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llvm-project/llvm/lib/Transforms/Scalar/InstructionCombining.cpp

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//===- InstructionCombining.cpp - Combine multiple instructions -----------===//
//
// InstructionCombining - Combine instructions to form fewer, simple
// instructions. This pass does not modify the CFG This pass is where algebraic
// simplification happens.
//
// This pass combines things like:
// %Y = add int 1, %X
// %Z = add int 1, %Y
// into:
// %Z = add int 2, %X
//
// This is a simple worklist driven algorithm.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/ConstantHandling.h"
#include "llvm/iMemory.h"
#include "llvm/iOther.h"
#include "llvm/iPHINode.h"
#include "llvm/iOperators.h"
#include "llvm/Pass.h"
#include "llvm/Support/InstIterator.h"
#include "llvm/Support/InstVisitor.h"
#include "Support/StatisticReporter.h"
#include <algorithm>
static Statistic<> NumCombined ("instcombine\t- Number of insts combined");
static Statistic<> NumConstProp("instcombine\t- Number of constant folds");
static Statistic<> NumDeadInst ("instcombine\t- Number of dead inst eliminate");
namespace {
class InstCombiner : public FunctionPass,
public InstVisitor<InstCombiner, Instruction*> {
// Worklist of all of the instructions that need to be simplified.
std::vector<Instruction*> WorkList;
void AddUsesToWorkList(Instruction &I) {
// The instruction was simplified, add all users of the instruction to
// the work lists because they might get more simplified now...
//
for (Value::use_iterator UI = I.use_begin(), UE = I.use_end();
UI != UE; ++UI)
WorkList.push_back(cast<Instruction>(*UI));
}
// removeFromWorkList - remove all instances of I from the worklist.
void removeFromWorkList(Instruction *I);
public:
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.preservesCFG();
}
// Visitation implementation - Implement instruction combining for different
// instruction types. The semantics are as follows:
// Return Value:
// null - No change was made
// I - Change was made, I is still valid, I may be dead though
// otherwise - Change was made, replace I with returned instruction
//
Instruction *visitAdd(BinaryOperator &I);
Instruction *visitSub(BinaryOperator &I);
Instruction *visitMul(BinaryOperator &I);
Instruction *visitDiv(BinaryOperator &I);
Instruction *visitRem(BinaryOperator &I);
Instruction *visitAnd(BinaryOperator &I);
Instruction *visitOr (BinaryOperator &I);
Instruction *visitXor(BinaryOperator &I);
Instruction *visitSetCondInst(BinaryOperator &I);
Instruction *visitShiftInst(Instruction &I);
Instruction *visitCastInst(CastInst &CI);
Instruction *visitPHINode(PHINode &PN);
Instruction *visitGetElementPtrInst(GetElementPtrInst &GEP);
// visitInstruction - Specify what to return for unhandled instructions...
Instruction *visitInstruction(Instruction &I) { return 0; }
// InsertNewInstBefore - insert an instruction New before instruction Old
// in the program. Add the new instruction to the worklist.
//
void InsertNewInstBefore(Instruction *New, Instruction &Old) {
assert(New && New->getParent() == 0 &&
"New instruction already inserted into a basic block!");
BasicBlock *BB = Old.getParent();
BB->getInstList().insert(&Old, New); // Insert inst
WorkList.push_back(New); // Add to worklist
}
// ReplaceInstUsesWith - This method is to be used when an instruction is
// found to be dead, replacable with another preexisting expression. Here
// we add all uses of I to the worklist, replace all uses of I with the new
// value, then return I, so that the inst combiner will know that I was
// modified.
//
Instruction *ReplaceInstUsesWith(Instruction &I, Value *V) {
AddUsesToWorkList(I); // Add all modified instrs to worklist
I.replaceAllUsesWith(V);
return &I;
}
};
RegisterOpt<InstCombiner> X("instcombine", "Combine redundant instructions");
}
// Make sure that this instruction has a constant on the right hand side if it
// has any constant arguments. If not, fix it an return true.
