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"
41 #include "llvm/Support/Compiler.h"
46 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
49 struct VISIBILITY_HIDDEN LCSSA : public FunctionPass {
50 // Cached analysis information for the current function.
53 std::vector<BasicBlock*> LoopBlocks;
55 virtual bool runOnFunction(Function &F);
56 bool visitSubloop(Loop* L);
57 void ProcessInstruction(Instruction* Instr,
58 const std::vector<BasicBlock*>& exitBlocks);
60 /// This transformation requires natural loop information & requires that
61 /// loop preheaders be inserted into the CFG. It maintains both of these,
62 /// as well as the CFG. It also requires dominator information.
64 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
66 AU.addRequiredID(LoopSimplifyID);
67 AU.addPreservedID(LoopSimplifyID);
68 AU.addRequired<LoopInfo>();
69 AU.addRequired<DominatorTree>();
72 void getLoopValuesUsedOutsideLoop(Loop *L,
73 SetVector<Instruction*> &AffectedValues);
75 Value *GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
76 std::map<DominatorTree::Node*, Value*> &Phis);
78 /// inLoop - returns true if the given block is within the current loop
79 const bool inLoop(BasicBlock* B) {
80 return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
84 RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
87 FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
88 const PassInfo *llvm::LCSSAID = X.getPassInfo();
90 /// runOnFunction - Process all loops in the function, inner-most out.
91 bool LCSSA::runOnFunction(Function &F) {
94 LI = &getAnalysis<LoopInfo>();
95 DT = &getAnalysis<DominatorTree>();
97 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
98 changed |= visitSubloop(*I);
103 /// visitSubloop - Recursively process all subloops, and then process the given
104 /// loop if it has live-out values.
105 bool LCSSA::visitSubloop(Loop* L) {
106 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
109 // Speed up queries by creating a sorted list of blocks
111 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
112 std::sort(LoopBlocks.begin(), LoopBlocks.end());
114 SetVector<Instruction*> AffectedValues;
115 getLoopValuesUsedOutsideLoop(L, AffectedValues);
117 // If no values are affected, we can save a lot of work, since we know that
118 // nothing will be changed.
119 if (AffectedValues.empty())
122 std::vector<BasicBlock*> exitBlocks;
123 L->getExitBlocks(exitBlocks);
126 // Iterate over all affected values for this loop and insert Phi nodes
127 // for them in the appropriate exit blocks
129 for (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
130 E = AffectedValues.end(); I != E; ++I)
131 ProcessInstruction(*I, exitBlocks);
133 assert(L->isLCSSAForm());
138 /// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
139 /// eliminate all out-of-loop uses.
140 void LCSSA::ProcessInstruction(Instruction *Instr,
141 const std::vector<BasicBlock*>& exitBlocks) {
142 ++NumLCSSA; // We are applying the transformation
144 // Keep track of the blocks that have the value available already.
145 std::map<DominatorTree::Node*, Value*> Phis;
147 DominatorTree::Node *InstrNode = DT->getNode(Instr->getParent());
149 // Insert the LCSSA phi's into the exit blocks (dominated by the value), and
150 // add them to the Phi's map.
151 for (std::vector<BasicBlock*>::const_iterator BBI = exitBlocks.begin(),
152 BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
153 BasicBlock *BB = *BBI;
154 DominatorTree::Node *ExitBBNode = DT->getNode(BB);
155 Value *&Phi = Phis[ExitBBNode];
156 if (!Phi && InstrNode->dominates(ExitBBNode)) {
157 PHINode *PN = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
159 PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
161 // Remember that this phi makes the value alive in this block.
164 // Add inputs from inside the loop for this PHI.
165 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
166 PN->addIncoming(Instr, *PI);
171 // Record all uses of Instr outside the loop. We need to rewrite these. The
172 // LCSSA phis won't be included because they use the value in the loop.
173 for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
175 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
176 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
177 unsigned OperandNo = UI.getOperandNo();
178 UserBB = P->getIncomingBlock(OperandNo/2);
181 // If the user is in the loop, don't rewrite it!
182 if (UserBB == Instr->getParent() || inLoop(UserBB)) {
187 // Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
188 // inserting PHI nodes into join points where needed.
189 Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
191 // Preincrement the iterator to avoid invalidating it when we change the
193 Use &U = UI.getUse();
199 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
200 /// are used by instructions outside of it.
201 void LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
202 SetVector<Instruction*> &AffectedValues) {
203 // FIXME: For large loops, we may be able to avoid a lot of use-scanning
204 // by using dominance information. In particular, if a block does not
205 // dominate any of the loop exits, then none of the values defined in the
206 // block could be used outside the loop.
207 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
209 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
210 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
212 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
213 if (PHINode* p = dyn_cast<PHINode>(*UI)) {
214 unsigned OperandNo = UI.getOperandNo();
215 UserBB = p->getIncomingBlock(OperandNo/2);
218 if (*BB != UserBB && !inLoop(UserBB)) {
219 AffectedValues.insert(I);
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);