1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by Owen Anderson and is distributed under the
6 // University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass transforms loops by placing phi nodes at the end of the loops for
11 // all values that are live across the loop boundary. For example, it turns
12 // the left into the right code:
14 // for (...) for (...)
19 // X3 = phi(X1, X2) X3 = phi(X1, X2)
20 // ... = X3 + 4 X4 = phi(X3)
23 // This is still valid LLVM; the extra phi nodes are purely redundant, and will
24 // be trivially eliminated by InstCombine. The major benefit of this
25 // transformation is that it makes many other loop optimizations, such as
26 // LoopUnswitching, simpler.
28 //===----------------------------------------------------------------------===//
30 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Pass.h"
32 #include "llvm/Function.h"
33 #include "llvm/Instructions.h"
34 #include "llvm/ADT/SetVector.h"
35 #include "llvm/ADT/Statistic.h"
36 #include "llvm/Analysis/Dominators.h"
37 #include "llvm/Analysis/LoopInfo.h"
38 #include "llvm/Support/CFG.h"
45 static Statistic<> NumLCSSA("lcssa",
46 "Number of live out of a loop variables");
48 class LCSSA : public FunctionPass {
52 LoopInfo *LI; // Loop information
53 DominatorTree *DT; // Dominator Tree for the current Function...
54 DominanceFrontier *DF; // Current Dominance Frontier
55 std::vector<BasicBlock*> LoopBlocks;
57 virtual bool runOnFunction(Function &F);
58 bool visitSubloop(Loop* L);
59 void processInstruction(Instruction* Instr,
60 const std::vector<BasicBlock*>& exitBlocks);
62 /// This transformation requires natural loop information & requires that
63 /// loop preheaders be inserted into the CFG. It maintains both of these,
64 /// as well as the CFG. It also requires dominator information.
66 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
68 AU.addRequiredID(LoopSimplifyID);
69 AU.addPreservedID(LoopSimplifyID);
70 AU.addRequired<LoopInfo>();
71 AU.addRequired<DominatorTree>();
72 AU.addRequired<DominanceFrontier>();
75 SetVector<Instruction*> getLoopValuesUsedOutsideLoop(Loop *L);
76 Instruction *getValueDominatingBlock(BasicBlock *BB,
77 std::map<BasicBlock*, Instruction*>& PotDoms);
79 /// inLoop - returns true if the given block is within the current loop
80 const bool inLoop(BasicBlock* B) {
81 return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
85 RegisterOpt<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
88 FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
89 const PassInfo *llvm::LCSSAID = X.getPassInfo();
91 /// runOnFunction - Process all loops in the function, inner-most out.
92 bool LCSSA::runOnFunction(Function &F) {
94 LI = &getAnalysis<LoopInfo>();
95 DF = &getAnalysis<DominanceFrontier>();
96 DT = &getAnalysis<DominatorTree>();
98 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) {
99 changed |= visitSubloop(*I);
105 /// visitSubloop - Recursively process all subloops, and then process the given
106 /// loop if it has live-out values.
107 bool LCSSA::visitSubloop(Loop* L) {
108 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
111 // Speed up queries by creating a sorted list of blocks
113 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
114 std::sort(LoopBlocks.begin(), LoopBlocks.end());
116 SetVector<Instruction*> AffectedValues = getLoopValuesUsedOutsideLoop(L);
118 // If no values are affected, we can save a lot of work, since we know that
119 // nothing will be changed.
120 if (AffectedValues.empty())
123 std::vector<BasicBlock*> exitBlocks;
124 L->getExitBlocks(exitBlocks);
127 // Iterate over all affected values for this loop and insert Phi nodes
128 // for them in the appropriate exit blocks
130 for (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
131 E = AffectedValues.end(); I != E; ++I) {
132 processInstruction(*I, exitBlocks);
135 assert(L->isLCSSAForm());
140 /// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
141 /// eliminate all out-of-loop uses.
142 void LCSSA::processInstruction(Instruction* Instr,
143 const std::vector<BasicBlock*>& exitBlocks)
145 ++NumLCSSA; // We are applying the transformation
147 std::map<BasicBlock*, Instruction*> Phis;
149 // Add the base instruction to the Phis list. This makes tracking down
150 // the dominating values easier when we're filling in Phi nodes. This will
151 // be removed later, before we perform use replacement.
