lib/Analysis/ScalarEvolutionExpander.cpp - llvm - Git at Google

 //===- ScalarEvolutionExpander.cpp - Scalar Evolution Analysis --*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file contains the implementation of the scalar evolution expander,
 // which is used to generate the code corresponding to a given scalar evolution
 // expression.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Analysis/ScalarEvolutionExpander.h"
 #include "llvm/Analysis/LoopInfo.h"
 using namespace llvm;

 /// InsertCastOfTo - Insert a cast of V to the specified type, doing what
 /// we can to share the casts.
 Value *SCEVExpander::InsertCastOfTo(Instruction::CastOps opcode, Value *V,
                                     const Type *Ty) {
   // FIXME: keep track of the cast instruction.
   if (Constant *C = dyn_cast<Constant>(V))
     return ConstantExpr::getCast(opcode, C, Ty);

   if (Argument *A = dyn_cast<Argument>(V)) {
     // Check to see if there is already a cast!
     for (Value::use_iterator UI = A->use_begin(), E = A->use_end();
          UI != E; ++UI) {
       if ((*UI)->getType() == Ty)
         if (CastInst *CI = dyn_cast<CastInst>(cast<Instruction>(*UI)))
           if (CI->getOpcode() == opcode) {
             // If the cast isn't the first instruction of the function, move it.
             if (BasicBlock::iterator(CI) !=
                 A->getParent()->getEntryBlock().begin()) {
               CI->moveBefore(A->getParent()->getEntryBlock().begin());
             }
             return CI;
           }
     }
     return CastInst::Create(opcode, V, Ty, V->getName(),
                             A->getParent()->getEntryBlock().begin());
   }

   Instruction *I = cast<Instruction>(V);

   // Check to see if there is already a cast.  If there is, use it.
   for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
        UI != E; ++UI) {
     if ((*UI)->getType() == Ty)
       if (CastInst *CI = dyn_cast<CastInst>(cast<Instruction>(*UI)))
         if (CI->getOpcode() == opcode) {
           BasicBlock::iterator It = I; ++It;
           if (isa<InvokeInst>(I))
             It = cast<InvokeInst>(I)->getNormalDest()->begin();
           while (isa<PHINode>(It)) ++It;
           if (It != BasicBlock::iterator(CI)) {
             // Splice the cast immediately after the operand in question.
             CI->moveBefore(It);
           }
           return CI;
         }
   }
   BasicBlock::iterator IP = I; ++IP;
   if (InvokeInst *II = dyn_cast<InvokeInst>(I))
     IP = II->getNormalDest()->begin();
   while (isa<PHINode>(IP)) ++IP;
   return CastInst::Create(opcode, V, Ty, V->getName(), IP);
 }

 /// InsertBinop - Insert the specified binary operator, doing a small amount
 /// of work to avoid inserting an obviously redundant operation.
 Value *SCEVExpander::InsertBinop(Instruction::BinaryOps Opcode, Value *LHS,
                                  Value *RHS, Instruction *InsertPt) {
   // Fold a binop with constant operands.
   if (Constant *CLHS = dyn_cast<Constant>(LHS))
     if (Constant *CRHS = dyn_cast<Constant>(RHS))
       return ConstantExpr::get(Opcode, CLHS, CRHS);

   // Do a quick scan to see if we have this binop nearby.  If so, reuse it.
   unsigned ScanLimit = 6;
   BasicBlock::iterator BlockBegin = InsertPt->getParent()->begin();
   if (InsertPt != BlockBegin) {
     // Scanning starts from the last instruction before InsertPt.
     BasicBlock::iterator IP = InsertPt;
     --IP;
     for (; ScanLimit; --IP, --ScanLimit) {
       if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(IP))
         if (BinOp->getOpcode() == Opcode && BinOp->getOperand(0) == LHS &&
             BinOp->getOperand(1) == RHS)
           return BinOp;
       if (IP == BlockBegin) break;
     }
   }

   // If we haven't found this binop, insert it.
   return BinaryOperator::Create(Opcode, LHS, RHS, "tmp", InsertPt);
 }

