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
16 //===----------------------------------------------------------------------===//
18 #define DEBUG_TYPE "break-crit-edges"
19 #include "llvm/Transforms/Scalar.h"
20 #include "llvm/ADT/SmallVector.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/Analysis/CFG.h"
23 #include "llvm/Analysis/LoopInfo.h"
24 #include "llvm/IR/CFG.h"
25 #include "llvm/IR/Dominators.h"
26 #include "llvm/IR/Function.h"
27 #include "llvm/IR/Instructions.h"
28 #include "llvm/IR/Type.h"
29 #include "llvm/Support/ErrorHandling.h"
30 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
33 STATISTIC(NumBroken, "Number of blocks inserted");
36 struct BreakCriticalEdges : public FunctionPass {
37 static char ID; // Pass identification, replacement for typeid
38 BreakCriticalEdges() : FunctionPass(ID) {
39 initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
42 bool runOnFunction(Function &F) override;
44 void getAnalysisUsage(AnalysisUsage &AU) const override {
45 AU.addPreserved<DominatorTreeWrapperPass>();
46 AU.addPreserved<LoopInfo>();
48 // No loop canonicalization guarantees are broken by this pass.
49 AU.addPreservedID(LoopSimplifyID);
54 char BreakCriticalEdges::ID = 0;
55 INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
56 "Break critical edges in CFG", false, false)
58 // Publicly exposed interface to pass...
59 char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
60 FunctionPass *llvm::createBreakCriticalEdgesPass() {
61 return new BreakCriticalEdges();
64 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
65 // edges as they are found.
67 bool BreakCriticalEdges::runOnFunction(Function &F) {
69 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
70 TerminatorInst *TI = I->getTerminator();
71 if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
72 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
73 if (SplitCriticalEdge(TI, i, this)) {
82 //===----------------------------------------------------------------------===//
83 // Implementation of the external critical edge manipulation functions
84 //===----------------------------------------------------------------------===//
86 /// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
87 /// may require new PHIs in the new exit block. This function inserts the
88 /// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
89 /// is the new loop exit block, and DestBB is the old loop exit, now the
90 /// successor of SplitBB.
91 static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
94 // SplitBB shouldn't have anything non-trivial in it yet.
95 assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() ||
96 SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");
98 // For each PHI in the destination block.
99 for (BasicBlock::iterator I = DestBB->begin();
100 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
101 unsigned Idx = PN->getBasicBlockIndex(SplitBB);
102 Value *V = PN->getIncomingValue(Idx);
104 // If the input is a PHI which already satisfies LCSSA, don't create
106 if (const PHINode *VP = dyn_cast<PHINode>(V))
107 if (VP->getParent() == SplitBB)
110 // Otherwise a new PHI is needed. Create one and populate it.
112 PHINode::Create(PN->getType(), Preds.size(), "split",
113 SplitBB->isLandingPad() ?
114 SplitBB->begin() : SplitBB->getTerminator());
115 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
116 NewPN->addIncoming(V, Preds[i]);
118 // Update the original PHI.
119 PN->setIncomingValue(Idx, NewPN);
123 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
124 /// split the critical edge. This will update DominatorTree information if it
125 /// is available, thus calling this pass will not invalidate either of them.
126 /// This returns the new block if the edge was split, null otherwise.
128 /// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the
129 /// specified successor will be merged into the same critical edge block.
130 /// This is most commonly interesting with switch instructions, which may
131 /// have many edges to any one destination. This ensures that all edges to that
132 /// dest go to one block instead of each going to a different block, but isn't
133 /// the standard definition of a "critical edge".
135 /// It is invalid to call this function on a critical edge that starts at an
136 /// IndirectBrInst. Splitting these edges will almost always create an invalid
137 /// program because the address of the new block won't be the one that is jumped
140 BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
141 Pass *P, bool MergeIdenticalEdges,
142 bool DontDeleteUselessPhis,
143 bool SplitLandingPads) {
144 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
146 assert(!isa<IndirectBrInst>(TI) &&
147 "Cannot split critical edge from IndirectBrInst");
149 BasicBlock *TIBB = TI->getParent();
150 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
152 // Splitting the critical edge to a landing pad block is non-trivial. Don't do
153 // it in this generic function.
154 if (DestBB->isLandingPad()) return 0;
156 // Create a new basic block, linking it into the CFG.
157 BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
158 TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
159 // Create our unconditional branch.
160 BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
161 NewBI->setDebugLoc(TI->getDebugLoc());
163 // Branch to the new block, breaking the edge.
164 TI->setSuccessor(SuccNum, NewBB);
166 // Insert the block into the function... right after the block TI lives in.
167 Function &F = *TIBB->getParent();
168 Function::iterator FBBI = TIBB;
169 F.getBasicBlockList().insert(++FBBI, NewBB);
171 // If there are any PHI nodes in DestBB, we need to update them so that they
172 // merge incoming values from NewBB instead of from TIBB.
175 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
176 // We no longer enter through TIBB, now we come in through NewBB.
177 // Revector exactly one entry in the PHI node that used to come from
178 // TIBB to come from NewBB.
179 PHINode *PN = cast<PHINode>(I);
181 // Reuse the previous value of BBIdx if it lines up. In cases where we
182 // have multiple phi nodes with *lots* of predecessors, this is a speed
183 // win because we don't have to scan the PHI looking for TIBB. This
184 // happens because the BB list of PHI nodes are usually in the same
186 if (PN->getIncomingBlock(BBIdx) != TIBB)
187 BBIdx = PN->getBasicBlockIndex(TIBB);
188 PN->setIncomingBlock(BBIdx, NewBB);
192 // If there are any other edges from TIBB to DestBB, update those to go
193 // through the split block, making those edges non-critical as well (and
194 // reducing the number of phi entries in the DestBB if relevant).
