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//===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===//
//
// This transformation implements the well known scalar replacement of
// aggregates transformation. This xform breaks up alloca instructions of
// aggregate type (structure or array) into individual alloca instructions for
// each member (if possible).
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Function.h"
#include "llvm/Pass.h"
#include "llvm/iMemory.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Constants.h"
#include "Support/StringExtras.h"
#include "Support/Statistic.h"
namespace {
Statistic<> NumReplaced("scalarrepl", "Number of alloca's broken up");
struct SROA : public FunctionPass {
bool runOnFunction(Function &F);
private:
AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base);
};
RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates");
}
Pass *createScalarReplAggregatesPass() { return new SROA(); }
// runOnFunction - This algorithm is a simple worklist driven algorithm, which
// runs on all of the malloc/alloca instructions in the function, removing them
// if they are only used by getelementptr instructions.
//
bool SROA::runOnFunction(Function &F) {
std::vector<AllocationInst*> WorkList;
// Scan the entry basic block, adding any alloca's and mallocs to the worklist
BasicBlock &BB = F.getEntryNode();
for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
if (AllocationInst *A = dyn_cast<AllocationInst>(I))
WorkList.push_back(A);
// Process the worklist
bool Changed = false;
while (!WorkList.empty()) {
AllocationInst *AI = WorkList.back();
WorkList.pop_back();
// We cannot transform the allocation instruction if it is an array
// allocation (allocations OF arrays are ok though), and an allocation of a
// scalar value cannot be decomposed at all.
//
if (AI->isArrayAllocation() ||
(!isa<StructType>(AI->getAllocatedType()) &&
!isa<ArrayType>(AI->getAllocatedType()))) continue;
const ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType());
// Loop over the use list of the alloca. We can only transform it if there
// are only getelementptr instructions (with a zero first index) and free
// instructions.
//
bool CannotTransform = false;
for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
I != E; ++I) {
Instruction *User = cast<Instruction>(*I);
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
// The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst>
if (GEPI->getNumOperands() <= 2 ||
GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) ||
!isa<Constant>(GEPI->getOperand(2)) ||
isa<ConstantExpr>(GEPI->getOperand(2))) {
DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
<< User);
CannotTransform = true;
break;
}
// If this is an array access, check to make sure that index falls
// within the array. If not, something funny is going on, so we won't
// do the optimization.
if (AT && cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >=
AT->getNumElements()) {
DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
<< User);
CannotTransform = true;
break;
}
} else {
DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
<< User);
CannotTransform = true;
break;
}
}
if (CannotTransform) continue;
DEBUG(std::cerr << "Found inst to xform: " << *AI);
Changed = true;
std::vector<AllocaInst*> ElementAllocas;
if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) {
ElementAllocas.reserve(ST->getNumContainedTypes());
for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) {
AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0,
AI->getName() + "." + utostr(i), AI);
ElementAllocas.push_back(NA);
WorkList.push_back(NA); // Add to worklist for recursive processing
}
} else {
ElementAllocas.reserve(AT->getNumElements());
const Type *ElTy = AT->getElementType();
for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) {
AllocaInst *NA = new AllocaInst(ElTy, 0,
AI->getName() + "." + utostr(i), AI);
ElementAllocas.push_back(NA);
WorkList.push_back(NA); // Add to worklist for recursive processing
}
}
// Now that we have created the alloca instructions that we want to use,
// expand the getelementptr instructions to use them.
//
for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
I != E; ++I) {
Instruction *User = cast<Instruction>(*I);
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
// We now know that the GEP is of the form: GEP <ptr>, 0, <cst>
uint64_t Idx;
if (ConstantSInt *CSI = dyn_cast<ConstantSInt>(GEPI->getOperand(2)))
Idx = CSI->getValue();
else
Idx = cast<ConstantUInt>(GEPI->getOperand(2))->getValue();
assert(Idx < ElementAllocas.size() && "Index out of range?");
AllocaInst *AllocaToUse = ElementAllocas[Idx];
Value *RepValue;
if (GEPI->getNumOperands() == 3) {
// Do not insert a new getelementptr instruction with zero indices,
// only to have it optimized out later.
RepValue = AllocaToUse;
} else {
// We are indexing deeply into the structure, so we still need a
// getelement ptr instruction to finish the indexing. This may be
// expanded itself once the worklist is rerun.
//
std::string OldName = GEPI->getName(); // Steal the old name...
GEPI->setName("");
RepValue =
new GetElementPtrInst(AllocaToUse,
std::vector<Value*>(GEPI->op_begin()+3,
GEPI->op_end()),
OldName, GEPI);
}
// Move all of the users over to the new GEP.
GEPI->replaceAllUsesWith(RepValue);
// Delete the old GEP
GEPI->getParent()->getInstList().erase(GEPI);
} else {
assert(0 && "Unexpected instruction type!");
}
}
// Finally, delete the Alloca instruction
AI->getParent()->getInstList().erase(AI);
NumReplaced++;
}
return Changed;
}
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