1 //===- PromoteMemoryToRegister.cpp - Convert memory refs to regs ----------===//
3 // This pass is used to promote memory references to be register references. A
4 // simple example of the transformation performed by this pass is:
7 // %X = alloca int, uint 1 ret int 42
8 // store int 42, int *%X
12 // To do this transformation, a simple analysis is done to ensure it is safe.
13 // Currently this just loops over all alloca instructions, looking for
14 // instructions that are only used in simple load and stores.
16 // After this, the code is transformed by...something magical :)
18 //===----------------------------------------------------------------------===//
20 #include "llvm/Transforms/Scalar/PromoteMemoryToRegister.h"
21 #include "llvm/Analysis/Dominators.h"
22 #include "llvm/iMemory.h"
23 #include "llvm/iPHINode.h"
24 #include "llvm/iTerminators.h"
25 #include "llvm/Function.h"
26 #include "llvm/BasicBlock.h"
27 #include "llvm/Constant.h"
34 struct PromotePass : public FunctionPass {
35 vector<AllocaInst*> Allocas; // the alloca instruction..
36 map<Instruction*, unsigned> AllocaLookup; // reverse mapping of above
38 vector<vector<BasicBlock*> > PhiNodes; // index corresponds to Allocas
40 // List of instructions to remove at end of pass
41 vector<Instruction *> KillList;
43 map<BasicBlock*,vector<PHINode*> > NewPhiNodes; // the PhiNodes we're adding
46 const char *getPassName() const { return "Promote Memory to Register"; }
48 // runOnFunction - To run this pass, first we calculate the alloca
49 // instructions that are safe for promotion, then we promote each one.
51 virtual bool runOnFunction(Function *F);
53 // getAnalysisUsage - We need dominance frontiers
55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 AU.addRequired(DominanceFrontier::ID);
61 void Traverse(BasicBlock *BB, BasicBlock *Pred, vector<Value*> &IncVals,
62 set<BasicBlock*> &Visited);
63 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx);
64 void FindSafeAllocas(Function *F);
67 } // end of anonymous namespace
70 // isSafeAlloca - This predicate controls what types of alloca instructions are
71 // allowed to be promoted...
73 static inline bool isSafeAlloca(const AllocaInst *AI) {
74 if (AI->isArrayAllocation()) return false;
76 for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
77 UI != UE; ++UI) { // Loop over all of the uses of the alloca
79 // Only allow nonindexed memory access instructions...
80 if (MemAccessInst *MAI = dyn_cast<MemAccessInst>(*UI)) {
81 if (MAI->hasIndices()) { // indexed?
82 // Allow the access if there is only one index and the index is
84 if (*MAI->idx_begin() != Constant::getNullValue(Type::UIntTy) ||
85 MAI->idx_begin()+1 != MAI->idx_end())
89 return false; // Not a load or store?
96 // FindSafeAllocas - Find allocas that are safe to promote
98 void PromotePass::FindSafeAllocas(Function *F) {
99 BasicBlock *BB = F->getEntryNode(); // Get the entry node for the function
101 // Look at all instructions in the entry node
102 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
103 if (AllocaInst *AI = dyn_cast<AllocaInst>(*I)) // Is it an alloca?
104 if (isSafeAlloca(AI)) { // If safe alloca, add alloca to safe list
105 AllocaLookup[AI] = Allocas.size(); // Keep reverse mapping
106 Allocas.push_back(AI);
112 bool PromotePass::runOnFunction(Function *F) {
113 // Calculate the set of safe allocas
116 // If there is nothing to do, bail out...
117 if (Allocas.empty()) return false;
119 // Add each alloca to the KillList. Note: KillList is destroyed MOST recently
120 // added to least recently.
121 KillList.assign(Allocas.begin(), Allocas.end());
123 // Calculate the set of write-locations for each alloca. This is analogous to
124 // counting the number of 'redefinitions' of each variable.
125 vector<vector<BasicBlock*> > WriteSets; // index corresponds to Allocas
126 WriteSets.resize(Allocas.size());
127 for (unsigned i = 0; i != Allocas.size(); ++i) {
128 AllocaInst *AI = Allocas[i];
129 for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E; ++U)
130 if (StoreInst *SI = dyn_cast<StoreInst>(*U))
131 // jot down the basic-block it came from
132 WriteSets[i].push_back(SI->getParent());
135 // Get dominance frontier information...