//
static bool SimplifyBinOp(BinaryOperator &I) {
if (isa<Constant>(I.getOperand(0)) && !isa<Constant>(I.getOperand(1)))
return !I.swapOperands();
return false;
}
// dyn_castNegInst - Given a 'sub' instruction, return the RHS of the
// instruction if the LHS is a constant zero (which is the 'negate' form).
//
static inline Value *dyn_castNegInst(Value *V) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I || I->getOpcode() != Instruction::Sub) return 0;
if (I->getOperand(0) == Constant::getNullValue(I->getType()))
return I->getOperand(1);
return 0;
}
static inline Value *dyn_castNotInst(Value *V) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I || I->getOpcode() != Instruction::Xor) return 0;
if (ConstantIntegral *CI = dyn_cast<ConstantIntegral>(I->getOperand(1)))
if (CI->isAllOnesValue())
return I->getOperand(0);
return 0;
}
Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
bool Changed = SimplifyBinOp(I);
Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
// Eliminate 'add int %X, 0'
if (RHS == Constant::getNullValue(I.getType()))
return ReplaceInstUsesWith(I, LHS);
// -A + B --> B - A
if (Value *V = dyn_castNegInst(LHS))
return BinaryOperator::create(Instruction::Sub, RHS, V);
// A + -B --> A - B
if (Value *V = dyn_castNegInst(RHS))
return BinaryOperator::create(Instruction::Sub, LHS, V);
// Simplify add instructions with a constant RHS...
if (Constant *Op2 = dyn_cast<Constant>(RHS)) {
if (BinaryOperator *ILHS = dyn_cast<BinaryOperator>(LHS)) {
if (ILHS->getOpcode() == Instruction::Add &&
isa<Constant>(ILHS->getOperand(1))) {
// Fold:
// %Y = add int %X, 1
// %Z = add int %Y, 1
// into:
// %Z = add int %X, 2
//
if (Constant *Val = *Op2 + *cast<Constant>(ILHS->getOperand(1))) {
I.setOperand(0, ILHS->getOperand(0));
I.setOperand(1, Val);
return &I;
}
}
}
}
return Changed ? &I : 0;
}
Instruction *InstCombiner::visitSub(BinaryOperator &I) {
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
if (Op0 == Op1) // sub X, X -> 0
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
// If this is a subtract instruction with a constant RHS, convert it to an add
// instruction of a negative constant
//
if (Constant *Op2 = dyn_cast<Constant>(Op1))
if (Constant *RHS = *Constant::getNullValue(I.getType()) - *Op2) // 0 - RHS
return BinaryOperator::create(Instruction::Add, Op0, RHS, I.getName());
// If this is a 'B = x-(-A)', change to B = x+A...
if (Value *V = dyn_castNegInst(Op1))
return BinaryOperator::create(Instruction::Add, Op0, V);
// Replace (x - (y - z)) with (x + (z - y)) if the (y - z) subexpression is
// not used by anyone else...
//
if (BinaryOperator *Op1I = dyn_cast<BinaryOperator>(Op1))
if (Op1I->use_size() == 1 && Op1I->getOpcode() == Instruction::Sub) {
// Swap the two operands of the subexpr...
Value *IIOp0 = Op1I->getOperand(0), *IIOp1 = Op1I->getOperand(1);
Op1I->setOperand(0, IIOp1);
Op1I->setOperand(1, IIOp0);
// Create the new top level add instruction...
return BinaryOperator::create(Instruction::Add, Op0, Op1);
}
return 0;
}
Instruction *InstCombiner::visitMul(BinaryOperator &I) {
bool Changed = SimplifyBinOp(I);
Value *Op1 = I.getOperand(0);
// Simplify mul instructions with a constant RHS...
