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/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((intptr_t)&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 std::vector<BasicBlock*>& 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>();
78 void getLoopValuesUsedOutsideLoop(Loop *L,
79 SetVector<Instruction*> &AffectedValues);
81 Value *GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
82 std::map<DomTreeNode*, Value*> &Phis);
84 /// inLoop - returns true if the given block is within the current loop
85 const bool inLoop(BasicBlock* B) {
86 return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
91 RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
94 LoopPass *llvm::createLCSSAPass() { return new LCSSA(); }
95 const PassInfo *llvm::LCSSAID = X.getPassInfo();
97 /// runOnFunction - Process all loops in the function, inner-most out.
98 bool LCSSA::runOnLoop(Loop *L, LPPassManager &LPM) {
100 LI = &LPM.getAnalysis<LoopInfo>();
101 DT = &getAnalysis<DominatorTree>();
103 // Speed up queries by creating a sorted list of blocks
105 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
106 std::sort(LoopBlocks.begin(), LoopBlocks.end());
108 SetVector<Instruction*> AffectedValues;
109 getLoopValuesUsedOutsideLoop(L, AffectedValues);
111 // If no values are affected, we can save a lot of work, since we know that
112 // nothing will be changed.
113 if (AffectedValues.empty())
116 std::vector<BasicBlock*> exitBlocks;
117 L->getExitBlocks(exitBlocks);
120 // Iterate over all affected values for this loop and insert Phi nodes
121 // for them in the appropriate exit blocks
123 for (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
124 E = AffectedValues.end(); I != E; ++I)
125 ProcessInstruction(*I, exitBlocks);
127 assert(L->isLCSSAForm());
132 /// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
133 /// eliminate all out-of-loop uses.
134 void LCSSA::ProcessInstruction(Instruction *Instr,
135 const std::vector<BasicBlock*>& exitBlocks) {
136 ++NumLCSSA; // We are applying the transformation
138 // Keep track of the blocks that have the value available already.
139 std::map<DomTreeNode*, Value*> Phis;
141 DomTreeNode *InstrNode = DT->getNode(Instr->getParent());
143 // Insert the LCSSA phi's into the exit blocks (dominated by the value), and
144 // add them to the Phi's map.
145 for (std::vector<BasicBlock*>::const_iterator BBI = exitBlocks.begin(),
146 BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
147 BasicBlock *BB = *BBI;
148 DomTreeNode *ExitBBNode = DT->getNode(BB);
149 Value *&Phi = Phis[ExitBBNode];
150 if (!Phi && DT->dominates(InstrNode, ExitBBNode)) {
151 PHINode *PN = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
153 PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
155 // Remember that this phi makes the value alive in this block.
158 // Add inputs from inside the loop for this PHI.
159 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
160 PN->addIncoming(Instr, *PI);
165 // Record all uses of Instr outside the loop. We need to rewrite these. The
166 // LCSSA phis won't be included because they use the value in the loop.
167 for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
169 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
170 if (PHINode *P = dyn_cast<PHINode>(*UI)) {
171 unsigned OperandNo = UI.getOperandNo();
172 UserBB = P->getIncomingBlock(OperandNo/2);
175 // If the user is in the loop, don't rewrite it!
176 if (UserBB == Instr->getParent() || inLoop(UserBB)) {
181 // Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
182 // inserting PHI nodes into join points where needed.
183 Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
185 // Preincrement the iterator to avoid invalidating it when we change the
187 Use &U = UI.getUse();
193 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
194 /// are used by instructions outside of it.
195 void LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
196 SetVector<Instruction*> &AffectedValues) {
197 // FIXME: For large loops, we may be able to avoid a lot of use-scanning
198 // by using dominance information. In particular, if a block does not
199 // dominate any of the loop exits, then none of the values defined in the
200 // block could be used outside the loop.
201 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
203 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
204 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
206 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
207 if (PHINode* p = dyn_cast<PHINode>(*UI)) {
208 unsigned OperandNo = UI.getOperandNo();
209 UserBB = p->getIncomingBlock(OperandNo/2);
212 if (*BB != UserBB && !inLoop(UserBB)) {
213 AffectedValues.insert(I);
220 /// GetValueForBlock - Get the value to use within the specified basic block.
221 /// available values are in Phis.
222 Value *LCSSA::GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
223 std::map<DomTreeNode*, Value*> &Phis) {
224 // If there is no dominator info for this BB, it is unreachable.
226 return UndefValue::get(OrigInst->getType());
228 // If we have already computed this value, return the previously computed val.
229 Value *&V = Phis[BB];
232 DomTreeNode *IDom = BB->getIDom();
234 // If the block has no dominator, bail
236 return V = UndefValue::get(OrigInst->getType());
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 return V = GetValueForBlock(IDom, OrigInst, Phis);
255 BasicBlock *BBN = BB->getBlock();
257 // Otherwise, the idom is the loop, so we need to insert a PHI node. Do so
258 // now, then get values to fill in the incoming values for the PHI.
259 PHINode *PN = new PHINode(OrigInst->getType(), OrigInst->getName()+".lcssa",
261 PN->reserveOperandSpace(std::distance(pred_begin(BBN), pred_end(BBN)));
264 // Fill in the incoming values for the block.
265 for (pred_iterator PI = pred_begin(BBN), E = pred_end(BBN); PI != E; ++PI)
266 PN->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);