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llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
Rafael Espindola 26c8dcc692 Always compute all the bits in ComputeMaskedBits. 
This allows us to keep passing reduced masks to SimplifyDemandedBits, but
know about all the bits if SimplifyDemandedBits fails. This allows instcombine
to simplify cases like the one in the included testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154011 91177308-0d34-0410-b5e6-96231b3b80d8
2012-04-04 12:51:34 +00:00

1153 lines
48 KiB
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//===- InstCombineSimplifyDemanded.cpp ------------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains logic for simplifying instructions based on information
// about how they are used.
//
//===----------------------------------------------------------------------===//
#include "InstCombine.h"
#include "llvm/Target/TargetData.h"
#include "llvm/IntrinsicInst.h"
using namespace llvm;
/// ShrinkDemandedConstant - Check to see if the specified operand of the
/// specified instruction is a constant integer. If so, check to see if there
/// are any bits set in the constant that are not demanded. If so, shrink the
/// constant and return true.
static bool ShrinkDemandedConstant(Instruction *I, unsigned OpNo,
APInt Demanded) {
assert(I && "No instruction?");
assert(OpNo < I->getNumOperands() && "Operand index too large");
// If the operand is not a constant integer, nothing to do.
ConstantInt *OpC = dyn_cast<ConstantInt>(I->getOperand(OpNo));
if (!OpC) return false;
// If there are no bits set that aren't demanded, nothing to do.
Demanded = Demanded.zextOrTrunc(OpC->getValue().getBitWidth());
if ((~Demanded & OpC->getValue()) == 0)
return false;
// This instruction is producing bits that are not demanded. Shrink the RHS.
Demanded &= OpC->getValue();
I->setOperand(OpNo, ConstantInt::get(OpC->getType(), Demanded));
return true;
}
/// SimplifyDemandedInstructionBits - Inst is an integer instruction that
/// SimplifyDemandedBits knows about. See if the instruction has any
/// properties that allow us to simplify its operands.
bool InstCombiner::SimplifyDemandedInstructionBits(Instruction &Inst) {
unsigned BitWidth = Inst.getType()->getScalarSizeInBits();
APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
APInt DemandedMask(APInt::getAllOnesValue(BitWidth));
Value *V = SimplifyDemandedUseBits(&Inst, DemandedMask,
KnownZero, KnownOne, 0);
if (V == 0) return false;
if (V == &Inst) return true;
ReplaceInstUsesWith(Inst, V);
return true;
}
/// SimplifyDemandedBits - This form of SimplifyDemandedBits simplifies the
/// specified instruction operand if possible, updating it in place. It returns
/// true if it made any change and false otherwise.
bool InstCombiner::SimplifyDemandedBits(Use &U, APInt DemandedMask,
APInt &KnownZero, APInt &KnownOne,
unsigned Depth) {
Value *NewVal = SimplifyDemandedUseBits(U.get(), DemandedMask,
KnownZero, KnownOne, Depth);
if (NewVal == 0) return false;
U = NewVal;
return true;
}
/// SimplifyDemandedUseBits - This function attempts to replace V with a simpler
/// value based on the demanded bits. When this function is called, it is known
/// that only the bits set in DemandedMask of the result of V are ever used
/// downstream. Consequently, depending on the mask and V, it may be possible
/// to replace V with a constant or one of its operands. In such cases, this
/// function does the replacement and returns true. In all other cases, it
/// returns false after analyzing the expression and setting KnownOne and known
/// to be one in the expression. KnownZero contains all the bits that are known
/// to be zero in the expression. These are provided to potentially allow the
/// caller (which might recursively be SimplifyDemandedBits itself) to simplify
/// the expression. KnownOne and KnownZero always follow the invariant that
/// KnownOne & KnownZero == 0. That is, a bit can't be both 1 and 0. Note that
/// the bits in KnownOne and KnownZero may only be accurate for those bits set
/// in DemandedMask. Note also that the bitwidth of V, DemandedMask, KnownZero
/// and KnownOne must all be the same.
///
/// This returns null if it did not change anything and it permits no
/// simplification. This returns V itself if it did some simplification of V's
/// operands based on the information about what bits are demanded. This returns
/// some other non-null value if it found out that V is equal to another value
/// in the context where the specified bits are demanded, but not for all users.
Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
APInt &KnownZero, APInt &KnownOne,
unsigned Depth) {
assert(V != 0 && "Null pointer of Value???");
assert(Depth <= 6 && "Limit Search Depth");
uint32_t BitWidth = DemandedMask.getBitWidth();
Type *VTy = V->getType();
assert((TD || !VTy->isPointerTy()) &&
"SimplifyDemandedBits needs to know bit widths!");
assert((!TD || TD->getTypeSizeInBits(VTy->getScalarType()) == BitWidth) &&
(!VTy->isIntOrIntVectorTy() ||
VTy->getScalarSizeInBits() == BitWidth) &&
KnownZero.getBitWidth() == BitWidth &&
KnownOne.getBitWidth() == BitWidth &&
"Value *V, DemandedMask, KnownZero and KnownOne "
"must have same BitWidth");
if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
// We know all of the bits for a constant!
KnownOne = CI->getValue() & DemandedMask;
KnownZero = ~KnownOne & DemandedMask;
return 0;
}
if (isa<ConstantPointerNull>(V)) {
// We know all of the bits for a constant!
KnownOne.clearAllBits();
KnownZero = DemandedMask;
return 0;
}
KnownZero.clearAllBits();
KnownOne.clearAllBits();
if (DemandedMask == 0) { // Not demanding any bits from V.
if (isa<UndefValue>(V))
return 0;
return UndefValue::get(VTy);
}
if (Depth == 6) // Limit search depth.
return 0;
APInt LHSKnownZero(BitWidth, 0), LHSKnownOne(BitWidth, 0);
APInt RHSKnownZero(BitWidth, 0), RHSKnownOne(BitWidth, 0);
Instruction *I = dyn_cast<Instruction>(V);
if (!I) {
ComputeMaskedBits(V, KnownZero, KnownOne, Depth);
return 0; // Only analyze instructions.
}
// If there are multiple uses of this value and we aren't at the root, then
// we can't do any simplifications of the operands, because DemandedMask
// only reflects the bits demanded by *one* of the users.
if (Depth != 0 && !I->hasOneUse()) {
// Despite the fact that we can't simplify this instruction in all User's
// context, we can at least compute the knownzero/knownone bits, and we can
// do simplifications that apply to *just* the one user if we know that
// this instruction has a simpler value in that context.
if (I->getOpcode() == Instruction::And) {
// If either the LHS or the RHS are Zero, the result is zero.
ComputeMaskedBits(I->getOperand(1), RHSKnownZero, RHSKnownOne, Depth+1);
ComputeMaskedBits(I->getOperand(0), LHSKnownZero, LHSKnownOne, Depth+1);
// If all of the demanded bits are known 1 on one side, return the other.
// These bits cannot contribute to the result of the 'and' in this
// context.
if ((DemandedMask & ~LHSKnownZero & RHSKnownOne) ==
(DemandedMask & ~LHSKnownZero))
return I->getOperand(0);
if ((DemandedMask & ~RHSKnownZero & LHSKnownOne) ==
(DemandedMask & ~RHSKnownZero))
return I->getOperand(1);
// If all of the demanded bits in the inputs are known zeros, return zero.
if ((DemandedMask & (RHSKnownZero|LHSKnownZero)) == DemandedMask)
return Constant::getNullValue(VTy);
} else if (I->getOpcode() == Instruction::Or) {
// We can simplify (X|Y) -> X or Y in the user's context if we know that
// only bits from X or Y are demanded.
// If either the LHS or the RHS are One, the result is One.
ComputeMaskedBits(I->getOperand(1), RHSKnownZero, RHSKnownOne, Depth+1);
ComputeMaskedBits(I->getOperand(0), LHSKnownZero, LHSKnownOne, Depth+1);
// If all of the demanded bits are known zero on one side, return the
// other. These bits cannot contribute to the result of the 'or' in this
// context.
if ((DemandedMask & ~LHSKnownOne & RHSKnownZero) ==
(DemandedMask & ~LHSKnownOne))
return I->getOperand(0);
if ((DemandedMask & ~RHSKnownOne & LHSKnownZero) ==
(DemandedMask & ~RHSKnownOne))
return I->getOperand(1);
// If all of the potentially set bits on one side are known to be set on
// the other side, just use the 'other' side.
if ((DemandedMask & (~RHSKnownZero) & LHSKnownOne) ==
(DemandedMask & (~RHSKnownZero)))
return I->getOperand(0);
if ((DemandedMask & (~LHSKnownZero) & RHSKnownOne) ==
(DemandedMask & (~LHSKnownZero)))
return I->getOperand(1);
}
// Compute the KnownZero/KnownOne bits to simplify things downstream.
