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 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Pass.h"
32 #include "llvm/Function.h"
33 #include "llvm/Instructions.h"
34 #include "llvm/ADT/Statistic.h"
35 #include "llvm/Analysis/Dominators.h"
36 #include "llvm/Analysis/LoopInfo.h"
37 #include "llvm/Support/CFG.h"
45 static Statistic<> NumLCSSA("lcssa", "Number of live out of a loop");
47 class LCSSA : public FunctionPass {
51 LoopInfo *LI; // Loop information
52 DominatorTree *DT; // Dominator Tree for the current Loop...
53 DominanceFrontier *DF; // Current Dominance Frontier
55 virtual bool runOnFunction(Function &F);
56 bool visitSubloop(Loop* L);
58 /// This transformation requires natural loop information & requires that
59 /// loop preheaders be inserted into the CFG. It maintains both of these,
60 /// as well as the CFG. It also requires dominator information.
62 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
64 AU.addRequiredID(LoopSimplifyID);
65 AU.addPreservedID(LoopSimplifyID);
66 AU.addRequired<LoopInfo>();
67 AU.addPreserved<LoopInfo>();
68 AU.addRequired<DominatorTree>();
69 AU.addRequired<DominanceFrontier>();
72 std::set<Instruction*> getLoopValuesUsedOutsideLoop(Loop *L,
73 std::vector<BasicBlock*> LoopBlocks);
76 RegisterOpt<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
79 FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
81 bool LCSSA::runOnFunction(Function &F) {
83 LI = &getAnalysis<LoopInfo>();
84 DF = &getAnalysis<DominanceFrontier>();
85 DT = &getAnalysis<DominatorTree>();
87 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) {
88 changed |= visitSubloop(*I);
94 bool LCSSA::visitSubloop(Loop* L) {
95 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
98 // Speed up queries by creating a sorted list of blocks
99 std::vector<BasicBlock*> LoopBlocks(L->block_begin(), L->block_end());
100 std::sort(LoopBlocks.begin(), LoopBlocks.end());
102 std::set<Instruction*> AffectedValues = getLoopValuesUsedOutsideLoop(L,
105 std::vector<BasicBlock*> exitBlocks;
106 L->getExitBlocks(exitBlocks);
108 // Phi nodes that need to be IDF-processed
109 std::vector<PHINode*> workList;
111 // Iterate over all affected values for this loop and insert Phi nodes
112 // for them in the appropriate exit blocks
113 for (std::set<Instruction*>::iterator I = AffectedValues.begin(),
114 E = AffectedValues.end(); I != E; ++I) {
115 ++NumLCSSA; // We are applying the transformation
116 for (std::vector<BasicBlock*>::iterator BBI = exitBlocks.begin(),
117 BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
118 PHINode *phi = new PHINode((*I)->getType(), "lcssa");
119 (*BBI)->getInstList().insert((*BBI)->front(), phi);
120 workList.push_back(phi);
122 // Since LoopSimplify has been run, we know that all of these predecessors
123 // are in the loop, so just hook them up in the obvious manner.
124 for (pred_iterator PI = pred_begin(*BBI), PE = pred_end(*BBI); PI != PE;
126 phi->addIncoming(*I, *PI);
130 // Calculate the IDF of these LCSSA Phi nodes, inserting new Phi's where
131 // necessary. Keep track of these new Phi's in DFPhis.
132 std::map<BasicBlock*, PHINode*> DFPhis;
133 for (std::vector<PHINode*>::iterator I = workList.begin(),
134 E = workList.end(); I != E; ++I) {
136 // Get the current Phi's DF, and insert Phi nodes. Add these new
137 // nodes to our worklist.
138 DominanceFrontier::const_iterator it = DF->find((*I)->getParent());
139 if (it != DF->end()) {
140 const DominanceFrontier::DomSetType &S = it->second;
141 for (DominanceFrontier::DomSetType::const_iterator P = S.begin(),
142 PE = S.end(); P != PE; ++P) {
143 if (DFPhis[*P] == 0) {
144 // Still doesn't have operands...
145 PHINode *phi = new PHINode((*I)->getType(), "lcssa");
146 (*P)->getInstList().insert((*P)->front(), phi);
149 workList.push_back(phi);
154 // Get the predecessor blocks of the current Phi, and use them to hook up
155 // the operands of the current Phi to any members of DFPhis that dominate
156 // it. This is a nop for the Phis inserted directly in the exit blocks,
157 // since they are not dominated by any members of DFPhis.
158 for (pred_iterator PI = pred_begin((*I)->getParent()),
159 E = pred_end((*I)->getParent()); PI != E; ++PI)
160 for (std::map<BasicBlock*, PHINode*>::iterator MI = DFPhis.begin(),
161 ME = DFPhis.end(); MI != ME; ++MI)
162 if (DT->getNode((*MI).first)->dominates(DT->getNode(*PI))) {
163 (*I)->addIncoming((*MI).second, *PI);
165 // Since dominate() is not cheap, don't do it more than we have to.
170 // FIXME: Should update all uses.
172 return true; // FIXME: Should be more intelligent in our return value.
175 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
176 /// are used by instructions outside of it.
177 std::set<Instruction*> LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
178 std::vector<BasicBlock*> LoopBlocks) {
180 std::set<Instruction*> AffectedValues;
181 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
183 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
184 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
186 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
187 if (!std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), UserBB))
188 AffectedValues.insert(I);
191 return AffectedValues;