1 //===- LoopPreheaders.cpp - Loop Preheader Insertion Pass -----------------===//
3 // Insert Loop pre-headers and exit blocks into the CFG for each function in the
4 // module. This pass updates loop information and dominator information.
6 // Loop pre-header insertion guarantees that there is a single, non-critical
7 // entry edge from outside of the loop to the loop header. This simplifies a
8 // number of analyses and transformations, such as LICM.
10 // Loop exit-block insertion guarantees that all exit blocks from the loop
11 // (blocks which are outside of the loop that have predecessors inside of the
12 // loop) are dominated by the loop header. This simplifies transformations such
13 // as store-sinking that are built into LICM.
15 // Note that the simplifycfg pass will clean up blocks which are split out but
16 // end up being unnecessary, so usage of this pass does not neccesarily
17 // pessimize generated code.
19 //===----------------------------------------------------------------------===//
21 #include "llvm/Transforms/Scalar.h"
22 #include "llvm/Analysis/Dominators.h"
23 #include "llvm/Analysis/LoopInfo.h"
24 #include "llvm/Function.h"
25 #include "llvm/iTerminators.h"
26 #include "llvm/iPHINode.h"
27 #include "llvm/Constant.h"
28 #include "llvm/Support/CFG.h"
29 #include "Support/SetOperations.h"
30 #include "Support/Statistic.h"
31 #include "Support/DepthFirstIterator.h"
34 Statistic<> NumInserted("preheaders", "Number of pre-header nodes inserted");
36 struct Preheaders : public FunctionPass {
37 virtual bool runOnFunction(Function &F);
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 // We need loop information to identify the loops...
41 AU.addRequired<LoopInfo>();
42 AU.addRequired<DominatorSet>();
44 AU.addPreserved<LoopInfo>();
45 AU.addPreserved<DominatorSet>();
46 AU.addPreserved<ImmediateDominators>();
47 AU.addPreserved<DominatorTree>();
48 AU.addPreserved<DominanceFrontier>();
49 AU.addPreservedID(BreakCriticalEdgesID); // No crit edges added....
52 bool ProcessLoop(Loop *L);
53 BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
54 const std::vector<BasicBlock*> &Preds);
55 void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
56 void InsertPreheaderForLoop(Loop *L);
59 RegisterOpt<Preheaders> X("preheaders", "Natural loop pre-header insertion");
62 // Publically exposed interface to pass...
63 const PassInfo *LoopPreheadersID = X.getPassInfo();
64 Pass *createLoopPreheaderInsertionPass() { return new Preheaders(); }
67 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
68 /// it in any convenient order) inserting preheaders...
70 bool Preheaders::runOnFunction(Function &F) {
72 LoopInfo &LI = getAnalysis<LoopInfo>();
74 for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
75 Changed |= ProcessLoop(LI.getTopLevelLoops()[i]);
81 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
82 /// all loops have preheaders.
84 bool Preheaders::ProcessLoop(Loop *L) {
87 // Does the loop already have a preheader? If so, don't modify the loop...
88 if (L->getLoopPreheader() == 0) {
89 InsertPreheaderForLoop(L);
94 // Regardless of whether or not we added a preheader to the loop we must
95 // guarantee that the preheader dominates all exit nodes. If there are any
96 // exit nodes not dominated, split them now.
97 DominatorSet &DS = getAnalysis<DominatorSet>();
98 BasicBlock *Header = L->getHeader();
99 for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i)
100 if (!DS.dominates(Header, L->getExitBlocks()[i])) {
101 RewriteLoopExitBlock(L, L->getExitBlocks()[i]);
102 assert(DS.dominates(Header, L->getExitBlocks()[i]) &&
103 "RewriteLoopExitBlock failed?");
108 const std::vector<Loop*> &SubLoops = L->getSubLoops();
109 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
110 Changed |= ProcessLoop(SubLoops[i]);
114 /// SplitBlockPredecessors - Split the specified block into two blocks. We want
115 /// to move the predecessors specified in the Preds list to point to the new
116 /// block, leaving the remaining predecessors pointing to BB. This method
117 /// updates the SSA PHINode's, but no other analyses.
119 BasicBlock *Preheaders::SplitBlockPredecessors(BasicBlock *BB,
121 const std::vector<BasicBlock*> &Preds) {
123 // Create new basic block, insert right before the original block...
124 BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);
126 // The preheader first gets an unconditional branch to the loop header...
127 BranchInst *BI = new BranchInst(BB);
128 NewBB->getInstList().push_back(BI);
130 // For every PHI node in the block, insert a PHI node into NewBB where the
131 // incoming values from the out of loop edges are moved to NewBB. We have two
132 // possible cases here. If the loop is dead, we just insert dummy entries
133 // into the PHI nodes for the new edge. If the loop is not dead, we move the
134 // incoming edges in BB into new PHI nodes in NewBB.
136 if (!Preds.empty()) { // Is the loop not obviously dead?