ComputeMaskedBits(I, KnownZero, KnownOne, Depth);
return 0;
}
// If this is the root being simplified, allow it to have multiple uses,
// just set the DemandedMask to all bits so that we can try to simplify the
// operands. This allows visitTruncInst (for example) to simplify the
// operand of a trunc without duplicating all the logic below.
if (Depth == 0 && !V->hasOneUse())
DemandedMask = APInt::getAllOnesValue(BitWidth);
switch (I->getOpcode()) {
default:
ComputeMaskedBits(I, KnownZero, KnownOne, Depth);
break;
case Instruction::And:
// If either the LHS or the RHS are Zero, the result is zero.
if (SimplifyDemandedBits(I->getOperandUse(1), DemandedMask,
RHSKnownZero, RHSKnownOne, Depth+1) ||
SimplifyDemandedBits(I->getOperandUse(0), DemandedMask & ~RHSKnownZero,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
assert(!(RHSKnownZero & RHSKnownOne) && "Bits known to be one AND zero?");
assert(!(LHSKnownZero & LHSKnownOne) && "Bits known to be one AND zero?");
// If all of the demanded bits are known 1 on one side, return the other.
// These bits cannot contribute to the result of the 'and'.
if ((DemandedMask & ~LHSKnownZero & RHSKnownOne) ==
(DemandedMask & ~LHSKnownZero))
return I->getOperand(0);
if ((DemandedMask & ~RHSKnownZero & LHSKnownOne) ==
(DemandedMask & ~RHSKnownZero))
return I->getOperand(1);
// If all of the demanded bits in the inputs are known zeros, return zero.
if ((DemandedMask & (RHSKnownZero|LHSKnownZero)) == DemandedMask)
return Constant::getNullValue(VTy);
// If the RHS is a constant, see if we can simplify it.
if (ShrinkDemandedConstant(I, 1, DemandedMask & ~LHSKnownZero))
return I;
// Output known-1 bits are only known if set in both the LHS & RHS.
KnownOne = RHSKnownOne & LHSKnownOne;
// Output known-0 are known to be clear if zero in either the LHS | RHS.
KnownZero = RHSKnownZero | LHSKnownZero;
break;
case Instruction::Or:
// If either the LHS or the RHS are One, the result is One.
if (SimplifyDemandedBits(I->getOperandUse(1), DemandedMask,
RHSKnownZero, RHSKnownOne, Depth+1) ||
SimplifyDemandedBits(I->getOperandUse(0), DemandedMask & ~RHSKnownOne,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
assert(!(RHSKnownZero & RHSKnownOne) && "Bits known to be one AND zero?");
assert(!(LHSKnownZero & LHSKnownOne) && "Bits known to be one AND zero?");
// If all of the demanded bits are known zero on one side, return the other.
// These bits cannot contribute to the result of the 'or'.
if ((DemandedMask & ~LHSKnownOne & RHSKnownZero) ==
(DemandedMask & ~LHSKnownOne))
return I->getOperand(0);
if ((DemandedMask & ~RHSKnownOne & LHSKnownZero) ==
(DemandedMask & ~RHSKnownOne))
return I->getOperand(1);
// If all of the potentially set bits on one side are known to be set on
// the other side, just use the 'other' side.
if ((DemandedMask & (~RHSKnownZero) & LHSKnownOne) ==
(DemandedMask & (~RHSKnownZero)))
return I->getOperand(0);
if ((DemandedMask & (~LHSKnownZero) & RHSKnownOne) ==
(DemandedMask & (~LHSKnownZero)))
return I->getOperand(1);
// If the RHS is a constant, see if we can simplify it.
if (ShrinkDemandedConstant(I, 1, DemandedMask))
return I;
// Output known-0 bits are only known if clear in both the LHS & RHS.
KnownZero = RHSKnownZero & LHSKnownZero;
// Output known-1 are known to be set if set in either the LHS | RHS.
KnownOne = RHSKnownOne | LHSKnownOne;
break;
case Instruction::Xor: {
if (SimplifyDemandedBits(I->getOperandUse(1), DemandedMask,
RHSKnownZero, RHSKnownOne, Depth+1) ||
SimplifyDemandedBits(I->getOperandUse(0), DemandedMask,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
assert(!(RHSKnownZero & RHSKnownOne) && "Bits known to be one AND zero?");
assert(!(LHSKnownZero & LHSKnownOne) && "Bits known to be one AND zero?");
// If all of the demanded bits are known zero on one side, return the other.
// These bits cannot contribute to the result of the 'xor'.
if ((DemandedMask & RHSKnownZero) == DemandedMask)
return I->getOperand(0);
if ((DemandedMask & LHSKnownZero) == DemandedMask)
return I->getOperand(1);
// If all of the demanded bits are known to be zero on one side or the
// other, turn this into an *inclusive* or.