136 DominanceFrontier &DF = getAnalysis<DominanceFrontier>();
138 // Compute the locations where PhiNodes need to be inserted. Look at the
139 // dominance frontier of EACH basic-block we have a write in
141 PhiNodes.resize(Allocas.size());
142 for (unsigned i = 0; i != Allocas.size(); ++i) {
143 for (unsigned j = 0; j != WriteSets[i].size(); j++) {
144 // Look up the DF for this write, add it to PhiNodes
145 DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]);
146 DominanceFrontier::DomSetType S = it->second;
147 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
152 // Perform iterative step
153 for (unsigned k = 0; k != PhiNodes[i].size(); k++) {
154 DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]);
155 DominanceFrontier::DomSetType S = it->second;
156 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
162 // Set the incoming values for the basic block to be null values for all of
163 // the alloca's. We do this in case there is a load of a value that has not
164 // been stored yet. In this case, it will get this null value.
166 vector<Value *> Values(Allocas.size());
167 for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
168 Values[i] = Constant::getNullValue(Allocas[i]->getType()->getElementType());
170 // Walks all basic blocks in the function performing the SSA rename algorithm
171 // and inserting the phi nodes we marked as necessary
173 set<BasicBlock*> Visited; // The basic blocks we've already visited
174 Traverse(F->front(), 0, Values, Visited);
176 // Remove all instructions marked by being placed in the KillList...
178 while (!KillList.empty()) {
179 Instruction *I = KillList.back();
182 I->getParent()->getInstList().remove(I);
186 // Purge data structurse so they are available the next iteration...
188 AllocaLookup.clear();
195 // QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
196 // Alloca returns true if there wasn't already a phi-node for that variable
198 bool PromotePass::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo) {
199 // Look up the basic-block in question
200 vector<PHINode*> &BBPNs = NewPhiNodes[BB];
201 if (BBPNs.empty()) BBPNs.resize(Allocas.size());
203 // If the BB already has a phi node added for the i'th alloca then we're done!
204 if (BBPNs[AllocaNo]) return false;
206 // Create a PhiNode using the dereferenced type...
207 PHINode *PN = new PHINode(Allocas[AllocaNo]->getType()->getElementType(),
208 Allocas[AllocaNo]->getName()+".mem2reg");
209 BBPNs[AllocaNo] = PN;
211 // Add the phi-node to the basic-block
212 BB->getInstList().push_front(PN);
214 PhiNodes[AllocaNo].push_back(BB);
218 void PromotePass::Traverse(BasicBlock *BB, BasicBlock *Pred,
219 vector<Value*> &IncomingVals,
220 set<BasicBlock*> &Visited) {
221 // If this is a BB needing a phi node, lookup/create the phinode for each
222 // variable we need phinodes for.
223 vector<PHINode *> &BBPNs = NewPhiNodes[BB];
224 for (unsigned k = 0; k != BBPNs.size(); ++k)
225 if (PHINode *PN = BBPNs[k]) {
226 // at this point we can assume that the array has phi nodes.. let's add
228 PN->addIncoming(IncomingVals[k], Pred);
230 // also note that the active variable IS designated by the phi node
231 IncomingVals[k] = PN;
234 // don't revisit nodes
235 if (Visited.count(BB)) return;
240 // keep track of the value of each variable we're watching.. how?
241 for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II) {
242 Instruction *I = *II; //get the instruction
244 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
245 Value *Ptr = LI->getPointerOperand();
247 if (AllocaInst *Src = dyn_cast<AllocaInst>(Ptr)) {
248 map<Instruction*, unsigned>::iterator AI = AllocaLookup.find(Src);
249 if (AI != AllocaLookup.end()) {
250 Value *V = IncomingVals[AI->second];
252 // walk the use list of this load and replace all uses with r
253 LI->replaceAllUsesWith(V);
254 KillList.push_back(LI); // Mark the load to be deleted
257 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
258 // delete this instruction and mark the name as the current holder of the
260 Value *Ptr = SI->getPointerOperand();
261 if (AllocaInst *Dest = dyn_cast<AllocaInst>(Ptr)) {
262 map<Instruction *, unsigned>::iterator ai = AllocaLookup.find(Dest);
263 if (ai != AllocaLookup.end()) {
264 // what value were we writing?
265 IncomingVals[ai->second] = SI->getOperand(0);
266 KillList.push_back(SI); // Mark the store to be deleted
270 } else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(I)) {
271 // Recurse across our successors
272 for (unsigned i = 0; i != TI->getNumSuccessors(); i++) {
273 vector<Value*> OutgoingVals(IncomingVals);
274 Traverse(TI->getSuccessor(i), BB, OutgoingVals, Visited);
281 // createPromoteMemoryToRegister - Provide an entry point to create this pass.
283 Pass *createPromoteMemoryToRegister() {
284 return new PromotePass();