if (Constant *Op2 = dyn_cast<Constant>(I.getOperand(1))) {
if (I.getType()->isInteger() && cast<ConstantInt>(Op2)->equalsInt(1))
return ReplaceInstUsesWith(I, Op1); // Eliminate 'mul int %X, 1'
if (I.getType()->isInteger() && cast<ConstantInt>(Op2)->equalsInt(2))
// Convert 'mul int %X, 2' to 'add int %X, %X'
return BinaryOperator::create(Instruction::Add, Op1, Op1, I.getName());
if (Op2->isNullValue())
return ReplaceInstUsesWith(I, Op2); // Eliminate 'mul int %X, 0'
}
return Changed ? &I : 0;
}
Instruction *InstCombiner::visitDiv(BinaryOperator &I) {
// div X, 1 == X
if (ConstantInt *RHS = dyn_cast<ConstantInt>(I.getOperand(1)))
if (RHS->equalsInt(1))
return ReplaceInstUsesWith(I, I.getOperand(0));
return 0;
}
Instruction *InstCombiner::visitRem(BinaryOperator &I) {
// rem X, 1 == 0
if (ConstantInt *RHS = dyn_cast<ConstantInt>(I.getOperand(1)))
if (RHS->equalsInt(1))
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
return 0;
}
// isMaxValueMinusOne - return true if this is Max-1
static bool isMaxValueMinusOne(const ConstantInt *C) {
if (const ConstantUInt *CU = dyn_cast<ConstantUInt>(C)) {
// Calculate -1 casted to the right type...
unsigned TypeBits = C->getType()->getPrimitiveSize()*8;
uint64_t Val = ~0ULL; // All ones
Val >>= 64-TypeBits; // Shift out unwanted 1 bits...
return CU->getValue() == Val-1;
}
const ConstantSInt *CS = cast<ConstantSInt>(C);
// Calculate 0111111111..11111
unsigned TypeBits = C->getType()->getPrimitiveSize()*8;
int64_t Val = INT64_MAX; // All ones
Val >>= 64-TypeBits; // Shift out unwanted 1 bits...
return CS->getValue() == Val-1;
}
// isMinValuePlusOne - return true if this is Min+1
static bool isMinValuePlusOne(const ConstantInt *C) {
if (const ConstantUInt *CU = dyn_cast<ConstantUInt>(C))
return CU->getValue() == 1;
const ConstantSInt *CS = cast<ConstantSInt>(C);
// Calculate 1111111111000000000000
unsigned TypeBits = C->getType()->getPrimitiveSize()*8;
int64_t Val = -1; // All ones
Val <<= TypeBits-1; // Shift over to the right spot
return CS->getValue() == Val+1;
}
Instruction *InstCombiner::visitAnd(BinaryOperator &I) {
bool Changed = SimplifyBinOp(I);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
// and X, X = X and X, 0 == 0
if (Op0 == Op1 || Op1 == Constant::getNullValue(I.getType()))
return ReplaceInstUsesWith(I, Op1);
// and X, -1 == X
if (ConstantIntegral *RHS = dyn_cast<ConstantIntegral>(Op1))
if (RHS->isAllOnesValue())
return ReplaceInstUsesWith(I, Op0);
// and (not A), (not B) == not (or A, B)
if (Op0->use_size() == 1 && Op1->use_size() == 1)
if (Value *A = dyn_castNotInst(Op0))
if (Value *B = dyn_castNotInst(Op1)) {
Instruction *Or = BinaryOperator::create(Instruction::Or, A, B,
I.getName()+".demorgan");
InsertNewInstBefore(Or, I);
return BinaryOperator::createNot(Or, I.getName());
}
return Changed ? &I : 0;
}
Instruction *InstCombiner::visitOr(BinaryOperator &I) {
bool Changed = SimplifyBinOp(I);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
// or X, X = X or X, 0 == X
if (Op0 == Op1 || Op1 == Constant::getNullValue(I.getType()))
return ReplaceInstUsesWith(I, Op0);
// or X, -1 == -1
if (ConstantIntegral *RHS = dyn_cast<ConstantIntegral>(Op1))
if (RHS->isAllOnesValue())
return ReplaceInstUsesWith(I, Op1);
return Changed ? &I : 0;
}
Instruction *InstCombiner::visitXor(BinaryOperator &I) {
bool Changed = SimplifyBinOp(I);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
// xor X, X = 0
if (Op0 == Op1)
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
if (ConstantIntegral *Op1C = dyn_cast<ConstantIntegral>(Op1)) {
// xor X, 0 == X
if (Op1C->isNullValue())
return ReplaceInstUsesWith(I, Op0);
// Is this a "NOT" instruction?
