Fix comment for consistency sake.
[llvm/avr.git] / lib / Transforms / Utils / BreakCriticalEdges.cpp
blobf5a136661f5bb581abbde15ab4d9f18470a7bbf1
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.
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/Function.h"
25 #include "llvm/Instructions.h"
26 #include "llvm/Type.h"
27 #include "llvm/Support/CFG.h"
28 #include "llvm/Support/Compiler.h"
29 #include "llvm/Support/ErrorHandling.h"
30 #include "llvm/ADT/SmallVector.h"
31 #include "llvm/ADT/Statistic.h"
32 using namespace llvm;
34 STATISTIC(NumBroken, "Number of blocks inserted");
36 namespace {
37 struct VISIBILITY_HIDDEN 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>();
48 // No loop canonicalization guarantees are broken by this pass.
49 AU.addPreservedID(LoopSimplifyID);
54 char BreakCriticalEdges::ID = 0;
55 static RegisterPass<BreakCriticalEdges>
56 X("break-crit-edges", "Break critical edges in CFG");
58 // Publically exposed interface to pass...
59 const PassInfo *const llvm::BreakCriticalEdgesID = &X;
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) {
68 bool Changed = false;
69 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
70 TerminatorInst *TI = I->getTerminator();
71 if (TI->getNumSuccessors() > 1)
72 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
73 if (SplitCriticalEdge(TI, i, this)) {
74 ++NumBroken;
75 Changed = true;
79 return Changed;
82 //===----------------------------------------------------------------------===//
83 // Implementation of the external critical edge manipulation functions
84 //===----------------------------------------------------------------------===//
86 // isCriticalEdge - Return true if the specified edge is a critical edge.
87 // Critical edges are edges from a block with multiple successors to a block
88 // with multiple predecessors.
90 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
91 bool AllowIdenticalEdges) {
92 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
93 if (TI->getNumSuccessors() == 1) return false;
95 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
96 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
98 // If there is more than one predecessor, this is a critical edge...
99 assert(I != E && "No preds, but we have an edge to the block?");
100 const BasicBlock *FirstPred = *I;
101 ++I; // Skip one edge due to the incoming arc from TI.
102 if (!AllowIdenticalEdges)
103 return I != E;
105 // If AllowIdenticalEdges is true, then we allow this edge to be considered
106 // non-critical iff all preds come from TI's block.
107 while (I != E) {
108 if (*I != FirstPred)
109 return true;
110 // Note: leave this as is until no one ever compiles with either gcc 4.0.1
111 // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
112 E = pred_end(*I);
113 ++I;
115 return false;
118 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
119 /// split the critical edge. This will update DominatorTree and
120 /// DominatorFrontier information if it is available, thus calling this pass
121 /// will not invalidate any of them. This returns true if the edge was split,
122 /// false otherwise. This ensures that all edges to that dest go to one block
123 /// instead of each going to a different block.
125 BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
126 Pass *P, bool MergeIdenticalEdges) {
127 if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
128 BasicBlock *TIBB = TI->getParent();
129 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
131 // Create a new basic block, linking it into the CFG.
132 BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
133 TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
134 // Create our unconditional branch...
135 BranchInst::Create(DestBB, NewBB);
137 // Branch to the new block, breaking the edge.
138 TI->setSuccessor(SuccNum, NewBB);
140 // Insert the block into the function... right after the block TI lives in.
141 Function &F = *TIBB->getParent();
142 Function::iterator FBBI = TIBB;
143 F.getBasicBlockList().insert(++FBBI, NewBB);
145 // If there are any PHI nodes in DestBB, we need to update them so that they
146 // merge incoming values from NewBB instead of from TIBB.
148 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
149 PHINode *PN = cast<PHINode>(I);
150 // We no longer enter through TIBB, now we come in through NewBB. Revector
151 // exactly one entry in the PHI node that used to come from TIBB to come
152 // from NewBB.
153 int BBIdx = PN->getBasicBlockIndex(TIBB);
154 PN->setIncomingBlock(BBIdx, NewBB);
157 // If there are any other edges from TIBB to DestBB, update those to go
158 // through the split block, making those edges non-critical as well (and
159 // reducing the number of phi entries in the DestBB if relevant).
160 if (MergeIdenticalEdges) {
161 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
162 if (TI->getSuccessor(i) != DestBB) continue;
164 // Remove an entry for TIBB from DestBB phi nodes.
165 DestBB->removePredecessor(TIBB);
167 // We found another edge to DestBB, go to NewBB instead.
168 TI->setSuccessor(i, NewBB);
174 // If we don't have a pass object, we can't update anything...
175 if (P == 0) return NewBB;
177 // Now update analysis information. Since the only predecessor of NewBB is
178 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
179 // anything, as there are other successors of DestBB. However, if all other
180 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
181 // loop header) then NewBB dominates DestBB.
182 SmallVector<BasicBlock*, 8> OtherPreds;
184 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
185 if (*I != NewBB)
186 OtherPreds.push_back(*I);
188 bool NewBBDominatesDestBB = true;
190 // Should we update DominatorTree information?
191 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
192 DomTreeNode *TINode = DT->getNode(TIBB);
194 // The new block is not the immediate dominator for any other nodes, but
195 // TINode is the immediate dominator for the new node.