// e.g. (A & C1)^(B & C2) -> (A & C1)|(B & C2) iff C1&C2 == 0
if ((DemandedMask & ~RHSKnownZero & ~LHSKnownZero) == 0) {
Instruction *Or =
BinaryOperator::CreateOr(I->getOperand(0), I->getOperand(1),
I->getName());
return InsertNewInstWith(Or, *I);
}
// If all of the demanded bits on one side are known, and all of the set
// bits on that side are also known to be set on the other side, turn this
// into an AND, as we know the bits will be cleared.
// e.g. (X | C1) ^ C2 --> (X | C1) & ~C2 iff (C1&C2) == C2
if ((DemandedMask & (RHSKnownZero|RHSKnownOne)) == DemandedMask) {
// all known
if ((RHSKnownOne & LHSKnownOne) == RHSKnownOne) {
Constant *AndC = Constant::getIntegerValue(VTy,
~RHSKnownOne & DemandedMask);
Instruction *And = BinaryOperator::CreateAnd(I->getOperand(0), AndC);
return InsertNewInstWith(And, *I);
}
}
// If the RHS is a constant, see if we can simplify it.
// FIXME: for XOR, we prefer to force bits to 1 if they will make a -1.
if (ShrinkDemandedConstant(I, 1, DemandedMask))
return I;
// If our LHS is an 'and' and if it has one use, and if any of the bits we
// are flipping are known to be set, then the xor is just resetting those
// bits to zero. We can just knock out bits from the 'and' and the 'xor',
// simplifying both of them.
if (Instruction *LHSInst = dyn_cast<Instruction>(I->getOperand(0)))
if (LHSInst->getOpcode() == Instruction::And && LHSInst->hasOneUse() &&
isa<ConstantInt>(I->getOperand(1)) &&
isa<ConstantInt>(LHSInst->getOperand(1)) &&
(LHSKnownOne & RHSKnownOne & DemandedMask) != 0) {
ConstantInt *AndRHS = cast<ConstantInt>(LHSInst->getOperand(1));
ConstantInt *XorRHS = cast<ConstantInt>(I->getOperand(1));
APInt NewMask = ~(LHSKnownOne & RHSKnownOne & DemandedMask);
Constant *AndC =
ConstantInt::get(I->getType(), NewMask & AndRHS->getValue());
Instruction *NewAnd = BinaryOperator::CreateAnd(I->getOperand(0), AndC);
InsertNewInstWith(NewAnd, *I);
Constant *XorC =
ConstantInt::get(I->getType(), NewMask & XorRHS->getValue());
Instruction *NewXor = BinaryOperator::CreateXor(NewAnd, XorC);
return InsertNewInstWith(NewXor, *I);
}
// Output known-0 bits are known if clear or set in both the LHS & RHS.
KnownZero= (RHSKnownZero & LHSKnownZero) | (RHSKnownOne & LHSKnownOne);
// Output known-1 are known to be set if set in only one of the LHS, RHS.
KnownOne = (RHSKnownZero & LHSKnownOne) | (RHSKnownOne & LHSKnownZero);
break;
}
case Instruction::Select:
if (SimplifyDemandedBits(I->getOperandUse(2), DemandedMask,
RHSKnownZero, RHSKnownOne, Depth+1) ||
SimplifyDemandedBits(I->getOperandUse(1), DemandedMask,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
assert(!(RHSKnownZero & RHSKnownOne) && "Bits known to be one AND zero?");
assert(!(LHSKnownZero & LHSKnownOne) && "Bits known to be one AND zero?");
// If the operands are constants, see if we can simplify them.
if (ShrinkDemandedConstant(I, 1, DemandedMask) ||
ShrinkDemandedConstant(I, 2, DemandedMask))
return I;
// Only known if known in both the LHS and RHS.