137 for (BasicBlock::iterator I = BB->begin();
138 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
140 // Create the new PHI node, insert it into NewBB at the end of the block
141 PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
143 // Move all of the edges from blocks outside the loop to the new PHI
144 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
145 Value *V = PN->removeIncomingValue(Preds[i]);
146 NewPHI->addIncoming(V, Preds[i]);
149 // Add an incoming value to the PHI node in the loop for the preheader
151 PN->addIncoming(NewPHI, NewBB);
154 // Now that the PHI nodes are updated, actually move the edges from
155 // Preds to point to NewBB instead of BB.
157 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
158 TerminatorInst *TI = Preds[i]->getTerminator();
159 for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
160 if (TI->getSuccessor(s) == BB)
161 TI->setSuccessor(s, NewBB);
164 } else { // Otherwise the loop is dead...
165 for (BasicBlock::iterator I = BB->begin();
166 PHINode *PN = dyn_cast<PHINode>(I); ++I)
167 // Insert dummy values as the incoming value...
168 PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
173 // ChangeExitBlock - This recursive function is used to change any exit blocks
174 // that use OldExit to use NewExit instead. This is recursive because children
175 // may need to be processed as well.
177 static void ChangeExitBlock(Loop *L, BasicBlock *OldExit, BasicBlock *NewExit) {
178 if (L->hasExitBlock(OldExit)) {
179 L->changeExitBlock(OldExit, NewExit);
180 const std::vector<Loop*> &SubLoops = L->getSubLoops();
181 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
182 ChangeExitBlock(SubLoops[i], OldExit, NewExit);
187 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
188 /// preheader, this method is called to insert one. This method has two phases:
189 /// preheader insertion and analysis updating.
191 void Preheaders::InsertPreheaderForLoop(Loop *L) {
192 BasicBlock *Header = L->getHeader();
194 // Compute the set of predecessors of the loop that are not in the loop.
195 std::vector<BasicBlock*> OutsideBlocks;
196 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
198 if (!L->contains(*PI)) // Coming in from outside the loop?
199 OutsideBlocks.push_back(*PI); // Keep track of it...
201 // Split out the loop pre-header
203 SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
205 //===--------------------------------------------------------------------===//
206 // Update analysis results now that we have preformed the transformation
209 // We know that we have loop information to update... update it now.
210 if (Loop *Parent = L->getParentLoop())
211 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
213 // If the header for the loop used to be an exit node for another loop, then
214 // we need to update this to know that the loop-preheader is now the exit
215 // node. Note that the only loop that could have our header as an exit node
216 // is a sibling loop, ie, one with the same parent loop, or one if it's
219 const std::vector<Loop*> *ParentSubLoops;
220 if (Loop *Parent = L->getParentLoop())
221 ParentSubLoops = &Parent->getSubLoops();
222 else // Must check top-level loops...
223 ParentSubLoops = &getAnalysis<LoopInfo>().getTopLevelLoops();
225 // Loop over all sibling loops, performing the substitution (recursively to
226 // include child loops)...
227 for (unsigned i = 0, e = ParentSubLoops->size(); i != e; ++i)
228 ChangeExitBlock((*ParentSubLoops)[i], Header, NewBB);
230 DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info
232 // The blocks that dominate NewBB are the blocks that dominate Header,
233 // minus Header, plus NewBB.
234 DominatorSet::DomSetType DomSet = DS.getDominators(Header);
235 DomSet.insert(NewBB); // We dominate ourself
236 DomSet.erase(Header); // Header does not dominate us...
237 DS.addBasicBlock(NewBB, DomSet);
239 // The newly created basic block dominates all nodes dominated by Header.
240 for (Function::iterator I = Header->getParent()->begin(),
241 E = Header->getParent()->end(); I != E; ++I)
242 if (DS.dominates(Header, I))
243 DS.addDominator(I, NewBB);
246 // Update immediate dominator information if we have it...
247 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
248 // Whatever i-dominated the header node now immediately dominates NewBB
249 ID->addNewBlock(NewBB, ID->get(Header));
251 // The preheader now is the immediate dominator for the header node...
252 ID->setImmediateDominator(Header, NewBB);
255 // Update DominatorTree information if it is active.
256 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
257 // The immediate dominator of the preheader is the immediate dominator of
260 DominatorTree::Node *HeaderNode = DT->getNode(Header);
261 DominatorTree::Node *PHNode = DT->createNewNode(NewBB,
262 HeaderNode->getIDom());
264 // Change the header node so that PNHode is the new immediate dominator
265 DT->changeImmediateDominator(HeaderNode, PHNode);
268 // Update dominance frontier information...
269 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
270 // The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
271 // everything that Header does, and it strictly dominates Header in
273 assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
274 DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
275 NewDFSet.erase(Header);
276 DF->addBasicBlock(NewBB, NewDFSet);
278 // Now we must loop over all of the dominance frontiers in the function,
279 // replacing occurrences of Header with NewBB in some cases. If a block
280 // dominates a (now) predecessor of NewBB, but did not strictly dominate
281 // Header, it will have Header in it's DF set, but should now have NewBB in
283 for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
284 // Get all of the dominators of the predecessor...