197 if (TINode) { // Don't break unreachable code!
198 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
199 DomTreeNode *DestBBNode = 0;
201 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
202 if (!OtherPreds.empty()) {
203 DestBBNode = DT->getNode(DestBB);
204 while (!OtherPreds.empty() && NewBBDominatesDestBB) {
205 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
206 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
207 OtherPreds.pop_back();
209 OtherPreds.clear();
212 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
213 // doesn't dominate anything.
214 if (NewBBDominatesDestBB) {
215 if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
216 DT->changeImmediateDominator(DestBBNode, NewBBNode);
221 // Should we update DominanceFrontier information?
222 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
223 // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
224 if (!OtherPreds.empty()) {
225 // FIXME: IMPLEMENT THIS!
226 llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset."
227 " not implemented yet!");
230 // Since the new block is dominated by its only predecessor TIBB,
231 // it cannot be in any block's dominance frontier. If NewBB dominates
232 // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
233 // just {DestBB}.
234 DominanceFrontier::DomSetType NewDFSet;
235 if (NewBBDominatesDestBB) {
236 DominanceFrontier::iterator I = DF->find(DestBB);
237 if (I != DF->end()) {
238 DF->addBasicBlock(NewBB, I->second);
240 if (I->second.count(DestBB)) {
241 // However NewBB's frontier does not include DestBB.
242 DominanceFrontier::iterator NF = DF->find(NewBB);
243 DF->removeFromFrontier(NF, DestBB);
246 else
247 DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
248 } else {
249 DominanceFrontier::DomSetType NewDFSet;
250 NewDFSet.insert(DestBB);
251 DF->addBasicBlock(NewBB, NewDFSet);
255 // Update LoopInfo if it is around.
256 if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
257 if (Loop *TIL = LI->getLoopFor(TIBB)) {
258 // If one or the other blocks were not in a loop, the new block is not
259 // either, and thus LI doesn't need to be updated.
260 if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
261 if (TIL == DestLoop) {
262 // Both in the same loop, the NewBB joins loop.
263 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
264 } else if (TIL->contains(DestLoop->getHeader())) {
265 // Edge from an outer loop to an inner loop. Add to the outer loop.
266 TIL->addBasicBlockToLoop(NewBB, LI->getBase());
267 } else if (DestLoop->contains(TIL->getHeader())) {
268 // Edge from an inner loop to an outer loop. Add to the outer loop.
269 DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
270 } else {
271 // Edge from two loops with no containment relation. Because these
272 // are natural loops, we know that the destination block must be the
273 // header of its loop (adding a branch into a loop elsewhere would
274 // create an irreducible loop).
275 assert(DestLoop->getHeader() == DestBB &&
276 "Should not create irreducible loops!");
277 if (Loop *P = DestLoop->getParentLoop())
278 P->addBasicBlockToLoop(NewBB, LI->getBase());
281 // If TIBB is in a loop and DestBB is outside of that loop, split the
282 // other exit blocks of the loop that also have predecessors outside
283 // the loop, to maintain a LoopSimplify guarantee.
284 if (!TIL->contains(DestBB) &&
285 P->mustPreserveAnalysisID(LoopSimplifyID)) {
286 // For each unique exit block...
287 SmallVector<BasicBlock *, 4> ExitBlocks;
288 TIL->getExitBlocks(ExitBlocks);
289 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
290 // Collect all the preds that are inside the loop, and note
291 // whether there are any preds outside the loop.
292 SmallVector<BasicBlock *, 4> Preds;
293 bool AllPredsInLoop = false;
294 BasicBlock *Exit = ExitBlocks[i];
295 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
296 I != E; ++I)
297 if (TIL->contains(*I))
298 Preds.push_back(*I);
299 else
300 AllPredsInLoop = true;
301 // If there are any preds not in the loop, we'll need to split
302 // the edges. The Preds.empty() check is needed because a block
303 // may appear multiple times in the list. We can't use
304 // getUniqueExitBlocks above because that depends on LoopSimplify
305 // form, which we're in the process of restoring!
306 if (Preds.empty() || !AllPredsInLoop) continue;
307 BasicBlock *NewBB = SplitBlockPredecessors(Exit,
308 Preds.data(), Preds.size(),
309 "split", P);
310 // Update LCSSA form. This is fairly simple in LoopSimplify form:
311 // just move the existing LCSSA-mandated PHI nodes from the old exit
312 // block to the new one.
313 if (P->mustPreserveAnalysisID(LCSSAID))
314 for (BasicBlock::iterator I = Exit->begin();
315 PHINode *PN = dyn_cast<PHINode>(I); ++I)
316 PN->moveBefore(NewBB->getTerminator());
319 // LCSSA form was updated above for the case where LoopSimplify is
320 // available, which means that all predecessors of loop exit blocks
321 // are within the loop. Without LoopSimplify form, it would be
322 // necessary to insert a new phi.
323 assert((!P->mustPreserveAnalysisID(LCSSAID) ||
324 P->mustPreserveAnalysisID(LoopSimplifyID)) &&
325 "SplitCriticalEdge doesn't know how to update LCCSA form "
326 "without LoopSimplify!");
331 return NewBB;