 Value *SCEVExpander::visitAddExpr(SCEVAddExpr *S) {
   Value *V = expand(S->getOperand(S->getNumOperands()-1));

   // Emit a bunch of add instructions
   for (int i = S->getNumOperands()-2; i >= 0; --i)
     V = InsertBinop(Instruction::Add, V, expand(S->getOperand(i)),
                     InsertPt);
   return V;
 }

 Value *SCEVExpander::visitMulExpr(SCEVMulExpr *S) {
   int FirstOp = 0;  // Set if we should emit a subtract.
   if (SCEVConstant *SC = dyn_cast<SCEVConstant>(S->getOperand(0)))
     if (SC->getValue()->isAllOnesValue())
       FirstOp = 1;

   int i = S->getNumOperands()-2;
   Value *V = expand(S->getOperand(i+1));

   // Emit a bunch of multiply instructions
   for (; i >= FirstOp; --i)
     V = InsertBinop(Instruction::Mul, V, expand(S->getOperand(i)),
                     InsertPt);
   // -1 * ...  --->  0 - ...
   if (FirstOp == 1)
     V = InsertBinop(Instruction::Sub, Constant::getNullValue(V->getType()), V,
                     InsertPt);
   return V;
 }

 Value *SCEVExpander::visitUDivExpr(SCEVUDivExpr *S) {
   Value *LHS = expand(S->getLHS());
   if (SCEVConstant *SC = dyn_cast<SCEVConstant>(S->getRHS())) {
     const APInt &RHS = SC->getValue()->getValue();
     if (RHS.isPowerOf2())
       return InsertBinop(Instruction::LShr, LHS,
                          ConstantInt::get(S->getType(), RHS.logBase2()),
                          InsertPt);
   }

   Value *RHS = expand(S->getRHS());
   return InsertBinop(Instruction::UDiv, LHS, RHS, InsertPt);
 }

 Value *SCEVExpander::visitAddRecExpr(SCEVAddRecExpr *S) {
   const Type *Ty = S->getType();
   const Loop *L = S->getLoop();
   // We cannot yet do fp recurrences, e.g. the xform of {X,+,F} --> X+{0,+,F}
   assert(Ty->isInteger() && "Cannot expand fp recurrences yet!");

   // {X,+,F} --> X + {0,+,F}
   if (!S->getStart()->isZero()) {
     Value *Start = expand(S->getStart());
     std::vector<SCEVHandle> NewOps(S->op_begin(), S->op_end());
     NewOps[0] = SE.getIntegerSCEV(0, Ty);
     Value *Rest = expand(SE.getAddRecExpr(NewOps, L));

     // FIXME: look for an existing add to use.
     return InsertBinop(Instruction::Add, Rest, Start, InsertPt);
   }

   // {0,+,1} --> Insert a canonical induction variable into the loop!
   if (S->isAffine() &&
       S->getOperand(1) == SE.getIntegerSCEV(1, Ty)) {
     // Create and insert the PHI node for the induction variable in the
     // specified loop.
     BasicBlock *Header = L->getHeader();
     PHINode *PN = PHINode::Create(Ty, "indvar", Header->begin());
     PN->addIncoming(Constant::getNullValue(Ty), L->getLoopPreheader());

     pred_iterator HPI = pred_begin(Header);
     assert(HPI != pred_end(Header) && "Loop with zero preds???");
     if (!L->contains(*HPI)) ++HPI;
     assert(HPI != pred_end(Header) && L->contains(*HPI) &&
            "No backedge in loop?");

     // Insert a unit add instruction right before the terminator corresponding
     // to the back-edge.
     Constant *One = ConstantInt::get(Ty, 1);
     Instruction *Add = BinaryOperator::CreateAdd(PN, One, "indvar.next",
                                                  (*HPI)->getTerminator());

     pred_iterator PI = pred_begin(Header);
     if (*PI == L->getLoopPreheader())
       ++PI;
     PN->addIncoming(Add, *PI);
     return PN;
   }

   // Get the canonical induction variable I for this loop.
   Value *I = getOrInsertCanonicalInductionVariable(L, Ty);

   // If this is a simple linear addrec, emit it now as a special case.
   if (S->isAffine()) {   // {0,+,F} --> i*F
     Value *F = expand(S->getOperand(1));

