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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:
bool isSafeElementUse(Value *Ptr);
bool isSafeUseOfAllocation(Instruction *User);
bool isSafeStructAllocaToPromote(AllocationInst *AI);
bool isSafeArrayAllocaToPromote(AllocationInst *AI);
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;
// Check that all of the users of the allocation are capable of being
// transformed.
if (isa<StructType>(AI->getAllocatedType())) {
if (!isSafeStructAllocaToPromote(AI))
continue;
} else if (!isSafeArrayAllocaToPromote(AI))
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 {
const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
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...
std::vector<Value*> NewArgs;
NewArgs.push_back(Constant::getNullValue(Type::LongTy));
NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end());
GEPI->setName("");
RepValue =
new GetElementPtrInst(AllocaToUse, NewArgs, 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;
}
/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an
/// aggregate allocation.
///
bool SROA::isSafeUseOfAllocation(Instruction *User) {
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)))
return false;
} else {
return false;
}
return true;
}
/// isSafeElementUse - Check to see if this use is an allowed use for a
/// getelementptr instruction of an array aggregate allocation.
///
bool SROA::isSafeElementUse(Value *Ptr) {
for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end();
I != E; ++I) {
Instruction *User = cast<Instruction>(*I);
switch (User->getOpcode()) {
case Instruction::Load: return true;
case Instruction::Store: return User->getOperand(0) != Ptr;
case Instruction::GetElementPtr: {
GetElementPtrInst *GEP = cast<GetElementPtrInst>(User);
if (GEP->getNumOperands() > 1) {
if (!isa<Constant>(GEP->getOperand(1)) ||
!cast<Constant>(GEP->getOperand(1))->isNullValue())
return false; // Using pointer arithmetic to navigate the array...
}
return isSafeElementUse(GEP);
}
default:
DEBUG(std::cerr << " Transformation preventing inst: " << *User);
return false;
}
}
return true; // All users look ok :)
}
/// isSafeStructAllocaToPromote - Check to see if the specified allocation of a
/// structure can be broken down into elements.
///
bool SROA::isSafeStructAllocaToPromote(AllocationInst *AI) {
// Loop over the use list of the alloca. We can only transform it if all of
// the users are safe to transform.
//
for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
I != E; ++I) {
if (!isSafeUseOfAllocation(cast<Instruction>(*I))) {
DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
<< *I);
return false;
}
// Pedantic check to avoid breaking broken programs...
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*I))
if (GEPI->getNumOperands() == 3 && !isSafeElementUse(GEPI))
return false;
}
return true;
}
/// isSafeArrayAllocaToPromote - Check to see if the specified allocation of a
/// structure can be broken down into elements.
///
bool SROA::isSafeArrayAllocaToPromote(AllocationInst *AI) {
const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
int64_t NumElements = AT->getNumElements();
// Loop over the use list of the alloca. We can only transform it if all of
// the users are safe to transform. Array allocas have extra constraints to
// meet though.
//
for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
I != E; ++I) {
Instruction *User = cast<Instruction>(*I);
if (!isSafeUseOfAllocation(User)) {
DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
<< User);
return false;
}
// Check to make sure that getelementptr follow the extra rules for arrays:
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
// 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 (cast<ConstantSInt>(GEPI->getOperand(2))->getValue() >= NumElements)
return false;
// Check to make sure that the only thing that uses the resultant pointer
// is safe for an array access. For example, code that looks like:
// P = &A[0]; P = P + 1
// is legal, and should prevent promotion.
//
if (!isSafeElementUse(GEPI)) {
DEBUG(std::cerr << "Cannot transform: " << *AI
<< " due to uses of user: " << *GEPI);
return false;
}
}
}
return true;
}
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