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26d2ca1d13
behavior is technically undefined git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6438 91177308-0d34-0410-b5e6-96231b3b80d8
298 lines
11 KiB
C++
298 lines
11 KiB
C++
//===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===//
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//
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// This transformation implements the well known scalar replacement of
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// aggregates transformation. This xform breaks up alloca instructions of
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// aggregate type (structure or array) into individual alloca instructions for
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// each member (if possible).
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Function.h"
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#include "llvm/Pass.h"
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#include "llvm/iMemory.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Constants.h"
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#include "Support/StringExtras.h"
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#include "Support/Statistic.h"
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namespace {
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Statistic<> NumReplaced("scalarrepl", "Number of alloca's broken up");
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struct SROA : public FunctionPass {
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bool runOnFunction(Function &F);
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private:
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bool isSafeStructElementUse(Value *Ptr);
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bool isSafeArrayElementUse(Value *Ptr);
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bool isSafeUseOfAllocation(Instruction *User);
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bool isSafeStructAllocaToPromote(AllocationInst *AI);
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bool isSafeArrayAllocaToPromote(AllocationInst *AI);
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AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base);
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};
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RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates");
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}
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Pass *createScalarReplAggregatesPass() { return new SROA(); }
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// runOnFunction - This algorithm is a simple worklist driven algorithm, which
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// runs on all of the malloc/alloca instructions in the function, removing them
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// if they are only used by getelementptr instructions.
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//
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bool SROA::runOnFunction(Function &F) {
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std::vector<AllocationInst*> WorkList;
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// Scan the entry basic block, adding any alloca's and mallocs to the worklist
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BasicBlock &BB = F.getEntryNode();
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for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
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if (AllocationInst *A = dyn_cast<AllocationInst>(I))
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WorkList.push_back(A);
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// Process the worklist
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bool Changed = false;
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while (!WorkList.empty()) {
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AllocationInst *AI = WorkList.back();
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WorkList.pop_back();
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// We cannot transform the allocation instruction if it is an array
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// allocation (allocations OF arrays are ok though), and an allocation of a
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// scalar value cannot be decomposed at all.
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//
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if (AI->isArrayAllocation() ||
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(!isa<StructType>(AI->getAllocatedType()) &&
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!isa<ArrayType>(AI->getAllocatedType()))) continue;
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// Check that all of the users of the allocation are capable of being
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// transformed.
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if (isa<StructType>(AI->getAllocatedType())) {
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if (!isSafeStructAllocaToPromote(AI))
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continue;
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} else if (!isSafeArrayAllocaToPromote(AI))
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continue;
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DEBUG(std::cerr << "Found inst to xform: " << *AI);
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Changed = true;
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std::vector<AllocaInst*> ElementAllocas;
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if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) {
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ElementAllocas.reserve(ST->getNumContainedTypes());
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for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) {
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AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0,
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AI->getName() + "." + utostr(i), AI);
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ElementAllocas.push_back(NA);
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WorkList.push_back(NA); // Add to worklist for recursive processing
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}
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} else {
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const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
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ElementAllocas.reserve(AT->getNumElements());
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const Type *ElTy = AT->getElementType();
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for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) {
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AllocaInst *NA = new AllocaInst(ElTy, 0,
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AI->getName() + "." + utostr(i), AI);
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ElementAllocas.push_back(NA);
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WorkList.push_back(NA); // Add to worklist for recursive processing
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}
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}
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// Now that we have created the alloca instructions that we want to use,
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// expand the getelementptr instructions to use them.
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//
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for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
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I != E; ++I) {
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Instruction *User = cast<Instruction>(*I);
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if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
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// We now know that the GEP is of the form: GEP <ptr>, 0, <cst>
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uint64_t Idx;
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if (ConstantSInt *CSI = dyn_cast<ConstantSInt>(GEPI->getOperand(2)))
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Idx = CSI->getValue();
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else
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Idx = cast<ConstantUInt>(GEPI->getOperand(2))->getValue();
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assert(Idx < ElementAllocas.size() && "Index out of range?");
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AllocaInst *AllocaToUse = ElementAllocas[Idx];
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Value *RepValue;
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if (GEPI->getNumOperands() == 3) {
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// Do not insert a new getelementptr instruction with zero indices,
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// only to have it optimized out later.
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RepValue = AllocaToUse;
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} else {
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// We are indexing deeply into the structure, so we still need a
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// getelement ptr instruction to finish the indexing. This may be
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// expanded itself once the worklist is rerun.
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//
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std::string OldName = GEPI->getName(); // Steal the old name...
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std::vector<Value*> NewArgs;
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NewArgs.push_back(Constant::getNullValue(Type::LongTy));
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NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end());
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GEPI->setName("");
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RepValue =
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new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI);
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}
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// Move all of the users over to the new GEP.