if (Op1C->isAllOnesValue()) {
// xor (xor X, -1), -1 = not (not X) = X
if (Value *X = dyn_castNotInst(Op0))
return ReplaceInstUsesWith(I, X);
// xor (setcc A, B), true = not (setcc A, B) = setncc A, B
if (SetCondInst *SCI = dyn_cast<SetCondInst>(Op0))
if (SCI->use_size() == 1)
return new SetCondInst(SCI->getInverseCondition(),
SCI->getOperand(0), SCI->getOperand(1));
}
}
return Changed ? &I : 0;
}
// AddOne, SubOne - Add or subtract a constant one from an integer constant...
static Constant *AddOne(ConstantInt *C) {
Constant *Result = *C + *ConstantInt::get(C->getType(), 1);
assert(Result && "Constant folding integer addition failed!");
return Result;
}
static Constant *SubOne(ConstantInt *C) {
Constant *Result = *C - *ConstantInt::get(C->getType(), 1);
assert(Result && "Constant folding integer addition failed!");
return Result;
}
// isTrueWhenEqual - Return true if the specified setcondinst instruction is
// true when both operands are equal...
//
static bool isTrueWhenEqual(Instruction &I) {
return I.getOpcode() == Instruction::SetEQ ||
I.getOpcode() == Instruction::SetGE ||
I.getOpcode() == Instruction::SetLE;
}
Instruction *InstCombiner::visitSetCondInst(BinaryOperator &I) {
bool Changed = SimplifyBinOp(I);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
const Type *Ty = Op0->getType();
// setcc X, X
if (Op0 == Op1)
return ReplaceInstUsesWith(I, ConstantBool::get(isTrueWhenEqual(I)));
// setcc <global*>, 0 - Global value addresses are never null!
if (isa<GlobalValue>(Op0) && isa<ConstantPointerNull>(Op1))
return ReplaceInstUsesWith(I, ConstantBool::get(!isTrueWhenEqual(I)));
// setcc's with boolean values can always be turned into bitwise operations
if (Ty == Type::BoolTy) {
// If this is <, >, or !=, we can change this into a simple xor instruction
if (!isTrueWhenEqual(I))
return BinaryOperator::create(Instruction::Xor, Op0, Op1, I.getName());
// Otherwise we need to make a temporary intermediate instruction and insert
// it into the instruction stream. This is what we are after:
//
// seteq bool %A, %B -> ~(A^B)
// setle bool %A, %B -> ~A | B
// setge bool %A, %B -> A | ~B
//
if (I.getOpcode() == Instruction::SetEQ) { // seteq case
Instruction *Xor = BinaryOperator::create(Instruction::Xor, Op0, Op1,
I.getName()+"tmp");
InsertNewInstBefore(Xor, I);
return BinaryOperator::createNot(Xor, I.getName());
}
// Handle the setXe cases...
assert(I.getOpcode() == Instruction::SetGE ||
I.getOpcode() == Instruction::SetLE);
if (I.getOpcode() == Instruction::SetGE)
std::swap(Op0, Op1); // Change setge -> setle
// Now we just have the SetLE case.
Instruction *Not = BinaryOperator::createNot(Op0, I.getName()+"tmp");
InsertNewInstBefore(Not, I);
return BinaryOperator::create(Instruction::Or, Not, Op1, I.getName());
}
// Check to see if we are doing one of many comparisons against constant
// integers at the end of their ranges...
//
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op1)) {
// Check to see if we are comparing against the minimum or maximum value...