195 if (MergeIdenticalEdges) {
196 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
197 if (TI->getSuccessor(i) != DestBB) continue;
199 // Remove an entry for TIBB from DestBB phi nodes.
200 DestBB->removePredecessor(TIBB, DontDeleteUselessPhis);
202 // We found another edge to DestBB, go to NewBB instead.
203 TI->setSuccessor(i, NewBB);
209 // If we don't have a pass object, we can't update anything...
210 if (P == 0) return NewBB;
212 DominatorTreeWrapperPass *DTWP =
213 P->getAnalysisIfAvailable<DominatorTreeWrapperPass>();
214 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : 0;
215 LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
217 // If we have nothing to update, just return.
218 if (DT == 0 && LI == 0)
221 // Now update analysis information. Since the only predecessor of NewBB is
222 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
223 // anything, as there are other successors of DestBB. However, if all other
224 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
225 // loop header) then NewBB dominates DestBB.
226 SmallVector<BasicBlock*, 8> OtherPreds;
228 // If there is a PHI in the block, loop over predecessors with it, which is
229 // faster than iterating pred_begin/end.
230 if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
231 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
232 if (PN->getIncomingBlock(i) != NewBB)
233 OtherPreds.push_back(PN->getIncomingBlock(i));
235 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB);
239 OtherPreds.push_back(P);
243 bool NewBBDominatesDestBB = true;
245 // Should we update DominatorTree information?
247 DomTreeNode *TINode = DT->getNode(TIBB);
249 // The new block is not the immediate dominator for any other nodes, but
250 // TINode is the immediate dominator for the new node.
252 if (TINode) { // Don't break unreachable code!
253 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
254 DomTreeNode *DestBBNode = 0;
256 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
257 if (!OtherPreds.empty()) {
258 DestBBNode = DT->getNode(DestBB);
259 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
260 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
261 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
262 OtherPreds.pop_back();
267 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
268 // doesn't dominate anything.
269 if (NewBBDominatesDestBB) {
270 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
271 DT->changeImmediateDominator(DestBBNode, NewBBNode);
276 // Update LoopInfo if it is around.
278 if (Loop *TIL = LI->getLoopFor(TIBB)) {
279 // If one or the other blocks were not in a loop, the new block is not
280 // either, and thus LI doesn't need to be updated.
281 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
282 if (TIL == DestLoop) {
283 // Both in the same loop, the NewBB joins loop.
284 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
285 } else if (TIL->contains(DestLoop)) {
286 // Edge from an outer loop to an inner loop. Add to the outer loop.
287 TIL->addBasicBlockToLoop(NewBB, LI->getBase());
288 } else if (DestLoop->contains(TIL)) {
289 // Edge from an inner loop to an outer loop. Add to the outer loop.
290 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
292 // Edge from two loops with no containment relation. Because these
293 // are natural loops, we know that the destination block must be the
294 // header of its loop (adding a branch into a loop elsewhere would
295 // create an irreducible loop).
296 assert(DestLoop->getHeader() == DestBB &&
297 "Should not create irreducible loops!");
298 if (Loop *P = DestLoop->getParentLoop())
299 P->addBasicBlockToLoop(NewBB, LI->getBase());
302 // If TIBB is in a loop and DestBB is outside of that loop, we may need
303 // to update LoopSimplify form and LCSSA form.
304 if (!TIL->contains(DestBB) &&
305 P->mustPreserveAnalysisID(LoopSimplifyID)) {
306 assert(!TIL->contains(NewBB) &&
307 "Split point for loop exit is contained in loop!");
309 // Update LCSSA form in the newly created exit block.
310 if (P->mustPreserveAnalysisID(LCSSAID))
311 createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);
313 // The only that we can break LoopSimplify form by splitting a critical
314 // edge is if after the split there exists some edge from TIL to DestBB
315 // *and* the only edge into DestBB from outside of TIL is that of
316 // NewBB. If the first isn't true, then LoopSimplify still holds, NewBB
317 // is the new exit block and it has no non-loop predecessors. If the
318 // second isn't true, then DestBB was not in LoopSimplify form prior to
319 // the split as it had a non-loop predecessor. In both of these cases,
320 // the predecessor must be directly in TIL, not in a subloop, or again
321 // LoopSimplify doesn't hold.
322 SmallVector<BasicBlock *, 4> LoopPreds;
323 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E;
327 continue; // The new block is known.
328 if (LI->getLoopFor(P) != TIL) {
329 // No need to re-simplify, it wasn't to start with.
333 LoopPreds.push_back(P);
335 if (!LoopPreds.empty()) {
336 assert(!DestBB->isLandingPad() &&
337 "We don't split edges to landing pads!");
338 BasicBlock *NewExitBB =
339 SplitBlockPredecessors(DestBB, LoopPreds, "split", P);
340 if (P->mustPreserveAnalysisID(LCSSAID))
341 createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB);
344 // LCSSA form was updated above for the case where LoopSimplify is
345 // available, which means that all predecessors of loop exit blocks
346 // are within the loop. Without LoopSimplify form, it would be
347 // necessary to insert a new phi.
348 assert((!P->mustPreserveAnalysisID(LCSSAID) ||
349 P->mustPreserveAnalysisID(LoopSimplifyID)) &&
350 "SplitCriticalEdge doesn't know how to update LCCSA form "
351 "without LoopSimplify!");