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 #define DEBUG_TYPE "lcssa"
31 #include "llvm/Transforms/Scalar.h"
32 #include "llvm/Constants.h"
33 #include "llvm/Pass.h"
34 #include "llvm/Function.h"
35 #include "llvm/Instructions.h"
36 #include "llvm/ADT/SetVector.h"
37 #include "llvm/ADT/Statistic.h"
38 #include "llvm/Analysis/Dominators.h"
39 #include "llvm/Analysis/LoopInfo.h"
40 #include "llvm/Support/CFG.h"
45 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
48 struct LCSSA : public FunctionPass {
49 // Cached analysis information for the current function.
52 std::vector<BasicBlock*> LoopBlocks;
54 virtual bool runOnFunction(Function &F);
55 bool visitSubloop(Loop* L);
56 void ProcessInstruction(Instruction* Instr,
57 const std::vector<BasicBlock*>& exitBlocks);
59 /// This transformation requires natural loop information & requires that
60 /// loop preheaders be inserted into the CFG. It maintains both of these,
61 /// as well as the CFG. It also requires dominator information.
63 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
65 AU.addRequiredID(LoopSimplifyID);
66 AU.addPreservedID(LoopSimplifyID);
67 AU.addRequired<LoopInfo>();
68 AU.addRequired<DominatorTree>();
71 SetVector<Instruction*> getLoopValuesUsedOutsideLoop(Loop *L);
73 Value *GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
74 std::map<DominatorTree::Node*, Value*> &Phis);
76 /// inLoop - returns true if the given block is within the current loop
77 const bool inLoop(BasicBlock* B) {
78 return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
82 RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
85 FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
86 const PassInfo *llvm::LCSSAID = X.getPassInfo();
88 /// runOnFunction - Process all loops in the function, inner-most out.
89 bool LCSSA::runOnFunction(Function &F) {
92 LI = &getAnalysis<LoopInfo>();
93 DT = &getAnalysis<DominatorTree>();
95 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
96 changed |= visitSubloop(*I);
101 /// visitSubloop - Recursively process all subloops, and then process the given
102 /// loop if it has live-out values.
103 bool LCSSA::visitSubloop(Loop* L) {
104 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
107 // Speed up queries by creating a sorted list of blocks
109 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
110 std::sort(LoopBlocks.begin(), LoopBlocks.end());
112 SetVector<Instruction*> AffectedValues = getLoopValuesUsedOutsideLoop(L);
114 // If no values are affected, we can save a lot of work, since we know that
115 // nothing will be changed.
116 if (AffectedValues.empty())
119 std::vector<BasicBlock*> exitBlocks;
120 L->getExitBlocks(exitBlocks);
123 // Iterate over all affected values for this loop and insert Phi nodes
124 // for them in the appropriate exit blocks
126 for (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
127 E = AffectedValues.end(); I != E; ++I)
128 ProcessInstruction(*I, exitBlocks);
130 assert(L->isLCSSAForm());
135 /// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
136 /// eliminate all out-of-loop uses.
137 void LCSSA::ProcessInstruction(Instruction *Instr,
138 const std::vector<BasicBlock*>& exitBlocks) {
139 ++NumLCSSA; // We are applying the transformation
141 // Keep track of the blocks that have the value available already.
142 std::map<DominatorTree::Node*, Value*> Phis;
144 DominatorTree::Node *InstrNode = DT->getNode(Instr->getParent());
146 // Insert the LCSSA phi's into the exit blocks (dominated by the value), and
147 // add them to the Phi's map.
148 for (std::vector<BasicBlock*>::const_iterator BBI = exitBlocks.begin(),
149 BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
150 BasicBlock *BB = *BBI;
151 DominatorTree::Node *ExitBBNode = DT->getNode(BB);
152 Value *&Phi = Phis[ExitBBNode];
153 if (!Phi && InstrNode->dominates(ExitBBNode)) {
154 PHINode *PN = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
156 PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
158 // Remember that this phi makes the value alive in this block.
161 // Add inputs from inside the loop for this PHI.
162 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
163 PN->addIncoming(Instr, *PI);
168 // Record all uses of Instr outside the loop. We need to rewrite these. The
169 // LCSSA phis won't be included because they use the value in the loop.
170 for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
172 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
173 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
174 unsigned OperandNo = UI.getOperandNo();
175 UserBB = P->getIncomingBlock(OperandNo/2);
178 // If the user is in the loop, don't rewrite it!
179 if (UserBB == Instr->getParent() || inLoop(UserBB)) {
184 // Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
185 // inserting PHI nodes into join points where needed.
186 Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
188 // Preincrement the iterator to avoid invalidating it when we change the
190 Use &U = UI.getUse();
196 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
197 /// are used by instructions outside of it.
198 SetVector<Instruction*> LCSSA::getLoopValuesUsedOutsideLoop(Loop *L) {
200 // FIXME: For large loops, we may be able to avoid a lot of use-scanning
201 // by using dominance information. In particular, if a block does not
202 // dominate any of the loop exits, then none of the values defined in the
203 // block could be used outside the loop.
205 SetVector<Instruction*> AffectedValues;
206 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
208 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
209 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
211 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
212 if (PHINode* p = dyn_cast<PHINode>(*UI)) {
213 unsigned OperandNo = UI.getOperandNo();
214 UserBB = p->getIncomingBlock(OperandNo/2);
217 if (*BB != UserBB && !inLoop(UserBB)) {
218 AffectedValues.insert(I);
223 return AffectedValues;
226 /// GetValueForBlock - Get the value to use within the specified basic block.
227 /// available values are in Phis.
228 Value *LCSSA::GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
229 std::map<DominatorTree::Node*, Value*> &Phis) {
230 // If there is no dominator info for this BB, it is unreachable.
232 return UndefValue::get(OrigInst->getType());
234 // If we have already computed this value, return the previously computed val.
235 Value *&V = Phis[BB];
238 DominatorTree::Node *IDom = BB->getIDom();
240 // Otherwise, there are two cases: we either have to insert a PHI node or we
241 // don't. We need to insert a PHI node if this block is not dominated by one
242 // of the exit nodes from the loop (the loop could have multiple exits, and
243 // though the value defined *inside* the loop dominated all its uses, each
244 // exit by itself may not dominate all the uses).
246 // The simplest way to check for this condition is by checking to see if the
247 // idom is in the loop. If so, we *know* that none of the exit blocks
248 // dominate this block. Note that we *know* that the block defining the
249 // original instruction is in the idom chain, because if it weren't, then the
250 // original value didn't dominate this use.
251 if (!inLoop(IDom->getBlock())) {
252 // Idom is not in the loop, we must still be "below" the exit block and must
253 // be fully dominated by the value live in the idom.
254 return V = GetValueForBlock(IDom, OrigInst, Phis);
257 BasicBlock *BBN = BB->getBlock();
259 // Otherwise, the idom is the loop, so we need to insert a PHI node. Do so
260 // now, then get values to fill in the incoming values for the PHI.
261 PHINode *PN = new PHINode(OrigInst->getType(), OrigInst->getName()+".lcssa",
263 PN->reserveOperandSpace(std::distance(pred_begin(BBN), pred_end(BBN)));
266 // Fill in the incoming values for the block.
267 for (pred_iterator PI = pred_begin(BBN), E = pred_end(BBN); PI != E; ++PI)
268 PN->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);