152 Phis[Instr->getParent()] = Instr;
154 // Phi nodes that need to be IDF-processed
155 std::vector<PHINode*> workList;
157 for (std::vector<BasicBlock*>::const_iterator BBI = exitBlocks.begin(),
158 BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
159 Instruction*& phi = Phis[*BBI];
161 DT->getNode(Instr->getParent())->dominates(DT->getNode(*BBI))) {
162 phi = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
164 workList.push_back(cast<PHINode>(phi));
169 // Phi nodes that need to have their incoming values filled.
170 std::vector<PHINode*> needIncomingValues;
172 // Calculate the IDF of these LCSSA Phi nodes, inserting new Phi's where
173 // necessary. Keep track of these new Phi's in the "Phis" map.
174 while (!workList.empty()) {
175 PHINode *CurPHI = workList.back();
178 // Even though we've removed this Phi from the work list, we still need
179 // to fill in its incoming values.
180 needIncomingValues.push_back(CurPHI);
182 // Get the current Phi's DF, and insert Phi nodes. Add these new
183 // nodes to our worklist.
184 DominanceFrontier::const_iterator it = DF->find(CurPHI->getParent());
185 if (it != DF->end()) {
186 const DominanceFrontier::DomSetType &S = it->second;
187 for (DominanceFrontier::DomSetType::const_iterator P = S.begin(),
188 PE = S.end(); P != PE; ++P) {
189 if (DT->getNode(Instr->getParent())->dominates(DT->getNode(*P))) {
190 Instruction *&Phi = Phis[*P];
192 // Still doesn't have operands...
193 Phi = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
196 workList.push_back(cast<PHINode>(Phi));
203 // Fill in all Phis we've inserted that need their incoming values filled in.
204 for (std::vector<PHINode*>::iterator IVI = needIncomingValues.begin(),
205 IVE = needIncomingValues.end(); IVI != IVE; ++IVI) {
206 for (pred_iterator PI = pred_begin((*IVI)->getParent()),
207 E = pred_end((*IVI)->getParent()); PI != E; ++PI)
208 (*IVI)->addIncoming(getValueDominatingBlock(*PI, Phis),
212 // Find all uses of the affected value, and replace them with the
214 std::vector<Instruction*> Uses;
215 for (Instruction::use_iterator UI = Instr->use_begin(), UE = Instr->use_end();
217 Instruction* use = cast<Instruction>(*UI);
218 // Don't need to update uses within the loop body.
219 if (!inLoop(use->getParent()))
223 for (std::vector<Instruction*>::iterator II = Uses.begin(), IE = Uses.end();
225 if (PHINode* phi = dyn_cast<PHINode>(*II)) {
226 for (unsigned int i = 0; i < phi->getNumIncomingValues(); ++i) {
227 if (phi->getIncomingValue(i) == Instr) {
228 Instruction* dominator =
229 getValueDominatingBlock(phi->getIncomingBlock(i), Phis);
230 phi->setIncomingValue(i, dominator);
234 Value *NewVal = getValueDominatingBlock((*II)->getParent(), Phis);
235 (*II)->replaceUsesOfWith(Instr, NewVal);
240 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
241 /// are used by instructions outside of it.
242 SetVector<Instruction*> LCSSA::getLoopValuesUsedOutsideLoop(Loop *L) {
244 // FIXME: For large loops, we may be able to avoid a lot of use-scanning
245 // by using dominance information. In particular, if a block does not
246 // dominate any of the loop exits, then none of the values defined in the
247 // block could be used outside the loop.
249 SetVector<Instruction*> AffectedValues;
250 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
252 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
253 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
255 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
256 if (!inLoop(UserBB)) {
257 AffectedValues.insert(I);
262 return AffectedValues;
265 /// getValueDominatingBlock - Return the value within the potential dominators
266 /// map that dominates the given block.
267 Instruction *LCSSA::getValueDominatingBlock(BasicBlock *BB,
268 std::map<BasicBlock*, Instruction*>& PotDoms) {
269 DominatorTree::Node* bbNode = DT->getNode(BB);
270 while (bbNode != 0) {
271 std::map<BasicBlock*, Instruction*>::iterator I =
272 PotDoms.find(bbNode->getBlock());
273 if (I != PotDoms.end()) {
276 bbNode = bbNode->getIDom();
279 assert(0 && "No dominating value found.");