llvm/lib/Transforms/Scalar/ScalarReplAggregates.cpp

//===- 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;
}