1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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/LoopPass.h"
40 #include "llvm/Analysis/ScalarEvolution.h"
41 #include "llvm/Support/CFG.h"
42 #include "llvm/Support/Compiler.h"
47 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
50 struct VISIBILITY_HIDDEN LCSSA : public LoopPass {
51 static char ID; // Pass identification, replacement for typeid
52 LCSSA() : LoopPass(&ID) {}
54 // Cached analysis information for the current function.
57 std::vector<BasicBlock*> LoopBlocks;
59 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
61 void ProcessInstruction(Instruction* Instr,
62 const SmallVector<BasicBlock*, 8>& exitBlocks);
64 /// This transformation requires natural loop information & requires that
65 /// loop preheaders be inserted into the CFG. It maintains both of these,
66 /// as well as the CFG. It also requires dominator information.
68 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
70 AU.addRequiredID(LoopSimplifyID);
71 AU.addPreservedID(LoopSimplifyID);
72 AU.addRequired<LoopInfo>();
73 AU.addPreserved<LoopInfo>();
74 AU.addRequired<DominatorTree>();
75 AU.addPreserved<ScalarEvolution>();
76 AU.addPreserved<DominatorTree>();
78 // Request DominanceFrontier now, even though LCSSA does
79 // not use it. This allows Pass Manager to schedule Dominance
80 // Frontier early enough such that one LPPassManager can handle
81 // multiple loop transformation passes.
82 AU.addRequired<DominanceFrontier>();
83 AU.addPreserved<DominanceFrontier>();
86 void getLoopValuesUsedOutsideLoop(Loop *L,
87 SetVector<Instruction*> &AffectedValues);
89 Value *GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
90 DenseMap<DomTreeNode*, Value*> &Phis);
92 /// inLoop - returns true if the given block is within the current loop
93 bool inLoop(BasicBlock* B) {
94 return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
100 static RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
102 Pass *llvm::createLCSSAPass() { return new LCSSA(); }
103 const PassInfo *const llvm::LCSSAID = &X;
105 /// runOnFunction - Process all loops in the function, inner-most out.
106 bool LCSSA::runOnLoop(Loop *L, LPPassManager &LPM) {
108 LI = &LPM.getAnalysis<LoopInfo>();
109 DT = &getAnalysis<DominatorTree>();
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;
117 getLoopValuesUsedOutsideLoop(L, AffectedValues);
119 // If no values are affected, we can save a lot of work, since we know that
120 // nothing will be changed.
121 if (AffectedValues.empty())
124 SmallVector<BasicBlock*, 8> exitBlocks;
125 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);
134 assert(L->isLCSSAForm());
139 /// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
140 /// eliminate all out-of-loop uses.
141 void LCSSA::ProcessInstruction(Instruction *Instr,
142 const SmallVector<BasicBlock*, 8>& exitBlocks) {
143 ++NumLCSSA; // We are applying the transformation
145 // Keep track of the blocks that have the value available already.
146 DenseMap<DomTreeNode*, Value*> Phis;
148 DomTreeNode *InstrNode = DT->getNode(Instr->getParent());
150 // Insert the LCSSA phi's into the exit blocks (dominated by the value), and
151 // add them to the Phi's map.
152 for (SmallVector<BasicBlock*, 8>::const_iterator BBI = exitBlocks.begin(),
153 BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
154 BasicBlock *BB = *BBI;
155 DomTreeNode *ExitBBNode = DT->getNode(BB);
156 Value *&Phi = Phis[ExitBBNode];
157 if (!Phi && DT->dominates(InstrNode, ExitBBNode)) {
158 PHINode *PN = PHINode::Create(Instr->getType(), Instr->getName()+".lcssa",
160 PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
162 // Remember that this phi makes the value alive in this block.
165 // Add inputs from inside the loop for this PHI.
166 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
167 PN->addIncoming(Instr, *PI);
172 // Record all uses of Instr outside the loop. We need to rewrite these. The
173 // LCSSA phis won't be included because they use the value in the loop.
174 for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
176 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
177 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
178 UserBB = P->getIncomingBlock(UI);
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 UserBB = p->getIncomingBlock(UI);
217 if (*BB != UserBB && !inLoop(UserBB)) {
218 AffectedValues.insert(I);
225 /// GetValueForBlock - Get the value to use within the specified basic block.
226 /// available values are in Phis.
227 Value *LCSSA::GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
228 DenseMap<DomTreeNode*, Value*> &Phis) {
229 // If there is no dominator info for this BB, it is unreachable.
231 return UndefValue::get(OrigInst->getType());
233 // If we have already computed this value, return the previously computed val.
234 if (Phis.count(BB)) return Phis[BB];
236 DomTreeNode *IDom = BB->getIDom();
238 // Otherwise, there are two cases: we either have to insert a PHI node or we
239 // don't. We need to insert a PHI node if this block is not dominated by one
240 // of the exit nodes from the loop (the loop could have multiple exits, and
241 // though the value defined *inside* the loop dominated all its uses, each
242 // exit by itself may not dominate all the uses).
244 // The simplest way to check for this condition is by checking to see if the
245 // idom is in the loop. If so, we *know* that none of the exit blocks
246 // dominate this block. Note that we *know* that the block defining the
247 // original instruction is in the idom chain, because if it weren't, then the
248 // original value didn't dominate this use.
249 if (!inLoop(IDom->getBlock())) {
250 // Idom is not in the loop, we must still be "below" the exit block and must
251 // be fully dominated by the value live in the idom.
252 Value* val = GetValueForBlock(IDom, OrigInst, Phis);
253 Phis.insert(std::make_pair(BB, val));
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 = PHINode::Create(OrigInst->getType(),
262 OrigInst->getName() + ".lcssa", BBN->begin());
263 PN->reserveOperandSpace(std::distance(pred_begin(BBN), pred_end(BBN)));
264 Phis.insert(std::make_pair(BB, PN));
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);