forked from OSchip/llvm-project
Implementation of the simple "scalar replacement of aggregates" transformation
llvm-svn: 6346
This commit is contained in:
Chris Lattner
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11fc2f6885
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fb41a50d15
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//===- 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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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, and an allocation of a scalar value cannot be decomposed
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if (AI->isArrayAllocation() ||
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(!isa<StructType>(AI->getAllocatedType()) /*&&
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!isa<ArrayType>(AI->getAllocatedType())*/
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)) continue;
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// Loop over the use list of the alloca. We can only transform it if there
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// are only getelementptr instructions (with a zero first index) and free
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// instructions.
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//
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bool CannotTransform = false;
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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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// 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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DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
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<< User);
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CannotTransform = true;
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break;
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}
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} else {
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DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
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<< User);
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CannotTransform = true;
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break;
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}
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}
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if (CannotTransform) 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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GEPI->setName("");
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RepValue =
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new GetElementPtrInst(AllocaToUse,
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std::vector<Value*>(GEPI->op_begin()+3,
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GEPI->op_end()),
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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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}
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return Changed;
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}
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