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/Pass.h"
26 #include "llvm/Function.h"
27 #include "llvm/BasicBlock.h"
28 #include "llvm/Constant.h"
35 struct PromotePass : public FunctionPass {
36 vector<AllocaInst*> Allocas; // the alloca instruction..
37 map<Instruction*, unsigned> AllocaLookup; // reverse mapping of above
39 vector<vector<BasicBlock*> > PhiNodes; // index corresponds to Allocas
41 // List of instructions to remove at end of pass
42 vector<Instruction *> KillList;
44 map<BasicBlock*,vector<PHINode*> > NewPhiNodes; // the PhiNodes we're adding
47 // runOnFunction - To run this pass, first we calculate the alloca
48 // instructions that are safe for promotion, then we promote each one.
50 virtual bool runOnFunction(Function *F);
52 // getAnalysisUsage - We need dominance frontiers
54 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
55 AU.addRequired(DominanceFrontier::ID);
59 void Traverse(BasicBlock *BB, BasicBlock *Pred, vector<Value*> &IncVals,
60 set<BasicBlock*> &Visited);
61 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx);
62 void FindSafeAllocas(Function *F);
65 } // end of anonymous namespace
68 // isSafeAlloca - This predicate controls what types of alloca instructions are
69 // allowed to be promoted...
71 static inline bool isSafeAlloca(const AllocaInst *AI) {
72 if (AI->isArrayAllocation()) return false;
74 for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
75 UI != UE; ++UI) { // Loop over all of the uses of the alloca
77 // Only allow nonindexed memory access instructions...
78 if (MemAccessInst *MAI = dyn_cast<MemAccessInst>(*UI)) {
79 if (MAI->hasIndices()) { // indexed?
80 // Allow the access if there is only one index and the index is
82 if (*MAI->idx_begin() != Constant::getNullValue(Type::UIntTy) ||
83 MAI->idx_begin()+1 != MAI->idx_end())
87 return false; // Not a load or store?
94 // FindSafeAllocas - Find allocas that are safe to promote
96 void PromotePass::FindSafeAllocas(Function *F) {
97 BasicBlock *BB = F->getEntryNode(); // Get the entry node for the function
99 // Look at all instructions in the entry node
100 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
101 if (AllocaInst *AI = dyn_cast<AllocaInst>(*I)) // Is it an alloca?
102 if (isSafeAlloca(AI)) { // If safe alloca, add alloca to safe list
103 AllocaLookup[AI] = Allocas.size(); // Keep reverse mapping
104 Allocas.push_back(AI);
110 bool PromotePass::runOnFunction(Function *F) {
111 // Calculate the set of safe allocas
114 // If there is nothing to do, bail out...
115 if (Allocas.empty()) return false;
117 // Add each alloca to the KillList. Note: KillList is destroyed MOST recently
118 // added to least recently.
119 KillList.assign(Allocas.begin(), Allocas.end());
121 // Calculate the set of write-locations for each alloca. This is analogous to
122 // counting the number of 'redefinitions' of each variable.
123 vector<vector<BasicBlock*> > WriteSets; // index corresponds to Allocas
124 WriteSets.resize(Allocas.size());
125 for (unsigned i = 0; i != Allocas.size(); ++i) {
126 AllocaInst *AI = Allocas[i];
127 for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E; ++U)
128 if (StoreInst *SI = dyn_cast<StoreInst>(*U))
129 // jot down the basic-block it came from
130 WriteSets[i].push_back(SI->getParent());
133 // Get dominance frontier information...
134 DominanceFrontier &DF = getAnalysis<DominanceFrontier>();
136 // Compute the locations where PhiNodes need to be inserted. Look at the
137 // dominance frontier of EACH basic-block we have a write in
139 PhiNodes.resize(Allocas.size());
140 for (unsigned i = 0; i != Allocas.size(); ++i) {
141 for (unsigned j = 0; j != WriteSets[i].size(); j++) {
142 // Look up the DF for this write, add it to PhiNodes
143 DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]);
144 DominanceFrontier::DomSetType S = it->second;
145 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
150 // Perform iterative step
151 for (unsigned k = 0; k != PhiNodes[i].size(); k++) {
152 DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]);
153 DominanceFrontier::DomSetType S = it->second;
154 for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
160 // Set the incoming values for the basic block to be null values for all of
161 // the alloca's. We do this in case there is a load of a value that has not
162 // been stored yet. In this case, it will get this null value.
