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 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Analysis/CFG.h"
22 #include "llvm/Analysis/LoopInfo.h"
23 #include "llvm/IR/CFG.h"
24 #include "llvm/IR/Dominators.h"
25 #include "llvm/IR/Function.h"
26 #include "llvm/IR/Instructions.h"
27 #include "llvm/IR/Type.h"
28 #include "llvm/Support/ErrorHandling.h"
29 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
32 #define DEBUG_TYPE "break-crit-edges"
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) {
40 initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
43 bool runOnFunction(Function &F) override {
44 unsigned N = SplitAllCriticalEdges(F, this);
49 void getAnalysisUsage(AnalysisUsage &AU) const override {
50 AU.addPreserved<DominatorTreeWrapperPass>();
51 AU.addPreserved<LoopInfo>();
53 // No loop canonicalization guarantees are broken by this pass.
54 AU.addPreservedID(LoopSimplifyID);
59 char BreakCriticalEdges::ID = 0;
60 INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
61 "Break critical edges in CFG", false, false)
63 // Publicly exposed interface to pass...
64 char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
65 FunctionPass *llvm::createBreakCriticalEdgesPass() {
66 return new BreakCriticalEdges();
69 //===----------------------------------------------------------------------===//
70 // Implementation of the external critical edge manipulation functions
71 //===----------------------------------------------------------------------===//
73 /// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
74 /// may require new PHIs in the new exit block. This function inserts the
75 /// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
76 /// is the new loop exit block, and DestBB is the old loop exit, now the
77 /// successor of SplitBB.
78 static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
81 // SplitBB shouldn't have anything non-trivial in it yet.
82 assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() ||
83 SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");
85 // For each PHI in the destination block.
86 for (BasicBlock::iterator I = DestBB->begin();
87 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
88 unsigned Idx = PN->getBasicBlockIndex(SplitBB);
89 Value *V = PN->getIncomingValue(Idx);
91 // If the input is a PHI which already satisfies LCSSA, don't create
93 if (const PHINode *VP = dyn_cast<PHINode>(V))
94 if (VP->getParent() == SplitBB)
97 // Otherwise a new PHI is needed. Create one and populate it.
99 PHINode::Create(PN->getType(), Preds.size(), "split",
100 SplitBB->isLandingPad() ?
101 SplitBB->begin() : SplitBB->getTerminator());
102 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
103 NewPN->addIncoming(V, Preds[i]);
105 // Update the original PHI.
106 PN->setIncomingValue(Idx, NewPN);
110 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
111 /// split the critical edge. This will update DominatorTree information if it
112 /// is available, thus calling this pass will not invalidate either of them.
113 /// This returns the new block if the edge was split, null otherwise.
115 /// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the
116 /// specified successor will be merged into the same critical edge block.
117 /// This is most commonly interesting with switch instructions, which may
118 /// have many edges to any one destination. This ensures that all edges to that
119 /// dest go to one block instead of each going to a different block, but isn't
120 /// the standard definition of a "critical edge".
122 /// It is invalid to call this function on a critical edge that starts at an
123 /// IndirectBrInst. Splitting these edges will almost always create an invalid
124 /// program because the address of the new block won't be the one that is jumped
127 BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
128 Pass *P, bool MergeIdenticalEdges,
129 bool DontDeleteUselessPhis,
130 bool SplitLandingPads) {
131 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return nullptr;
133 assert(!isa<IndirectBrInst>(TI) &&
134 "Cannot split critical edge from IndirectBrInst");
136 BasicBlock *TIBB = TI->getParent();
137 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
139 // Splitting the critical edge to a landing pad block is non-trivial. Don't do
140 // it in this generic function.
141 if (DestBB->isLandingPad()) return nullptr;
143 // Create a new basic block, linking it into the CFG.
144 BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
145 TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
146 // Create our unconditional branch.
147 BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
148 NewBI->setDebugLoc(TI->getDebugLoc());
150 // Branch to the new block, breaking the edge.
151 TI->setSuccessor(SuccNum, NewBB);
153 // Insert the block into the function... right after the block TI lives in.
154 Function &F = *TIBB->getParent();
155 Function::iterator FBBI = TIBB;
156 F.getBasicBlockList().insert(++FBBI, NewBB);
158 // If there are any PHI nodes in DestBB, we need to update them so that they
159 // merge incoming values from NewBB instead of from TIBB.
162 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
163 // We no longer enter through TIBB, now we come in through NewBB.
164 // Revector exactly one entry in the PHI node that used to come from
165 // TIBB to come from NewBB.
166 PHINode *PN = cast<PHINode>(I);
168 // Reuse the previous value of BBIdx if it lines up. In cases where we
169 // have multiple phi nodes with *lots* of predecessors, this is a speed
170 // win because we don't have to scan the PHI looking for TIBB. This
171 // happens because the BB list of PHI nodes are usually in the same
173 if (PN->getIncomingBlock(BBIdx) != TIBB)
174 BBIdx = PN->getBasicBlockIndex(TIBB);
175 PN->setIncomingBlock(BBIdx, NewBB);
179 // If there are any other edges from TIBB to DestBB, update those to go
180 // through the split block, making those edges non-critical as well (and
181 // reducing the number of phi entries in the DestBB if relevant).
