1 //===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===//
3 // This transformation implements the well known scalar replacement of
4 // aggregates transformation. This xform breaks up alloca instructions of
5 // aggregate type (structure or array) into individual alloca instructions for
6 // each member (if possible). Then, if possible, it transforms the individual
7 // alloca instructions into nice clean scalar SSA form.
9 // This combines a simple SRoA algorithm with the Mem2Reg algorithm because
10 // often interact, especially for C++ programs. As such, iterating between
11 // SRoA, then Mem2Reg until we run out of things to promote works well.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/Constants.h"
17 #include "llvm/DerivedTypes.h"
18 #include "llvm/Function.h"
19 #include "llvm/Pass.h"
20 #include "llvm/iMemory.h"
21 #include "llvm/Analysis/Dominators.h"
22 #include "llvm/Target/TargetData.h"
23 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
24 #include "Support/Debug.h"
25 #include "Support/Statistic.h"
26 #include "Support/StringExtras.h"
29 Statistic<> NumReplaced("scalarrepl", "Number of allocas broken up");
30 Statistic<> NumPromoted("scalarrepl", "Number of allocas promoted");
32 struct SROA : public FunctionPass {
33 bool runOnFunction(Function &F);
35 bool performScalarRepl(Function &F);
36 bool performPromotion(Function &F);
38 // getAnalysisUsage - This pass does not require any passes, but we know it
39 // will not alter the CFG, so say so.
40 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
41 AU.addRequired<DominanceFrontier>();
42 AU.addRequired<TargetData>();
47 bool isSafeElementUse(Value *Ptr);
48 bool isSafeUseOfAllocation(Instruction *User);
49 bool isSafeStructAllocaToPromote(AllocationInst *AI);
50 bool isSafeArrayAllocaToPromote(AllocationInst *AI);
51 AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base);
54 RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates");
57 Pass *createScalarReplAggregatesPass() { return new SROA(); }
60 bool SROA::runOnFunction(Function &F) {
61 bool Changed = false, LocalChange;
63 LocalChange = performScalarRepl(F);
64 LocalChange |= performPromotion(F);
65 Changed |= LocalChange;
66 } while (LocalChange);
72 bool SROA::performPromotion(Function &F) {
73 std::vector<AllocaInst*> Allocas;
74 const TargetData &TD = getAnalysis<TargetData>();
76 BasicBlock &BB = F.getEntryNode(); // Get the entry node for the function
83 // Find allocas that are safe to promote, by looking at all instructions in
85 for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I)
86 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) // Is it an alloca?
87 if (isAllocaPromotable(AI, TD))
88 Allocas.push_back(AI);
90 if (Allocas.empty()) break;
92 PromoteMemToReg(Allocas, getAnalysis<DominanceFrontier>(), TD);
93 NumPromoted += Allocas.size();
101 // performScalarRepl - This algorithm is a simple worklist driven algorithm,
102 // which runs on all of the malloc/alloca instructions in the function, removing
103 // them if they are only used by getelementptr instructions.
105 bool SROA::performScalarRepl(Function &F) {
106 std::vector<AllocationInst*> WorkList;
108 // Scan the entry basic block, adding any alloca's and mallocs to the worklist
109 BasicBlock &BB = F.getEntryNode();
110 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
111 if (AllocationInst *A = dyn_cast<AllocationInst>(I))
112 WorkList.push_back(A);
114 // Process the worklist
115 bool Changed = false;
116 while (!WorkList.empty()) {
117 AllocationInst *AI = WorkList.back();
120 // We cannot transform the allocation instruction if it is an array
121 // allocation (allocations OF arrays are ok though), and an allocation of a
122 // scalar value cannot be decomposed at all.
124 if (AI->isArrayAllocation() ||
125 (!isa<StructType>(AI->getAllocatedType()) &&
126 !isa<ArrayType>(AI->getAllocatedType()))) continue;
128 // Check that all of the users of the allocation are capable of being
130 if (isa<StructType>(AI->getAllocatedType())) {
131 if (!isSafeStructAllocaToPromote(AI))
133 } else if (!isSafeArrayAllocaToPromote(AI))
136 DEBUG(std::cerr << "Found inst to xform: " << *AI);
139 std::vector<AllocaInst*> ElementAllocas;
140 if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) {
141 ElementAllocas.reserve(ST->getNumContainedTypes());
142 for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) {
143 AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0,
144 AI->getName() + "." + utostr(i), AI);
145 ElementAllocas.push_back(NA);
146 WorkList.push_back(NA); // Add to worklist for recursive processing
149 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
150 ElementAllocas.reserve(AT->getNumElements());
151 const Type *ElTy = AT->getElementType();
152 for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) {
153 AllocaInst *NA = new AllocaInst(ElTy, 0,
154 AI->getName() + "." + utostr(i), AI);
155 ElementAllocas.push_back(NA);
156 WorkList.push_back(NA); // Add to worklist for recursive processing
160 // Now that we have created the alloca instructions that we want to use,
161 // expand the getelementptr instructions to use them.