     // IF the step is by one, just return the inserted IV.
     if (ConstantInt *CI = dyn_cast<ConstantInt>(F))
       if (CI->getValue() == 1)
         return I;

     // If the insert point is directly inside of the loop, emit the multiply at
     // the insert point.  Otherwise, L is a loop that is a parent of the insert
     // point loop.  If we can, move the multiply to the outer most loop that it
     // is safe to be in.
     Instruction *MulInsertPt = InsertPt;
     Loop *InsertPtLoop = LI.getLoopFor(MulInsertPt->getParent());
     if (InsertPtLoop != L && InsertPtLoop &&
         L->contains(InsertPtLoop->getHeader())) {
       do {
         // If we cannot hoist the multiply out of this loop, don't.
         if (!InsertPtLoop->isLoopInvariant(F)) break;

         BasicBlock *InsertPtLoopPH = InsertPtLoop->getLoopPreheader();

         // If this loop hasn't got a preheader, we aren't able to hoist the
         // multiply.
         if (!InsertPtLoopPH)
           break;

         // Otherwise, move the insert point to the preheader.
         MulInsertPt = InsertPtLoopPH->getTerminator();
         InsertPtLoop = InsertPtLoop->getParentLoop();
       } while (InsertPtLoop != L);
     }

     return InsertBinop(Instruction::Mul, I, F, MulInsertPt);
   }

   // If this is a chain of recurrences, turn it into a closed form, using the
   // folders, then expandCodeFor the closed form.  This allows the folders to
   // simplify the expression without having to build a bunch of special code
   // into this folder.
   SCEVHandle IH = SE.getUnknown(I);   // Get I as a "symbolic" SCEV.

   SCEVHandle V = S->evaluateAtIteration(IH, SE);
   //cerr << "Evaluated: " << *this << "\n     to: " << *V << "\n";

   return expand(V);
 }

 Value *SCEVExpander::visitTruncateExpr(SCEVTruncateExpr *S) {
   Value *V = expand(S->getOperand());
   return CastInst::createTruncOrBitCast(V, S->getType(), "tmp.", InsertPt);
 }

 Value *SCEVExpander::visitZeroExtendExpr(SCEVZeroExtendExpr *S) {
   Value *V = expand(S->getOperand());
   return CastInst::createZExtOrBitCast(V, S->getType(), "tmp.", InsertPt);
 }

 Value *SCEVExpander::visitSignExtendExpr(SCEVSignExtendExpr *S) {
   Value *V = expand(S->getOperand());
   return CastInst::createSExtOrBitCast(V, S->getType(), "tmp.", InsertPt);
 }

 Value *SCEVExpander::visitSMaxExpr(SCEVSMaxExpr *S) {
   Value *LHS = expand(S->getOperand(0));
   for (unsigned i = 1; i < S->getNumOperands(); ++i) {
     Value *RHS = expand(S->getOperand(i));
     Value *ICmp = new ICmpInst(ICmpInst::ICMP_SGT, LHS, RHS, "tmp", InsertPt);
     LHS = SelectInst::Create(ICmp, LHS, RHS, "smax", InsertPt);
   }
   return LHS;
 }

 Value *SCEVExpander::visitUMaxExpr(SCEVUMaxExpr *S) {
   Value *LHS = expand(S->getOperand(0));
   for (unsigned i = 1; i < S->getNumOperands(); ++i) {
     Value *RHS = expand(S->getOperand(i));
     Value *ICmp = new ICmpInst(ICmpInst::ICMP_UGT, LHS, RHS, "tmp", InsertPt);
     LHS = SelectInst::Create(ICmp, LHS, RHS, "umax", InsertPt);
   }
   return LHS;
 }

 Value *SCEVExpander::expandCodeFor(SCEVHandle SH, Instruction *IP) {
   // Expand the code for this SCEV.
   this->InsertPt = IP;
   return expand(SH);
 }

 Value *SCEVExpander::expand(SCEV *S) {
   // Check to see if we already expanded this.
   std::map<SCEVHandle, Value*>::iterator I = InsertedExpressions.find(S);
   if (I != InsertedExpressions.end())
     return I->second;