285 const DominatorSet::DomSetType &PredDoms =
286 DS.getDominators(OutsideBlocks[i]);
287 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
288 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
289 BasicBlock *PredDom = *PDI;
290 // If the loop header is in DF(PredDom), then PredDom didn't dominate
291 // the header but did dominate a predecessor outside of the loop. Now
292 // we change this entry to include the preheader in the DF instead of
294 DominanceFrontier::iterator DFI = DF->find(PredDom);
295 assert(DFI != DF->end() && "No dominance frontier for node?");
296 if (DFI->second.count(Header)) {
297 DF->removeFromFrontier(DFI, Header);
298 DF->addToFrontier(DFI, NewBB);
305 void Preheaders::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
306 DominatorSet &DS = getAnalysis<DominatorSet>();
307 assert(!DS.dominates(L->getHeader(), Exit) &&
308 "Loop already dominates exit block??");
309 assert(std::find(L->getExitBlocks().begin(), L->getExitBlocks().end(), Exit)
310 != L->getExitBlocks().end() && "Not a current exit block!");
312 std::vector<BasicBlock*> LoopBlocks;
313 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
315 LoopBlocks.push_back(*I);
317 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
318 BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
320 // Update Loop Information - we know that the new block will be in the parent
322 if (Loop *Parent = L->getParentLoop())
323 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
325 // Replace any instances of Exit with NewBB in this and any nested loops...
326 for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
327 if (I->hasExitBlock(Exit))
328 I->changeExitBlock(Exit, NewBB); // Update exit block information
330 // Update dominator information... The blocks that dominate NewBB are the
331 // intersection of the dominators of predecessors, plus the block itself.
332 // The newly created basic block does not dominate anything except itself.
334 DominatorSet::DomSetType NewBBDomSet = DS.getDominators(LoopBlocks[0]);
335 for (unsigned i = 1, e = LoopBlocks.size(); i != e; ++i)
336 set_intersect(NewBBDomSet, DS.getDominators(LoopBlocks[i]));
337 NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
338 DS.addBasicBlock(NewBB, NewBBDomSet);
340 // Update immediate dominator information if we have it...
341 BasicBlock *NewBBIDom = 0;
342 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
343 // This block does not strictly dominate anything, so it is not an immediate
344 // dominator. To find the immediate dominator of the new exit node, we
345 // trace up the immediate dominators of a predecessor until we find a basic
346 // block that dominates the exit block.
348 BasicBlock *Dom = LoopBlocks[0]; // Some random predecessor...
349 while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
350 assert(Dom != 0 && "No shared dominator found???");
354 // Set the immediate dominator now...
355 ID->addNewBlock(NewBB, Dom);
356 NewBBIDom = Dom; // Reuse this if calculating DominatorTree info...
359 // Update DominatorTree information if it is active.
360 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
361 // NewBB doesn't dominate anything, so just create a node and link it into
362 // its immediate dominator. If we don't have ImmediateDominator info
363 // around, calculate the idom as above.
364 DominatorTree::Node *NewBBIDomNode;
366 NewBBIDomNode = DT->getNode(NewBBIDom);
368 NewBBIDomNode = DT->getNode(LoopBlocks[0]); // Random pred
369 while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) {
370 NewBBIDomNode = NewBBIDomNode->getIDom();
371 assert(NewBBIDomNode && "No shared dominator found??");
375 // Create the new dominator tree node...
376 DT->createNewNode(NewBB, NewBBIDomNode);
379 // Update dominance frontier information...
380 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
381 // DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
382 // does dominate itself (and there is an edge (NewBB -> Exit)).
383 DominanceFrontier::DomSetType NewDFSet;
384 NewDFSet.insert(Exit);
385 DF->addBasicBlock(NewBB, NewDFSet);
387 // Now we must loop over all of the dominance frontiers in the function,
388 // replacing occurrences of Exit with NewBB in some cases. If a block
389 // dominates a (now) predecessor of NewBB, but did not strictly dominate
390 // Exit, it will have Exit in it's DF set, but should now have NewBB in its
392 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
393 // Get all of the dominators of the predecessor...
394 const DominatorSet::DomSetType &PredDoms =DS.getDominators(LoopBlocks[i]);
395 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
396 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
397 BasicBlock *PredDom = *PDI;
398 // Make sure to only rewrite blocks that are part of the loop...
399 if (L->contains(PredDom)) {
400 // If the exit node is in DF(PredDom), then PredDom didn't dominate
401 // Exit but did dominate a predecessor inside of the loop. Now we
402 // change this entry to include NewBB in the DF instead of Exit.
403 DominanceFrontier::iterator DFI = DF->find(PredDom);
404 assert(DFI != DF->end() && "No dominance frontier for node?");
405 if (DFI->second.count(Exit)) {
406 DF->removeFromFrontier(DFI, Exit);
407 DF->addToFrontier(DFI, NewBB);