llvm-project/llvm/lib/Transforms/Utils/DemoteRegToStack.cpp

//===- DemoteRegToStack.cpp - Move a virtual reg. to stack ------*- C++ -*-===//
// 
// This file provide the function DemoteRegToStack().  This function takes a
// virtual register computed by an Instruction& X and replaces it with a slot in
// the stack frame, allocated via alloca. It returns the pointer to the
// AllocaInst inserted.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/DemoteRegToStack.h"
#include "llvm/Function.h"
#include "llvm/iMemory.h"
#include "llvm/iPHINode.h"
#include "llvm/iTerminators.h"
#include "llvm/Type.h"
#include "Support/hash_set"
#include <stack>

//---------------------------------------------------------------------------- 
// function DemoteRegToStack()
//
//---------------------------------------------------------------------------- 

typedef hash_set<PHINode*>           PhiSet;
typedef hash_set<PHINode*>::iterator PhiSetIterator;

// Helper function to push a phi *and* all its operands to the worklist!
// Do not push an instruction if it is already in the result set of Phis to go.
inline void PushOperandsOnWorkList(std::stack<Instruction*>& workList,
                                   PhiSet& phisToGo, PHINode* phiN) {
  for (User::op_iterator OI = phiN->op_begin(), OE = phiN->op_end();
       OI != OE; ++OI)
    if (Instruction* opI = dyn_cast<Instruction>(OI))
      if (!isa<PHINode>(opI) ||
          phisToGo.find(cast<PHINode>(opI)) == phisToGo.end())
        workList.push(opI);
}

void FindPhis(Instruction& X, PhiSet& phisToGo)
{
  std::stack<Instruction*> workList;
  workList.push(&X);

  // Handle the case that X itself is a Phi!
  if (PHINode* phiX = dyn_cast<PHINode>(&X))
    {
      phisToGo.insert(phiX);
      PushOperandsOnWorkList(workList, phisToGo, phiX);
    }

  // Now use a worklist to find all phis reachable from X, and
  // (recursively) all phis reachable from operands of such phis.
  for (Instruction* workI; !workList.empty(); workList.pop())
    {
      workI = workList.top();
      for (Value::use_iterator UI=workI->use_begin(), UE=workI->use_end();
           UI != UE; ++UI)
        if (PHINode* phiN = dyn_cast<PHINode>(*UI))
          if (phisToGo.find(phiN) == phisToGo.end())
            { // Seeing this phi for the first time: it must go!
              phisToGo.insert(phiN);
              workList.push(phiN);
              PushOperandsOnWorkList(workList, phisToGo, phiN);
            }
    }
}


// Create the Alloca for X
AllocaInst* CreateAllocaForX(Instruction& X)
{
  Function* parentFunc = X.getParent()->getParent();
  Instruction* entryInst = parentFunc->getEntryNode().begin();
  return new AllocaInst(X.getType(), /*arraySize*/ NULL,
                        X.hasName()? X.getName()+std::string("OnStack")
                                   : "DemotedTmp",
                        entryInst);
}

// Insert loads before all uses of I, except uses in Phis
// since all such Phis *must* be deleted.
void LoadBeforeUses(Instruction* def, AllocaInst* XSlot)
{
  for (unsigned nPhis = 0; def->use_size() - nPhis > 0; )
    {
      Instruction* useI = cast<Instruction>(def->use_back());
      if (!isa<PHINode>(useI))
        {
          LoadInst* loadI =
            new LoadInst(XSlot, std::string("Load")+XSlot->getName(), useI);
          useI->replaceUsesOfWith(def, loadI);
        }
      else
        ++nPhis;
    }
}

void AddLoadsAndStores(AllocaInst* XSlot, Instruction& X, PhiSet& phisToGo)
{
  for (PhiSetIterator PI=phisToGo.begin(), PE=phisToGo.end(); PI != PE; ++PI)
    {
      PHINode* pn = *PI;

      // First, insert loads before all uses except uses in Phis.
      // Do this first because new stores will appear as uses also!
      LoadBeforeUses(pn, XSlot);

      // For every incoming operand of the Phi, insert a store either
      // just after the instruction defining the value or just before the
      // predecessor of the Phi if the value is a formal, not an instruction.
      // 
      for (unsigned i=0, N=pn->getNumIncomingValues(); i < N; ++i)
        {
          Value* phiOp = pn->getIncomingValue(i);
          if (phiOp != &X &&
              (!isa<PHINode>(phiOp) ||
               phisToGo.find(cast<PHINode>(phiOp)) == phisToGo.end()))
            { // This operand is not a phi that will be deleted: need to store.
              assert(!isa<TerminatorInst>(phiOp));

              Instruction* storeBefore;
              if (Instruction* I = dyn_cast<Instruction>(phiOp))
                { // phiOp is an instruction, store its result right after it.
                  assert(I->getNext() && "Non-terminator without successor?");
                  storeBefore = I->getNext();
                }
              else
                { // If not, it must be a formal: store it at the end of the
                  // predecessor block of the Phi (*not* at function entry!).
                  storeBefore = pn->getIncomingBlock(i)->getTerminator();
                }
              
              // Create instr. to store the value of phiOp before `insertBefore'
              StoreInst* storeI = new StoreInst(phiOp, XSlot, storeBefore);
            }
        }
    }
}

void DeletePhis(PhiSet& phisToGo)
{
  for (PhiSetIterator PI=phisToGo.begin(), PE=phisToGo.end(); PI != PE; ++PI)
    {
      assert((*PI)->use_size() == 0 && "This PHI should be DEAD!");
      (*PI)->getParent()->getInstList().remove(*PI);
      delete *PI;
    }
  phisToGo.clear();
}

//---------------------------------------------------------------------------- 
// function DemoteRegToStack()
// 
// This function takes a virtual register computed by an
// Instruction& X and replaces it with a slot in the stack frame,
// allocated via alloca.  It has to:
// (1) Identify all Phi operations that have X as an operand and
//     transitively other Phis that use such Phis; 
// (2) Store all values merged with X via Phi operations to the stack slot;
// (3) Load the value from the stack slot just before any use of X or any
//     of the Phis that were eliminated; and
// (4) Delete all the Phis, which should all now be dead.
//
// Returns the pointer to the alloca inserted to create a stack slot for X.
//---------------------------------------------------------------------------- 

AllocaInst* DemoteRegToStack(Instruction& X)
{
  if (X.getType() == Type::VoidTy)
    return NULL;                             // nothing to do!

  // Find all Phis involving X or recursively using such Phis or Phis
  // involving operands of such Phis (essentially all Phis in the "web" of X)
  PhiSet phisToGo;
  FindPhis(X, phisToGo);

  // Create a stack slot to hold X
  AllocaInst* XSlot = CreateAllocaForX(X);

  // Insert loads before all uses of X and (*only then*) insert store after X
  assert(X.getNext() && "Non-terminator (since non-void) with no successor?");
  LoadBeforeUses(&X, XSlot);
  StoreInst* storeI = new StoreInst(&X, XSlot, X.getNext());

  // Do the same for all the phis that will be deleted
  AddLoadsAndStores(XSlot, X, phisToGo);

  // Delete the phis and return the alloca instruction
  DeletePhis(phisToGo);
  return XSlot;
}