   Value *V = visit(S);
   InsertedExpressions[S] = V;
   return V;
 }
	//===- ScalarEvolutionExpander.cpp - Scalar Evolution Analysis --- C++ --===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file contains the implementation of the scalar evolution expander,
	// which is used to generate the code corresponding to a given scalar evolution
	// expression.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Analysis/ScalarEvolutionExpander.h"
	#include "llvm/Analysis/LoopInfo.h"
	using namespace llvm;

	/// InsertCastOfTo - Insert a cast of V to the specified type, doing what
	/// we can to share the casts.
	Value SCEVExpander::InsertCastOfTo(Instruction::CastOps opcode, Value V,
	const Type *Ty) {
	// FIXME: keep track of the cast instruction.
	if (Constant *C = dyn_cast<Constant>(V))
	return ConstantExpr::getCast(opcode, C, Ty);

	if (Argument *A = dyn_cast<Argument>(V)) {
	// Check to see if there is already a cast!
	for (Value::use_iterator UI = A->use_begin(), E = A->use_end();
	UI != E; ++UI) {
	if ((*UI)->getType() == Ty)
	if (CastInst CI = dyn_cast<CastInst>(cast<Instruction>(UI)))
	if (CI->getOpcode() == opcode) {
	// If the cast isn't the first instruction of the function, move it.
	if (BasicBlock::iterator(CI) !=
	A->getParent()->getEntryBlock().begin()) {
	CI->moveBefore(A->getParent()->getEntryBlock().begin());
	}
	return CI;
	}
	}
	return CastInst::Create(opcode, V, Ty, V->getName(),
	A->getParent()->getEntryBlock().begin());
	}

	Instruction *I = cast<Instruction>(V);

	// Check to see if there is already a cast. If there is, use it.
	for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
	UI != E; ++UI) {
	if ((*UI)->getType() == Ty)
	if (CastInst CI = dyn_cast<CastInst>(cast<Instruction>(UI)))
	if (CI->getOpcode() == opcode) {
	BasicBlock::iterator It = I; ++It;
	if (isa<InvokeInst>(I))
	It = cast<InvokeInst>(I)->getNormalDest()->begin();
	while (isa<PHINode>(It)) ++It;
	if (It != BasicBlock::iterator(CI)) {
	// Splice the cast immediately after the operand in question.
	CI->moveBefore(It);
	}
	return CI;
	}
	}
	BasicBlock::iterator IP = I; ++IP;
	if (InvokeInst *II = dyn_cast<InvokeInst>(I))
	IP = II->getNormalDest()->begin();
	while (isa<PHINode>(IP)) ++IP;
	return CastInst::Create(opcode, V, Ty, V->getName(), IP);
	}

	/// InsertBinop - Insert the specified binary operator, doing a small amount
	/// of work to avoid inserting an obviously redundant operation.
	Value SCEVExpander::InsertBinop(Instruction::BinaryOps Opcode, Value LHS,
	Value RHS, Instruction InsertPt) {
	// Fold a binop with constant operands.
	if (Constant *CLHS = dyn_cast<Constant>(LHS))
	if (Constant *CRHS = dyn_cast<Constant>(RHS))
	return ConstantExpr::get(Opcode, CLHS, CRHS);

	// Do a quick scan to see if we have this binop nearby. If so, reuse it.
	unsigned ScanLimit = 6;
	BasicBlock::iterator BlockBegin = InsertPt->getParent()->begin();
	if (InsertPt != BlockBegin) {
	// Scanning starts from the last instruction before InsertPt.
	BasicBlock::iterator IP = InsertPt;
	--IP;
	for (; ScanLimit; --IP, --ScanLimit) {
	if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(IP))
	if (BinOp->getOpcode() == Opcode && BinOp->getOperand(0) == LHS &&
	BinOp->getOperand(1) == RHS)
	return BinOp;
	if (IP == BlockBegin) break;
	}
	}

	// If we haven't found this binop, insert it.
	return BinaryOperator::Create(Opcode, LHS, RHS, "tmp", InsertPt);
	}

	Value SCEVExpander::visitAddExpr(SCEVAddExpr S) {
	Value *V = expand(S->getOperand(S->getNumOperands()-1));