164 vector<Value *> Values(Allocas.size());
165 for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
166 Values[i] = Constant::getNullValue(Allocas[i]->getType()->getElementType());
168 // Walks all basic blocks in the function performing the SSA rename algorithm
169 // and inserting the phi nodes we marked as necessary
171 set<BasicBlock*> Visited; // The basic blocks we've already visited
172 Traverse(F->front(), 0, Values, Visited);
174 // Remove all instructions marked by being placed in the KillList...
176 while (!KillList.empty()) {
177 Instruction *I = KillList.back();
180 I->getParent()->getInstList().remove(I);
184 // Purge data structurse so they are available the next iteration...
186 AllocaLookup.clear();
193 // QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
194 // Alloca returns true if there wasn't already a phi-node for that variable
196 bool PromotePass::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo) {
197 // Look up the basic-block in question
198 vector<PHINode*> &BBPNs = NewPhiNodes[BB];
199 if (BBPNs.empty()) BBPNs.resize(Allocas.size());
201 // If the BB already has a phi node added for the i'th alloca then we're done!
202 if (BBPNs[AllocaNo]) return false;
204 // Create a PhiNode using the dereferenced type...
205 PHINode *PN = new PHINode(Allocas[AllocaNo]->getType()->getElementType(),
206 Allocas[AllocaNo]->getName()+".mem2reg");
207 BBPNs[AllocaNo] = PN;
209 // Add the phi-node to the basic-block
210 BB->getInstList().push_front(PN);
212 PhiNodes[AllocaNo].push_back(BB);
216 void PromotePass::Traverse(BasicBlock *BB, BasicBlock *Pred,
217 vector<Value*> &IncomingVals,
218 set<BasicBlock*> &Visited) {
219 // If this is a BB needing a phi node, lookup/create the phinode for each
220 // variable we need phinodes for.
221 vector<PHINode *> &BBPNs = NewPhiNodes[BB];
222 for (unsigned k = 0; k != BBPNs.size(); ++k)
223 if (PHINode *PN = BBPNs[k]) {
224 // at this point we can assume that the array has phi nodes.. let's add
226 PN->addIncoming(IncomingVals[k], Pred);
228 // also note that the active variable IS designated by the phi node
229 IncomingVals[k] = PN;
232 // don't revisit nodes
233 if (Visited.count(BB)) return;
238 // keep track of the value of each variable we're watching.. how?
239 for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II) {
240 Instruction *I = *II; //get the instruction
242 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
243 Value *Ptr = LI->getPointerOperand();
245 if (AllocaInst *Src = dyn_cast<AllocaInst>(Ptr)) {
246 map<Instruction*, unsigned>::iterator AI = AllocaLookup.find(Src);
247 if (AI != AllocaLookup.end()) {
248 Value *V = IncomingVals[AI->second];
250 // walk the use list of this load and replace all uses with r
251 LI->replaceAllUsesWith(V);
252 KillList.push_back(LI); // Mark the load to be deleted
255 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
256 // delete this instruction and mark the name as the current holder of the
258 Value *Ptr = SI->getPointerOperand();
259 if (AllocaInst *Dest = dyn_cast<AllocaInst>(Ptr)) {
260 map<Instruction *, unsigned>::iterator ai = AllocaLookup.find(Dest);
261 if (ai != AllocaLookup.end()) {
262 // what value were we writing?
263 IncomingVals[ai->second] = SI->getOperand(0);
264 KillList.push_back(SI); // Mark the store to be deleted
268 } else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(I)) {
269 // Recurse across our successors
270 for (unsigned i = 0; i != TI->getNumSuccessors(); i++) {
271 vector<Value*> OutgoingVals(IncomingVals);
272 Traverse(TI->getSuccessor(i), BB, OutgoingVals, Visited);
279 // createPromoteMemoryToRegister - Provide an entry point to create this pass.
281 Pass *createPromoteMemoryToRegister() {
282 return new PromotePass();