if (CI->isMinValue()) {
if (I.getOpcode() == Instruction::SetLT) // A < MIN -> FALSE
return ReplaceInstUsesWith(I, ConstantBool::False);
if (I.getOpcode() == Instruction::SetGE) // A >= MIN -> TRUE
return ReplaceInstUsesWith(I, ConstantBool::True);
if (I.getOpcode() == Instruction::SetLE) // A <= MIN -> A == MIN
return BinaryOperator::create(Instruction::SetEQ, Op0,Op1, I.getName());
if (I.getOpcode() == Instruction::SetGT) // A > MIN -> A != MIN
return BinaryOperator::create(Instruction::SetNE, Op0,Op1, I.getName());
} else if (CI->isMaxValue()) {
if (I.getOpcode() == Instruction::SetGT) // A > MAX -> FALSE
return ReplaceInstUsesWith(I, ConstantBool::False);
if (I.getOpcode() == Instruction::SetLE) // A <= MAX -> TRUE
return ReplaceInstUsesWith(I, ConstantBool::True);
if (I.getOpcode() == Instruction::SetGE) // A >= MAX -> A == MAX
return BinaryOperator::create(Instruction::SetEQ, Op0,Op1, I.getName());
if (I.getOpcode() == Instruction::SetLT) // A < MAX -> A != MAX
return BinaryOperator::create(Instruction::SetNE, Op0,Op1, I.getName());
// Comparing against a value really close to min or max?
} else if (isMinValuePlusOne(CI)) {
if (I.getOpcode() == Instruction::SetLT) // A < MIN+1 -> A == MIN
return BinaryOperator::create(Instruction::SetEQ, Op0,
SubOne(CI), I.getName());
if (I.getOpcode() == Instruction::SetGE) // A >= MIN-1 -> A != MIN
return BinaryOperator::create(Instruction::SetNE, Op0,
SubOne(CI), I.getName());
} else if (isMaxValueMinusOne(CI)) {
if (I.getOpcode() == Instruction::SetGT) // A > MAX-1 -> A == MAX
return BinaryOperator::create(Instruction::SetEQ, Op0,
AddOne(CI), I.getName());
if (I.getOpcode() == Instruction::SetLE) // A <= MAX-1 -> A != MAX
return BinaryOperator::create(Instruction::SetNE, Op0,
AddOne(CI), I.getName());
}
}
return Changed ? &I : 0;
}
Instruction *InstCombiner::visitShiftInst(Instruction &I) {
assert(I.getOperand(1)->getType() == Type::UByteTy);
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
// shl X, 0 == X and shr X, 0 == X
// shl 0, X == 0 and shr 0, X == 0
if (Op1 == Constant::getNullValue(Type::UByteTy) ||
Op0 == Constant::getNullValue(Op0->getType()))
return ReplaceInstUsesWith(I, Op0);
// shl uint X, 32 = 0 and shr ubyte Y, 9 = 0, ... just don't eliminate shr of
// a signed value.
//
if (ConstantUInt *CUI = dyn_cast<ConstantUInt>(Op1)) {
unsigned TypeBits = Op0->getType()->getPrimitiveSize()*8;
if (CUI->getValue() >= TypeBits &&
!(Op0->getType()->isSigned() && I.getOpcode() == Instruction::Shr))
return ReplaceInstUsesWith(I, Constant::getNullValue(Op0->getType()));
}
return 0;
}
// isEliminableCastOfCast - Return true if it is valid to eliminate the CI
// instruction.
//
static inline bool isEliminableCastOfCast(const CastInst &CI,
const CastInst *CSrc) {
assert(CI.getOperand(0) == CSrc);
const Type *SrcTy = CSrc->getOperand(0)->getType();
const Type *MidTy = CSrc->getType();
const Type *DstTy = CI.getType();
// It is legal to eliminate the instruction if casting A->B->A if the sizes
// are identical and the bits don't get reinterpreted (for example
// int->float->int would not be allowed)
if (SrcTy == DstTy && SrcTy->isLosslesslyConvertableTo(MidTy))
return true;
// Allow free casting and conversion of sizes as long as the sign doesn't
// change...
if (SrcTy->isIntegral() && MidTy->isIntegral() && DstTy->isIntegral()) {
unsigned SrcSize = SrcTy->getPrimitiveSize();
unsigned MidSize = MidTy->getPrimitiveSize();
unsigned DstSize = DstTy->getPrimitiveSize();
// Cases where we are monotonically decreasing the size of the type are
// always ok, regardless of what sign changes are going on.
//
if (SrcSize >= MidSize && MidSize >= DstSize)
return true;
// If we are monotonically growing, things are more complex.