	// Emit a bunch of add instructions
	for (int i = S->getNumOperands()-2; i >= 0; --i)
	V = InsertBinop(Instruction::Add, V, expand(S->getOperand(i)),
	InsertPt);
	return V;
	}

	Value SCEVExpander::visitMulExpr(SCEVMulExpr S) {
	int FirstOp = 0; // Set if we should emit a subtract.
	if (SCEVConstant *SC = dyn_cast<SCEVConstant>(S->getOperand(0)))
	if (SC->getValue()->isAllOnesValue())
	FirstOp = 1;

	int i = S->getNumOperands()-2;
	Value *V = expand(S->getOperand(i+1));

	// Emit a bunch of multiply instructions
	for (; i >= FirstOp; --i)
	V = InsertBinop(Instruction::Mul, V, expand(S->getOperand(i)),
	InsertPt);
	// -1 * ... ---> 0 - ...
	if (FirstOp == 1)
	V = InsertBinop(Instruction::Sub, Constant::getNullValue(V->getType()), V,
	InsertPt);
	return V;
	}

	Value SCEVExpander::visitUDivExpr(SCEVUDivExpr S) {
	Value *LHS = expand(S->getLHS());
	if (SCEVConstant *SC = dyn_cast<SCEVConstant>(S->getRHS())) {
	const APInt &RHS = SC->getValue()->getValue();
	if (RHS.isPowerOf2())
	return InsertBinop(Instruction::LShr, LHS,
	ConstantInt::get(S->getType(), RHS.logBase2()),
	InsertPt);
	}

	Value *RHS = expand(S->getRHS());
	return InsertBinop(Instruction::UDiv, LHS, RHS, InsertPt);
	}

	Value SCEVExpander::visitAddRecExpr(SCEVAddRecExpr S) {
	const Type *Ty = S->getType();
	const Loop *L = S->getLoop();
	// We cannot yet do fp recurrences, e.g. the xform of {X,+,F} --> X+{0,+,F}
	assert(Ty->isInteger() && "Cannot expand fp recurrences yet!");

	// {X,+,F} --> X + {0,+,F}
	if (!S->getStart()->isZero()) {
	Value *Start = expand(S->getStart());
	std::vector<SCEVHandle> NewOps(S->op_begin(), S->op_end());
	NewOps[0] = SE.getIntegerSCEV(0, Ty);
	Value *Rest = expand(SE.getAddRecExpr(NewOps, L));

	// FIXME: look for an existing add to use.
	return InsertBinop(Instruction::Add, Rest, Start, InsertPt);
	}

	// {0,+,1} --> Insert a canonical induction variable into the loop!
	if (S->isAffine() &&
	S->getOperand(1) == SE.getIntegerSCEV(1, Ty)) {
	// Create and insert the PHI node for the induction variable in the
	// specified loop.
	BasicBlock *Header = L->getHeader();
	PHINode *PN = PHINode::Create(Ty, "indvar", Header->begin());
	PN->addIncoming(Constant::getNullValue(Ty), L->getLoopPreheader());

	pred_iterator HPI = pred_begin(Header);
	assert(HPI != pred_end(Header) && "Loop with zero preds???");
	if (!L->contains(*HPI)) ++HPI;
	assert(HPI != pred_end(Header) && L->contains(*HPI) &&
	"No backedge in loop?");

	// Insert a unit add instruction right before the terminator corresponding
	// to the back-edge.
	Constant *One = ConstantInt::get(Ty, 1);
	Instruction *Add = BinaryOperator::CreateAdd(PN, One, "indvar.next",
	(*HPI)->getTerminator());

	pred_iterator PI = pred_begin(Header);
	if (*PI == L->getLoopPreheader())
	++PI;
	PN->addIncoming(Add, *PI);
	return PN;
	}

	// Get the canonical induction variable I for this loop.
	Value *I = getOrInsertCanonicalInductionVariable(L, Ty);

	// If this is a simple linear addrec, emit it now as a special case.
	if (S->isAffine()) { // {0,+,F} --> i*F
	Value *F = expand(S->getOperand(1));