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GEPI->replaceAllUsesWith(RepValue);
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// Delete the old GEP
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GEPI->getParent()->getInstList().erase(GEPI);
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} else {
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assert(0 && "Unexpected instruction type!");
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}
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}
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// Finally, delete the Alloca instruction
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AI->getParent()->getInstList().erase(AI);
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NumReplaced++;
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}
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return Changed;
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}
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/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an
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/// aggregate allocation.
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///
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bool SROA::isSafeUseOfAllocation(Instruction *User) {
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if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
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// The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst>
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if (GEPI->getNumOperands() <= 2 ||
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GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) ||
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!isa<Constant>(GEPI->getOperand(2)) ||
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isa<ConstantExpr>(GEPI->getOperand(2)))
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return false;
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} else {
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return false;
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}
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return true;
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}
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/// isSafeStructElementUse - It is illegal in C to take the address of a
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/// structure sub-element, and then use pointer arithmetic to access other
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/// elements of the struct. Despite the fact that this is illegal, some
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/// programs do this, so do at least a simple check to try to avoid breaking
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/// broken programs if possible.
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///
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bool SROA::isSafeStructElementUse(Value *Ptr) {
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for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end();
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I != E; ++I)
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if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*I)) {
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if (GEP->getNumOperands() > 1) {
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if (!isa<Constant>(GEP->getOperand(1)) ||
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!cast<Constant>(GEP->getOperand(1))->isNullValue()) {
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std::cerr << "WARNING: Undefined behavior found: " << *GEP
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<< " ... uses pointer arithmetic to access other struct "
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<< "elements!\n";
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return false;
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return false; // Using pointer arithmetic to navigate the array...
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}
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}
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}
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return true;
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}
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/// isSafeArrayElementUse - Check to see if this use is an allowed use for a
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/// getelementptr instruction of an array aggregate allocation.
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///
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bool SROA::isSafeArrayElementUse(Value *Ptr) {
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for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end();
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I != E; ++I) {
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Instruction *User = cast<Instruction>(*I);
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switch (User->getOpcode()) {
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case Instruction::Load: return true;
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case Instruction::Store: return User->getOperand(0) != Ptr;
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case Instruction::GetElementPtr: {
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GetElementPtrInst *GEP = cast<GetElementPtrInst>(User);
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if (GEP->getNumOperands() > 1) {
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if (!isa<Constant>(GEP->getOperand(1)) ||
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!cast<Constant>(GEP->getOperand(1))->isNullValue())
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return false; // Using pointer arithmetic to navigate the array...
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// Check to see if there are any structure indexes involved in this GEP.
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// If so, then we can safely break the array up until at least the
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// structure.
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for (unsigned i = 2, e = GEP->getNumOperands(); i != e; ++i)
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if (GEP->getOperand(i)->getType()->isUnsigned())
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break;
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}
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return isSafeArrayElementUse(GEP);
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}
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default:
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DEBUG(std::cerr << " Transformation preventing inst: " << *User);
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return false;
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}
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}
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return true; // All users look ok :)
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}
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/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a
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/// structure can be broken down into elements.
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///
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bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) {
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// Loop over the use list of the alloca. We can only transform it if all of
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// the users are safe to transform.
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//
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for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
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I != E; ++I) {
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if (!isSafeUseOfAllocation(cast<Instruction>(*I))) {
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DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
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<< *I);
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return false;
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}
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// Pedantic check to avoid breaking broken programs...
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if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*I))
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if (GEPI->getNumOperands() == 3 && !isSafeStructElementUse(GEPI))
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return false;
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}
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return true;
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}
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/// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a
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/// structure can be broken down into elements.
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///
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bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) {
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const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
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int64_t NumElements = AT->getNumElements();
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// Loop over the use list of the alloca. We can only transform it if all of
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// the users are safe to transform. Array allocas have extra constraints to
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// meet though.
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//
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for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
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I != E; ++I) {
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Instruction *User = cast<Instruction>(*I);
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if (!isSafeUseOfAllocation(User)) {
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DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
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<< User);
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return false;
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}
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// Check to make sure that getelementptr follow the extra rules for arrays:
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if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
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// Check to make sure that index falls within the array. If not,
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// something funny is going on, so we won't do the optimization.
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//
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if (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements)
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return false;
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// Check to make sure that the only thing that uses the resultant pointer
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// is safe for an array access. For example, code that looks like:
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// P = &A[0]; P = P + 1
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// is legal, and should prevent promotion.
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//
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if (!isSafeArrayElementUse(GEPI)) {
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DEBUG(std::cerr << "Cannot transform: " << *AI
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<< " due to uses of user: " << *GEPI);
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return false;
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}
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}
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}
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return true;
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}
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