//
if (SrcSize <= MidSize && MidSize <= DstSize) {
// We have eight combinations of signedness to worry about. Here's the
// table:
static const int SignTable[8] = {
// CODE, SrcSigned, MidSigned, DstSigned, Comment
1, // U U U Always ok
1, // U U S Always ok
3, // U S U Ok iff SrcSize != MidSize
3, // U S S Ok iff SrcSize != MidSize
0, // S U U Never ok
2, // S U S Ok iff MidSize == DstSize
1, // S S U Always ok
1, // S S S Always ok
};
// Choose an action based on the current entry of the signtable that this
// cast of cast refers to...
unsigned Row = SrcTy->isSigned()*4+MidTy->isSigned()*2+DstTy->isSigned();
switch (SignTable[Row]) {
case 0: return false; // Never ok
case 1: return true; // Always ok
case 2: return MidSize == DstSize; // Ok iff MidSize == DstSize
case 3: // Ok iff SrcSize != MidSize
return SrcSize != MidSize || SrcTy == Type::BoolTy;
default: assert(0 && "Bad entry in sign table!");
}
}
}
// Otherwise, we cannot succeed. Specifically we do not want to allow things
// like: short -> ushort -> uint, because this can create wrong results if
// the input short is negative!
//
return false;
}
// CastInst simplification
//
Instruction *InstCombiner::visitCastInst(CastInst &CI) {
// If the user is casting a value to the same type, eliminate this cast
// instruction...
if (CI.getType() == CI.getOperand(0)->getType())
return ReplaceInstUsesWith(CI, CI.getOperand(0));
// If casting the result of another cast instruction, try to eliminate this
// one!
//
if (CastInst *CSrc = dyn_cast<CastInst>(CI.getOperand(0))) {
if (isEliminableCastOfCast(CI, CSrc)) {
// This instruction now refers directly to the cast's src operand. This
// has a good chance of making CSrc dead.
CI.setOperand(0, CSrc->getOperand(0));
return &CI;
}
// If this is an A->B->A cast, and we are dealing with integral types, try
// to convert this into a logical 'and' instruction.
//
if (CSrc->getOperand(0)->getType() == CI.getType() &&
CI.getType()->isInteger() && CSrc->getType()->isInteger() &&
CI.getType()->isUnsigned() && CSrc->getType()->isUnsigned() &&
CSrc->getType()->getPrimitiveSize() < CI.getType()->getPrimitiveSize()){
assert(CSrc->getType() != Type::ULongTy &&
"Cannot have type bigger than ulong!");
unsigned AndValue = (1U << CSrc->getType()->getPrimitiveSize()*8)-1;
Constant *AndOp = ConstantUInt::get(CI.getType(), AndValue);
return BinaryOperator::create(Instruction::And, CSrc->getOperand(0),
AndOp);
}
}
return 0;
}
// PHINode simplification
//
Instruction *InstCombiner::visitPHINode(PHINode &PN) {
// If the PHI node only has one incoming value, eliminate the PHI node...
if (PN.getNumIncomingValues() == 0)
return ReplaceInstUsesWith(PN, Constant::getNullValue(PN.getType()));
if (PN.getNumIncomingValues() == 1)
return ReplaceInstUsesWith(PN, PN.getIncomingValue(0));
// Otherwise if all of the incoming values are the same for the PHI, replace
// the PHI node with the incoming value.
//
Value *InVal = 0;
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
if (PN.getIncomingValue(i) != &PN) // Not the PHI node itself...
if (InVal && PN.getIncomingValue(i) != InVal)
return 0; // Not the same, bail out.
else
InVal = PN.getIncomingValue(i);
// The only case that could cause InVal to be null is if we have a PHI node
// that only has entries for itself. In this case, there is no entry into the
// loop, so kill the PHI.
//
if (InVal == 0) InVal = Constant::getNullValue(PN.getType());
// All of the incoming values are the same, replace the PHI node now.
return ReplaceInstUsesWith(PN, InVal);
}
Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
// Is it 'getelementptr %P, uint 0' or 'getelementptr %P'
// If so, eliminate the noop.
if ((GEP.getNumOperands() == 2 &&
GEP.getOperand(1) == Constant::getNullValue(Type::UIntTy)) ||
GEP.getNumOperands() == 1)
return ReplaceInstUsesWith(GEP, GEP.getOperand(0));
// Combine Indices - If the source pointer to this getelementptr instruction
// is a getelementptr instruction, combine the indices of the two
// getelementptr instructions into a single instruction.