	// IF the step is by one, just return the inserted IV.
	if (ConstantInt *CI = dyn_cast<ConstantInt>(F))
	if (CI->getValue() == 1)
	return I;

	// If the insert point is directly inside of the loop, emit the multiply at
	// the insert point. Otherwise, L is a loop that is a parent of the insert
	// point loop. If we can, move the multiply to the outer most loop that it
	// is safe to be in.
	Instruction *MulInsertPt = InsertPt;
	Loop *InsertPtLoop = LI.getLoopFor(MulInsertPt->getParent());
	if (InsertPtLoop != L && InsertPtLoop &&
	L->contains(InsertPtLoop->getHeader())) {
	do {
	// If we cannot hoist the multiply out of this loop, don't.
	if (!InsertPtLoop->isLoopInvariant(F)) break;

	BasicBlock *InsertPtLoopPH = InsertPtLoop->getLoopPreheader();

	// If this loop hasn't got a preheader, we aren't able to hoist the
	// multiply.
	if (!InsertPtLoopPH)
	break;

	// Otherwise, move the insert point to the preheader.
	MulInsertPt = InsertPtLoopPH->getTerminator();
	InsertPtLoop = InsertPtLoop->getParentLoop();
	} while (InsertPtLoop != L);
	}

	return InsertBinop(Instruction::Mul, I, F, MulInsertPt);
	}

	// If this is a chain of recurrences, turn it into a closed form, using the
	// folders, then expandCodeFor the closed form. This allows the folders to
	// simplify the expression without having to build a bunch of special code
	// into this folder.
	SCEVHandle IH = SE.getUnknown(I); // Get I as a "symbolic" SCEV.

	SCEVHandle V = S->evaluateAtIteration(IH, SE);
	//cerr << "Evaluated: " << this << "\n to: " << V << "\n";

	return expand(V);
	}

	Value SCEVExpander::visitTruncateExpr(SCEVTruncateExpr S) {
	Value *V = expand(S->getOperand());
	return CastInst::createTruncOrBitCast(V, S->getType(), "tmp.", InsertPt);
	}

	Value SCEVExpander::visitZeroExtendExpr(SCEVZeroExtendExpr S) {
	Value *V = expand(S->getOperand());
	return CastInst::createZExtOrBitCast(V, S->getType(), "tmp.", InsertPt);
	}

	Value SCEVExpander::visitSignExtendExpr(SCEVSignExtendExpr S) {
	Value *V = expand(S->getOperand());
	return CastInst::createSExtOrBitCast(V, S->getType(), "tmp.", InsertPt);
	}

	Value SCEVExpander::visitSMaxExpr(SCEVSMaxExpr S) {
	Value *LHS = expand(S->getOperand(0));
	for (unsigned i = 1; i < S->getNumOperands(); ++i) {
	Value *RHS = expand(S->getOperand(i));
	Value *ICmp = new ICmpInst(ICmpInst::ICMP_SGT, LHS, RHS, "tmp", InsertPt);
	LHS = SelectInst::Create(ICmp, LHS, RHS, "smax", InsertPt);
	}
	return LHS;
	}

	Value SCEVExpander::visitUMaxExpr(SCEVUMaxExpr S) {
	Value *LHS = expand(S->getOperand(0));
	for (unsigned i = 1; i < S->getNumOperands(); ++i) {
	Value *RHS = expand(S->getOperand(i));
	Value *ICmp = new ICmpInst(ICmpInst::ICMP_UGT, LHS, RHS, "tmp", InsertPt);
	LHS = SelectInst::Create(ICmp, LHS, RHS, "umax", InsertPt);
	}
	return LHS;
	}

	Value SCEVExpander::expandCodeFor(SCEVHandle SH, Instruction IP) {
	// Expand the code for this SCEV.
	this->InsertPt = IP;
	return expand(SH);
	}

	Value SCEVExpander::expand(SCEV S) {
	// Check to see if we already expanded this.
	std::map<SCEVHandle, Value*>::iterator I = InsertedExpressions.find(S);
	if (I != InsertedExpressions.end())
	return I->second;

	Value *V = visit(S);
	InsertedExpressions[S] = V;
	return V;
	}