1 //===- PromoteMemoryToRegister.cpp - Convert memory refs to regs ----------===//
3 // This file is used to promote memory references to be register references. A
4 // simple example of the transformation performed by this function is:
7 // %X = alloca int, uint 1 ret int 42
8 // store int 42, int *%X
12 // The code is transformed by looping over all of the alloca instruction,
13 // calculating dominator frontiers, then inserting phi-nodes following the usual
14 // SSA construction algorithm. This code does not modify the CFG of the
17 //===----------------------------------------------------------------------===//
19 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
20 #include "llvm/Analysis/Dominators.h"
21 #include "llvm/iMemory.h"
22 #include "llvm/iPHINode.h"
23 #include "llvm/iTerminators.h"
24 #include "llvm/Function.h"
25 #include "llvm/Constant.h"
26 #include "llvm/Type.h"
28 /// isAllocaPromotable - Return true if this alloca is legal for promotion.
29 /// This is true if there are only loads and stores to the alloca...
31 bool isAllocaPromotable(const AllocaInst *AI) {
32 // Only allow direct loads and stores...
33 for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
34 UI != UE; ++UI) // Loop over all of the uses of the alloca
35 if (!isa<LoadInst>(*UI))
36 if (const StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
37 if (SI->getOperand(0) == AI)
38 return false; // Don't allow a store of the AI, only INTO the AI.
40 return false; // Not a load or store?
48 struct PromoteMem2Reg {
49 const std::vector<AllocaInst*> &Allocas; // the alloca instructions..
50 DominanceFrontier &DF;
52 std::map<Instruction*, unsigned> AllocaLookup; // reverse mapping of above
54 std::vector<std::vector<BasicBlock*> > PhiNodes;// Idx corresponds 2 Allocas
56 // List of instructions to remove at end of pass
57 std::vector<Instruction *> KillList;
60 std::vector<PHINode*> > NewPhiNodes; // the PhiNodes we're adding
63 PromoteMem2Reg(const std::vector<AllocaInst*> &A, DominanceFrontier &df)
64 :Allocas(A), DF(df) {}
69 void RenamePass(BasicBlock *BB, BasicBlock *Pred,
70 std::vector<Value*> &IncVals,
71 std::set<BasicBlock*> &Visited);
72 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx);
74 } // end of anonymous namespace
77 void PromoteMem2Reg::run() {
78 // If there is nothing to do, bail out...
79 if (Allocas.empty()) return;
81 Function &F = *DF.getRoot()->getParent();
83 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
84 assert(isAllocaPromotable(Allocas[i]) &&
85 "Cannot promote non-promotable alloca!");
86 assert(Allocas[i]->getParent()->getParent() == &F &&
87 "All allocas should be in the same function, which is same as DF!");
88 AllocaLookup[Allocas[i]] = i;
92 // Add each alloca to the KillList. Note: KillList is destroyed MOST recently
93 // added to least recently.
94 KillList.assign(Allocas.begin(), Allocas.end());
96 // Calculate the set of write-locations for each alloca. This is analogous to
97 // counting the number of 'redefinitions' of each variable.
98 std::vector<std::vector<BasicBlock*> > WriteSets;// Idx corresponds to Allocas
99 WriteSets.resize(Allocas.size());
100 for (unsigned i = 0; i != Allocas.size(); ++i) {
101 AllocaInst *AI = Allocas[i];
102 for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E; ++U)
103 if (StoreInst *SI = dyn_cast<StoreInst>(*U))
104 // jot down the basic-block it came from
105 WriteSets[i].push_back(SI->getParent());
108 // Compute the locations where PhiNodes need to be inserted. Look at the
109 // dominance frontier of EACH basic-block we have a write in
111 PhiNodes.resize(Allocas.size());
112 for (unsigned i = 0; i != Allocas.size(); ++i) {
113 for (unsigned j = 0; j != WriteSets[i].size(); j++) {
114 // Look up the DF for this write, add it to PhiNodes
115 DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]);
116 DominanceFrontier::DomSetType S = it->second;
117 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
122 // Perform iterative step
123 for (unsigned k = 0; k != PhiNodes[i].size(); k++) {
124 DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]);
125 DominanceFrontier::DomSetType S = it->second;
126 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
132 // Set the incoming values for the basic block to be null values for all of
133 // the alloca's. We do this in case there is a load of a value that has not
134 // been stored yet. In this case, it will get this null value.