//
if (GetElementPtrInst *Src = dyn_cast<GetElementPtrInst>(GEP.getOperand(0))) {
std::vector<Value *> Indices;
// Can we combine the two pointer arithmetics offsets?
if (Src->getNumOperands() == 2 && isa<Constant>(Src->getOperand(1)) &&
isa<Constant>(GEP.getOperand(1))) {
// Replace the index list on this GEP with the index on the getelementptr
Indices.insert(Indices.end(), GEP.idx_begin(), GEP.idx_end());
Indices[0] = *cast<Constant>(Src->getOperand(1)) +
*cast<Constant>(GEP.getOperand(1));
assert(Indices[0] != 0 && "Constant folding of uint's failed!?");
} else if (*GEP.idx_begin() == ConstantUInt::get(Type::UIntTy, 0)) {
// Otherwise we can do the fold if the first index of the GEP is a zero
Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
Indices.insert(Indices.end(), GEP.idx_begin()+1, GEP.idx_end());
}
if (!Indices.empty())
return new GetElementPtrInst(Src->getOperand(0), Indices, GEP.getName());
} else if (GlobalValue *GV = dyn_cast<GlobalValue>(GEP.getOperand(0))) {
// GEP of global variable. If all of the indices for this GEP are
// constants, we can promote this to a constexpr instead of an instruction.
// Scan for nonconstants...
std::vector<Constant*> Indices;
User::op_iterator I = GEP.idx_begin(), E = GEP.idx_end();
for (; I != E && isa<Constant>(*I); ++I)
Indices.push_back(cast<Constant>(*I));
if (I == E) { // If they are all constants...
ConstantExpr *CE =
ConstantExpr::getGetElementPtr(ConstantPointerRef::get(GV), Indices);
// Replace all uses of the GEP with the new constexpr...
return ReplaceInstUsesWith(GEP, CE);
}
}
return 0;
}
void InstCombiner::removeFromWorkList(Instruction *I) {
WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I),
WorkList.end());
}
bool InstCombiner::runOnFunction(Function &F) {
bool Changed = false;
WorkList.insert(WorkList.end(), inst_begin(F), inst_end(F));
while (!WorkList.empty()) {
Instruction *I = WorkList.back(); // Get an instruction from the worklist
WorkList.pop_back();
// Check to see if we can DCE or ConstantPropogate the instruction...
// Check to see if we can DIE the instruction...
if (isInstructionTriviallyDead(I)) {
// Add operands to the worklist...
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(i)))
WorkList.push_back(Op);
++NumDeadInst;
BasicBlock::iterator BBI = I;
if (dceInstruction(BBI)) {
removeFromWorkList(I);
continue;
}
}
// Instruction isn't dead, see if we can constant propogate it...
if (Constant *C = ConstantFoldInstruction(I)) {
// Add operands to the worklist...
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(i)))
WorkList.push_back(Op);
I->replaceAllUsesWith(C);
++NumConstProp;
BasicBlock::iterator BBI = I;
if (dceInstruction(BBI)) {
removeFromWorkList(I);
continue;
}
}
// Now that we have an instruction, try combining it to simplify it...
if (Instruction *Result = visit(*I)) {
++NumCombined;
// Should we replace the old instruction with a new one?
if (Result != I) {
// Instructions can end up on the worklist more than once. Make sure
// we do not process an instruction that has been deleted.
removeFromWorkList(I);
ReplaceInstWithInst(I, Result);
} else {
BasicBlock::iterator II = I;
// If the instruction was modified, it's possible that it is now dead.
// if so, remove it.
if (dceInstruction(II)) {
// Instructions may end up in the worklist more than once. Erase them
// all.
removeFromWorkList(I);
Result = 0;
}
}
if (Result) {
WorkList.push_back(Result);
AddUsesToWorkList(*Result);
}
Changed = true;
}
}
return Changed;
}
Pass *createInstructionCombiningPass() {
return new InstCombiner();
}
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