182 if (MergeIdenticalEdges) {
183 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
184 if (TI->getSuccessor(i) != DestBB) continue;
186 // Remove an entry for TIBB from DestBB phi nodes.
187 DestBB->removePredecessor(TIBB, DontDeleteUselessPhis);
189 // We found another edge to DestBB, go to NewBB instead.
190 TI->setSuccessor(i, NewBB);
196 // If we don't have a pass object, we can't update anything...
197 if (!P) return NewBB;
199 DominatorTreeWrapperPass *DTWP =
200 P->getAnalysisIfAvailable<DominatorTreeWrapperPass>();
201 DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
202 LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
204 // If we have nothing to update, just return.
208 // Now update analysis information. Since the only predecessor of NewBB is
209 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
210 // anything, as there are other successors of DestBB. However, if all other
211 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
212 // loop header) then NewBB dominates DestBB.
213 SmallVector<BasicBlock*, 8> OtherPreds;
215 // If there is a PHI in the block, loop over predecessors with it, which is
216 // faster than iterating pred_begin/end.
217 if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
218 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
219 if (PN->getIncomingBlock(i) != NewBB)
220 OtherPreds.push_back(PN->getIncomingBlock(i));
222 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB);
226 OtherPreds.push_back(P);
230 bool NewBBDominatesDestBB = true;
232 // Should we update DominatorTree information?
234 DomTreeNode *TINode = DT->getNode(TIBB);
236 // The new block is not the immediate dominator for any other nodes, but
237 // TINode is the immediate dominator for the new node.
239 if (TINode) { // Don't break unreachable code!
240 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
241 DomTreeNode *DestBBNode = nullptr;
243 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
244 if (!OtherPreds.empty()) {
245 DestBBNode = DT->getNode(DestBB);
246 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
247 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
248 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
249 OtherPreds.pop_back();
254 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
255 // doesn't dominate anything.
256 if (NewBBDominatesDestBB) {
257 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
258 DT->changeImmediateDominator(DestBBNode, NewBBNode);
263 // Update LoopInfo if it is around.
265 if (Loop *TIL = LI->getLoopFor(TIBB)) {
266 // If one or the other blocks were not in a loop, the new block is not
267 // either, and thus LI doesn't need to be updated.
268 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
269 if (TIL == DestLoop) {
270 // Both in the same loop, the NewBB joins loop.
271 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
272 } else if (TIL->contains(DestLoop)) {
273 // Edge from an outer loop to an inner loop. Add to the outer loop.
274 TIL->addBasicBlockToLoop(NewBB, LI->getBase());
275 } else if (DestLoop->contains(TIL)) {
276 // Edge from an inner loop to an outer loop. Add to the outer loop.
277 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
279 // Edge from two loops with no containment relation. Because these
280 // are natural loops, we know that the destination block must be the
281 // header of its loop (adding a branch into a loop elsewhere would
282 // create an irreducible loop).
283 assert(DestLoop->getHeader() == DestBB &&
284 "Should not create irreducible loops!");
285 if (Loop *P = DestLoop->getParentLoop())
286 P->addBasicBlockToLoop(NewBB, LI->getBase());
289 // If TIBB is in a loop and DestBB is outside of that loop, we may need
290 // to update LoopSimplify form and LCSSA form.
291 if (!TIL->contains(DestBB) &&
292 P->mustPreserveAnalysisID(LoopSimplifyID)) {
293 assert(!TIL->contains(NewBB) &&
294 "Split point for loop exit is contained in loop!");
296 // Update LCSSA form in the newly created exit block.
297 if (P->mustPreserveAnalysisID(LCSSAID))
298 createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);
300 // The only that we can break LoopSimplify form by splitting a critical
301 // edge is if after the split there exists some edge from TIL to DestBB
302 // *and* the only edge into DestBB from outside of TIL is that of
303 // NewBB. If the first isn't true, then LoopSimplify still holds, NewBB
304 // is the new exit block and it has no non-loop predecessors. If the
305 // second isn't true, then DestBB was not in LoopSimplify form prior to
306 // the split as it had a non-loop predecessor. In both of these cases,
307 // the predecessor must be directly in TIL, not in a subloop, or again
308 // LoopSimplify doesn't hold.
309 SmallVector<BasicBlock *, 4> LoopPreds;
310 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E;
314 continue; // The new block is known.
315 if (LI->getLoopFor(P) != TIL) {
316 // No need to re-simplify, it wasn't to start with.
320 LoopPreds.push_back(P);
322 if (!LoopPreds.empty()) {
323 assert(!DestBB->isLandingPad() &&
324 "We don't split edges to landing pads!");
325 BasicBlock *NewExitBB =
326 SplitBlockPredecessors(DestBB, LoopPreds, "split", P);
327 if (P->mustPreserveAnalysisID(LCSSAID))
328 createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB);
331 // LCSSA form was updated above for the case where LoopSimplify is
332 // available, which means that all predecessors of loop exit blocks
333 // are within the loop. Without LoopSimplify form, it would be
334 // necessary to insert a new phi.
335 assert((!P->mustPreserveAnalysisID(LCSSAID) ||
336 P->mustPreserveAnalysisID(LoopSimplifyID)) &&
337 "SplitCriticalEdge doesn't know how to update LCCSA form "
338 "without LoopSimplify!");