136 std::vector<Value *> Values(Allocas.size());
137 for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
138 Values[i] = Constant::getNullValue(Allocas[i]->getAllocatedType());
140 // Walks all basic blocks in the function performing the SSA rename algorithm
141 // and inserting the phi nodes we marked as necessary
143 std::set<BasicBlock*> Visited; // The basic blocks we've already visited
144 RenamePass(F.begin(), 0, Values, Visited);
146 // Remove all instructions marked by being placed in the KillList...
148 while (!KillList.empty()) {
149 Instruction *I = KillList.back();
152 I->getParent()->getInstList().erase(I);
157 // QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
158 // Alloca returns true if there wasn't already a phi-node for that variable
160 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo) {
161 // Look up the basic-block in question
162 std::vector<PHINode*> &BBPNs = NewPhiNodes[BB];
163 if (BBPNs.empty()) BBPNs.resize(Allocas.size());
165 // If the BB already has a phi node added for the i'th alloca then we're done!
166 if (BBPNs[AllocaNo]) return false;
168 // Create a PhiNode using the dereferenced type... and add the phi-node to the
170 PHINode *PN = new PHINode(Allocas[AllocaNo]->getAllocatedType(),
171 Allocas[AllocaNo]->getName()+".mem2reg",
173 BBPNs[AllocaNo] = PN;
174 PhiNodes[AllocaNo].push_back(BB);
178 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
179 std::vector<Value*> &IncomingVals,
180 std::set<BasicBlock*> &Visited) {
181 // If this is a BB needing a phi node, lookup/create the phinode for each
182 // variable we need phinodes for.
183 std::vector<PHINode *> &BBPNs = NewPhiNodes[BB];
184 for (unsigned k = 0; k != BBPNs.size(); ++k)
185 if (PHINode *PN = BBPNs[k]) {
186 // at this point we can assume that the array has phi nodes.. let's add
188 PN->addIncoming(IncomingVals[k], Pred);
190 // also note that the active variable IS designated by the phi node
191 IncomingVals[k] = PN;
194 // don't revisit nodes
195 if (Visited.count(BB)) return;
200 // keep track of the value of each variable we're watching.. how?
201 for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II) {
202 Instruction *I = II; // get the instruction
204 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
205 if (AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand())) {
206 std::map<Instruction*, unsigned>::iterator AI = AllocaLookup.find(Src);
207 if (AI != AllocaLookup.end()) {
208 Value *V = IncomingVals[AI->second];
210 // walk the use list of this load and replace all uses with r
211 LI->replaceAllUsesWith(V);
212 KillList.push_back(LI); // Mark the load to be deleted
215 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
216 // Delete this instruction and mark the name as the current holder of the
218 if (AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand())) {
219 std::map<Instruction *, unsigned>::iterator ai =AllocaLookup.find(Dest);
220 if (ai != AllocaLookup.end()) {
221 // what value were we writing?
222 IncomingVals[ai->second] = SI->getOperand(0);
223 KillList.push_back(SI); // Mark the store to be deleted
227 } else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(I)) {
228 // Recurse across our successors
229 for (unsigned i = 0; i != TI->getNumSuccessors(); i++) {
230 std::vector<Value*> OutgoingVals(IncomingVals);
231 RenamePass(TI->getSuccessor(i), BB, OutgoingVals, Visited);
237 /// PromoteMemToReg - Promote the specified list of alloca instructions into
238 /// scalar registers, inserting PHI nodes as appropriate. This function makes
239 /// use of DominanceFrontier information. This function does not modify the CFG
240 /// of the function at all. All allocas must be from the same function.
242 void PromoteMemToReg(const std::vector<AllocaInst*> &Allocas,
243 DominanceFrontier &DF) {
244 PromoteMem2Reg(Allocas, DF).run();