?
239 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
240 DomTreeNode *TINode = DT->getNode(TIBB);
241
242 // The new block is not the immediate dominator for any other nodes, but
243 // TINode is the immediate dominator for the new node.
244 //
245 if (TINode) { // Don't break unreachable code!
246 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
247 DomTreeNode *DestBBNode = 0;
248
249 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
250 if (!OtherPreds.empty()) {
251 DestBBNode = DT->getNode(DestBB);
252 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
253 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
254 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
255 OtherPreds.pop_back();
256 }
257 OtherPreds.clear();
258 }
259
260 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
261 // doesn't dominate anything.
262 if (NewBBDominatesDestBB) {
263 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
264 DT->changeImmediateDominator(DestBBNode, NewBBNode);
265 }
266 }
267 }
268
269 // Should we update DominanceFrontier information?
270 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
271 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
272 if (!OtherPreds.empty()) {
273 // FIXME: IMPLEMENT THIS!
274 llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset."
275 " not implemented yet!");
276 }
277
278 // Since the new block is dominated by its only predecessor TIBB,
279 // it cannot be in any block's dominance frontier. If NewBB dominates
280 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
281 // just {DestBB}.
282 DominanceFrontier::DomSetType NewDFSet;
283 if (NewBBDominatesDestBB) {
284 DominanceFrontier::iterator I = DF->find(DestBB);
285 if (I != DF->end()) {
286 DF->addBasicBlock(NewBB, I->second);
287
288 if (I->second.count(DestBB)) {
289 // However NewBB's frontier does not include DestBB.
290 DominanceFrontier::iterator NF = DF->find(NewBB);
291 DF->removeFromFrontier(NF, DestBB);
292 }
293 }
294 else
295 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
296 } else {
297 DominanceFrontier::DomSetType NewDFSet;
298 NewDFSet.insert(DestBB);
299 DF->addBasicBlock(NewBB, NewDFSet);
300 }
301 }
302
303 // Update LoopInfo if it is around.
304 if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
305 if (Loop *TIL = LI->getLoopFor(TIBB)) {
306 // If one or the other blocks were not in a loop, the new block is not
307 // either, and thus LI doesn't need to be updated.
308 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
309 if (TIL == DestLoop) {
310 // Both in the same loop, the NewBB joins loop.
311 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
312 } else if (TIL->contains(DestLoop)) {
313 // Edge from an outer loop to an inner loop. Add to the outer loop.
314 TIL->addBasicBlockToLoop(NewBB, LI->getBase());
315 } else if (DestLoop->contains(TIL)) {
316 // Edge from an inner loop to an outer loop. Add to the outer loop.
317 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
318 } else {
319 // Edge from two loops with no containment relation. Because these
320 // are natural loops, we know that the destination block must be the
321 // header of its loop (adding a branch into a loop elsewhere would
322 // create an irreducible loop).
323 assert(DestLoop->getHeader() == DestBB &&
324 "Should not create irreducible loops!");
325 if (Loop *P = DestLoop->getParentLoop())
326 P->addBasicBlockToLoop(NewBB, LI->getBase());
327 }
328 }
329 // If TIBB is in a loop and DestBB is outside of that loop, split the
330 // other exit blocks of the loop that also have predecessors outside
331 // the loop, to maintain a LoopSimplify guarantee.
332 if (!TIL->contains(DestBB) &&
333 P->mustPreserveAnalysisID(LoopSimplifyID)) {
334 assert(!TIL->contains(NewBB) &&
335 "Split point for loop exit is contained in loop!");
336
337 // Update LCSSA form in the newly created exit block.
338 if (P->mustPreserveAnalysisID(LCSSAID)) {
339 SmallVector<BasicBlock *, 1> OrigPred;
340 OrigPred.push_back(TIBB);
341 CreatePHIsForSplitLoopExit(OrigPred, NewBB, DestBB);
342 }
343
344 // For each unique exit block...
345 SmallVector<BasicBlock *, 4> ExitBlocks;
346 TIL->getExitBlocks(ExitBlocks);
347 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
348 // Collect all the preds that are inside the loop, and note
349 // whether there are any preds outside the loop.
350 SmallVector<BasicBlock *, 4> Preds;
351 bool HasPredOutsideOfLoop = false;
352 BasicBlock *Exit = ExitBlocks[i];
353 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
354 I != E; ++I)
355 if (TIL->contains(*I))
356 Preds.push_back(*I);
357 else
358 HasPredOutsideOfLoop = true;
359 // If there are any preds not in the loop, we'll need to split
360 // the edges. The Preds.empty() check is needed because a block
361 // may appear multiple times in the list. We can't use
362 // getUniqueExitBlocks above because that depends on LoopSimplify
363 // form, which we're in the process of restoring!
364 if (!Preds.empty() && HasPredOutsideOfLoop) {
365 BasicBlock *NewExitBB =
366 SplitBlockPredecessors(Exit, Preds.data(), Preds.size(),
367 "split", P);
368 if (P->mustPreserveAnalysisID(LCSSAID))
369 CreatePHIsForSplitLoopExit(Preds, NewExitBB, Exit);
370 }
371 }
372 }
373 // LCSSA form was updated above for the case where LoopSimplify is
374 // available, which means that all predecessors of loop exit blocks
375 // are within the loop. Without LoopSimplify form, it would be
376 // necessary to insert a new phi.
377 assert((!P->mustPreserveAnalysisID(LCSSAID) ||
378 P->mustPreserveAnalysisID(LoopSimplifyID)) &&
379 "SplitCriticalEdge doesn't know how to update LCCSA form "
380 "without LoopSimplify!");
381 }
382 }
383
384 // Update ProfileInfo if it is around.
385 if (ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>()) {
386 PI->splitEdge(TIBB,DestBB,NewBB,MergeIdenticalEdges);
387 }
388
389 return NewBB;
390}