163 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
165 Instruction *User = cast<Instruction>(*I);
166 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
167 // We now know that the GEP is of the form: GEP <ptr>, 0, <cst>
168 uint64_t Idx = cast<ConstantInt>(GEPI->getOperand(2))->getRawValue();
170 assert(Idx < ElementAllocas.size() && "Index out of range?");
171 AllocaInst *AllocaToUse = ElementAllocas[Idx];
174 if (GEPI->getNumOperands() == 3) {
175 // Do not insert a new getelementptr instruction with zero indices,
176 // only to have it optimized out later.
177 RepValue = AllocaToUse;
179 // We are indexing deeply into the structure, so we still need a
180 // getelement ptr instruction to finish the indexing. This may be
181 // expanded itself once the worklist is rerun.
183 std::string OldName = GEPI->getName(); // Steal the old name...
184 std::vector<Value*> NewArgs;
185 NewArgs.push_back(Constant::getNullValue(Type::LongTy));
186 NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end());
189 new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI);
192 // Move all of the users over to the new GEP.
193 GEPI->replaceAllUsesWith(RepValue);
194 // Delete the old GEP
195 GEPI->getParent()->getInstList().erase(GEPI);
197 assert(0 && "Unexpected instruction type!");
201 // Finally, delete the Alloca instruction
202 AI->getParent()->getInstList().erase(AI);
210 /// isSafeUseOfAllocation - Check to see if this user is an allowed use for an
211 /// aggregate allocation.
213 bool SROA::isSafeUseOfAllocation(Instruction *User) {
214 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
215 // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst>
216 if (GEPI->getNumOperands() <= 2 ||
217 GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) ||
218 !isa<Constant>(GEPI->getOperand(2)) ||
219 isa<ConstantExpr>(GEPI->getOperand(2)))
227 /// isSafeElementUse - Check to see if this use is an allowed use for a
228 /// getelementptr instruction of an array aggregate allocation.
230 bool SROA::isSafeElementUse(Value *Ptr) {
231 for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end();
233 Instruction *User = cast<Instruction>(*I);
234 switch (User->getOpcode()) {
235 case Instruction::Load: return true;
236 case Instruction::Store: return User->getOperand(0) != Ptr;
237 case Instruction::GetElementPtr: {
238 GetElementPtrInst *GEP = cast<GetElementPtrInst>(User);
239 if (GEP->getNumOperands() > 1) {
240 if (!isa<Constant>(GEP->getOperand(1)) ||
241 !cast<Constant>(GEP->getOperand(1))->isNullValue())
242 return false; // Using pointer arithmetic to navigate the array...
244 return isSafeElementUse(GEP);
247 DEBUG(std::cerr << " Transformation preventing inst: " << *User);
251 return true; // All users look ok :)
255 /// isSafeStructAllocaToPromote - Check to see if the specified allocation of a
256 /// structure can be broken down into elements.
258 bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) {
259 // Loop over the use list of the alloca. We can only transform it if all of
260 // the users are safe to transform.
262 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
264 if (!isSafeUseOfAllocation(cast<Instruction>(*I))) {
265 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
270 // Pedantic check to avoid breaking broken programs...
271 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*I))
272 if (GEPI->getNumOperands() == 3 && !isSafeElementUse(GEPI))
279 /// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a
280 /// structure can be broken down into elements.
282 bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) {
283 const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
284 int64_t NumElements = AT->getNumElements();
286 // Loop over the use list of the alloca. We can only transform it if all of
287 // the users are safe to transform. Array allocas have extra constraints to
290 for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
292 Instruction *User = cast<Instruction>(*I);
293 if (!isSafeUseOfAllocation(User)) {
294 DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
299 // Check to make sure that getelementptr follow the extra rules for arrays:
300 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
301 // Check to make sure that index falls within the array. If not,
302 // something funny is going on, so we won't do the optimization.
304 if (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements)
307 // Check to make sure that the only thing that uses the resultant pointer
308 // is safe for an array access. For example, code that looks like:
309 // P = &A[0]; P = P + 1
310 // is legal, and should prevent promotion.
312 if (!isSafeElementUse(GEPI)) {
313 DEBUG(std::cerr << "Cannot transform: " << *AI
314 << " due to uses of user: " << *GEPI);