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 static const int ID; // Pass identifcation, replacement for typeid
51 LCSSA() : FunctionPass((intptr_t)&ID) {}
53 // Cached analysis information for the current function.
56 std::vector<BasicBlock*> LoopBlocks;
58 virtual bool runOnFunction(Function &F);
59 bool visitSubloop(Loop* L);
60 void ProcessInstruction(Instruction* Instr,
61 const std::vector<BasicBlock*>& exitBlocks);
63 /// This transformation requires natural loop information & requires that
64 /// loop preheaders be inserted into the CFG. It maintains both of these,
65 /// as well as the CFG. It also requires dominator information.
67 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
69 AU.addRequiredID(LoopSimplifyID);
70 AU.addPreservedID(LoopSimplifyID);
71 AU.addRequired<LoopInfo>();
72 AU.addRequired<DominatorTree>();
75 void getLoopValuesUsedOutsideLoop(Loop *L,
76 SetVector<Instruction*> &AffectedValues);
78 Value *GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
79 std::map<DominatorTree::Node*, Value*> &Phis);
81 /// inLoop - returns true if the given block is within the current loop
82 const bool inLoop(BasicBlock* B) {
83 return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
87 const int LCSSA::ID = 0;
88 RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
91 FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
92 const PassInfo *llvm::LCSSAID = X.getPassInfo();
94 /// runOnFunction - Process all loops in the function, inner-most out.
95 bool LCSSA::runOnFunction(Function &F) {
98 LI = &getAnalysis<LoopInfo>();
99 DT = &getAnalysis<DominatorTree>();
101 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
102 changed |= visitSubloop(*I);
107 /// visitSubloop - Recursively process all subloops, and then process the given
108 /// loop if it has live-out values.
109 bool LCSSA::visitSubloop(Loop* L) {
110 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
113 // Speed up queries by creating a sorted list of blocks
115 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
116 std::sort(LoopBlocks.begin(), LoopBlocks.end());
118 SetVector<Instruction*> AffectedValues;
119 getLoopValuesUsedOutsideLoop(L, AffectedValues);
121 // If no values are affected, we can save a lot of work, since we know that
122 // nothing will be changed.
123 if (AffectedValues.empty())
126 std::vector<BasicBlock*> exitBlocks;
127 L->getExitBlocks(exitBlocks);
130 // Iterate over all affected values for this loop and insert Phi nodes
131 // for them in the appropriate exit blocks
133 for (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
134 E = AffectedValues.end(); I != E; ++I)
135 ProcessInstruction(*I, exitBlocks);
137 assert(L->isLCSSAForm());
142 /// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
143 /// eliminate all out-of-loop uses.
144 void LCSSA::ProcessInstruction(Instruction *Instr,
145 const std::vector<BasicBlock*>& exitBlocks) {
146 ++NumLCSSA; // We are applying the transformation
148 // Keep track of the blocks that have the value available already.
149 std::map<DominatorTree::Node*, Value*> Phis;
151 DominatorTree::Node *InstrNode = DT->getNode(Instr->getParent());
153 // Insert the LCSSA phi's into the exit blocks (dominated by the value), and
154 // add them to the Phi's map.
155 for (std::vector<BasicBlock*>::const_iterator BBI = exitBlocks.begin(),
156 BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
157 BasicBlock *BB = *BBI;
158 DominatorTree::Node *ExitBBNode = DT->getNode(BB);
159 Value *&Phi = Phis[ExitBBNode];
160 if (!Phi && InstrNode->dominates(ExitBBNode)) {
161 PHINode *PN = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
163 PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
165 // Remember that this phi makes the value alive in this block.
168 // Add inputs from inside the loop for this PHI.
169 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
170 PN->addIncoming(Instr, *PI);
175 // Record all uses of Instr outside the loop. We need to rewrite these. The
176 // LCSSA phis won't be included because they use the value in the loop.
177 for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
179 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
180 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
181 unsigned OperandNo = UI.getOperandNo();
182 UserBB = P->getIncomingBlock(OperandNo/2);
185 // If the user is in the loop, don't rewrite it!
186 if (UserBB == Instr->getParent() || inLoop(UserBB)) {
191 // Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
192 // inserting PHI nodes into join points where needed.
193 Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
195 // Preincrement the iterator to avoid invalidating it when we change the
197 Use &U = UI.getUse();
203 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
204 /// are used by instructions outside of it.
205 void LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
206 SetVector<Instruction*> &AffectedValues) {
207 // FIXME: For large loops, we may be able to avoid a lot of use-scanning
208 // by using dominance information. In particular, if a block does not
209 // dominate any of the loop exits, then none of the values defined in the
210 // block could be used outside the loop.
211 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
213 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
214 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
216 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
217 if (PHINode* p = dyn_cast<PHINode>(*UI)) {
218 unsigned OperandNo = UI.getOperandNo();
219 UserBB = p->getIncomingBlock(OperandNo/2);
222 if (*BB != UserBB && !inLoop(UserBB)) {
223 AffectedValues.insert(I);
230 /// GetValueForBlock - Get the value to use within the specified basic block.
231 /// available values are in Phis.
232 Value *LCSSA::GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
233 std::map<DominatorTree::Node*, Value*> &Phis) {
234 // If there is no dominator info for this BB, it is unreachable.
236 return UndefValue::get(OrigInst->getType());
238 // If we have already computed this value, return the previously computed val.
239 Value *&V = Phis[BB];
242 DominatorTree::Node *IDom = BB->getIDom();
244 // Otherwise, there are two cases: we either have to insert a PHI node or we
245 // don't. We need to insert a PHI node if this block is not dominated by one
246 // of the exit nodes from the loop (the loop could have multiple exits, and
247 // though the value defined *inside* the loop dominated all its uses, each
248 // exit by itself may not dominate all the uses).
250 // The simplest way to check for this condition is by checking to see if the
251 // idom is in the loop. If so, we *know* that none of the exit blocks
252 // dominate this block. Note that we *know* that the block defining the
253 // original instruction is in the idom chain, because if it weren't, then the
254 // original value didn't dominate this use.
255 if (!inLoop(IDom->getBlock())) {
256 // Idom is not in the loop, we must still be "below" the exit block and must
257 // be fully dominated by the value live in the idom.
258 return V = GetValueForBlock(IDom, OrigInst, Phis);
261 BasicBlock *BBN = BB->getBlock();
263 // Otherwise, the idom is the loop, so we need to insert a PHI node. Do so
264 // now, then get values to fill in the incoming values for the PHI.
265 PHINode *PN = new PHINode(OrigInst->getType(), OrigInst->getName()+".lcssa",
267 PN->reserveOperandSpace(std::distance(pred_begin(BBN), pred_end(BBN)));
270 // Fill in the incoming values for the block.
271 for (pred_iterator PI = pred_begin(BBN), E = pred_end(BBN); PI != E; ++PI)
272 PN->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);