KnownOne = RHSKnownOne & LHSKnownOne;
KnownZero = RHSKnownZero & LHSKnownZero;
break;
case Instruction::Trunc: {
unsigned truncBf = I->getOperand(0)->getType()->getScalarSizeInBits();
DemandedMask = DemandedMask.zext(truncBf);
KnownZero = KnownZero.zext(truncBf);
KnownOne = KnownOne.zext(truncBf);
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedMask,
KnownZero, KnownOne, Depth+1))
return I;
DemandedMask = DemandedMask.trunc(BitWidth);
KnownZero = KnownZero.trunc(BitWidth);
KnownOne = KnownOne.trunc(BitWidth);
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
break;
}
case Instruction::BitCast:
if (!I->getOperand(0)->getType()->isIntOrIntVectorTy())
return 0; // vector->int or fp->int?
if (VectorType *DstVTy = dyn_cast<VectorType>(I->getType())) {
if (VectorType *SrcVTy =
dyn_cast<VectorType>(I->getOperand(0)->getType())) {
if (DstVTy->getNumElements() != SrcVTy->getNumElements())
// Don't touch a bitcast between vectors of different element counts.
return 0;
} else
// Don't touch a scalar-to-vector bitcast.
return 0;
} else if (I->getOperand(0)->getType()->isVectorTy())
// Don't touch a vector-to-scalar bitcast.
return 0;
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedMask,
KnownZero, KnownOne, Depth+1))
return I;
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
break;
case Instruction::ZExt: {
// Compute the bits in the result that are not present in the input.
unsigned SrcBitWidth =I->getOperand(0)->getType()->getScalarSizeInBits();
DemandedMask = DemandedMask.trunc(SrcBitWidth);
KnownZero = KnownZero.trunc(SrcBitWidth);
KnownOne = KnownOne.trunc(SrcBitWidth);
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedMask,
KnownZero, KnownOne, Depth+1))
return I;
DemandedMask = DemandedMask.zext(BitWidth);
KnownZero = KnownZero.zext(BitWidth);
KnownOne = KnownOne.zext(BitWidth);
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
// The top bits are known to be zero.
KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth);
break;
}
case Instruction::SExt: {
// Compute the bits in the result that are not present in the input.
unsigned SrcBitWidth =I->getOperand(0)->getType()->getScalarSizeInBits();
APInt InputDemandedBits = DemandedMask &
APInt::getLowBitsSet(BitWidth, SrcBitWidth);
APInt NewBits(APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth));
// If any of the sign extended bits are demanded, we know that the sign
// bit is demanded.
if ((NewBits & DemandedMask) != 0)
InputDemandedBits.setBit(SrcBitWidth-1);
InputDemandedBits = InputDemandedBits.trunc(SrcBitWidth);
KnownZero = KnownZero.trunc(SrcBitWidth);
KnownOne = KnownOne.trunc(SrcBitWidth);
if (SimplifyDemandedBits(I->getOperandUse(0), InputDemandedBits,
KnownZero, KnownOne, Depth+1))
return I;
InputDemandedBits = InputDemandedBits.zext(BitWidth);
KnownZero = KnownZero.zext(BitWidth);
KnownOne = KnownOne.zext(BitWidth);
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
// If the sign bit of the input is known set or clear, then we know the
// top bits of the result.
// If the input sign bit is known zero, or if the NewBits are not demanded
// convert this into a zero extension.
if (KnownZero[SrcBitWidth-1] || (NewBits & ~DemandedMask) == NewBits) {
// Convert to ZExt cast
CastInst *NewCast = new ZExtInst(I->getOperand(0), VTy, I->getName());
return InsertNewInstWith(NewCast, *I);
} else if (KnownOne[SrcBitWidth-1]) { // Input sign bit known set
KnownOne |= NewBits;
}
break;
}
case Instruction::Add: {
// Figure out what the input bits are. If the top bits of the and result
// are not demanded, then the add doesn't demand them from its input
// either.
unsigned NLZ = DemandedMask.countLeadingZeros();
// If there is a constant on the RHS, there are a variety of xformations
// we can do.
if (ConstantInt *RHS = dyn_cast<ConstantInt>(I->getOperand(1))) {
// If null, this should be simplified elsewhere. Some of the xforms here
// won't work if the RHS is zero.
if (RHS->isZero())
break;
// If the top bit of the output is demanded, demand everything from the
// input. Otherwise, we demand all the input bits except NLZ top bits.
APInt InDemandedBits(APInt::getLowBitsSet(BitWidth, BitWidth - NLZ));
// Find information about known zero/one bits in the input.
if (SimplifyDemandedBits(I->getOperandUse(0), InDemandedBits,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
// If the RHS of the add has bits set that can't affect the input, reduce
// the constant.
if (ShrinkDemandedConstant(I, 1, InDemandedBits))
return I;
// Avoid excess work.
if (LHSKnownZero == 0 && LHSKnownOne == 0)
break;
// Turn it into OR if input bits are zero.
if ((LHSKnownZero & RHS->getValue()) == RHS->getValue()) {
Instruction *Or =
BinaryOperator::CreateOr(I->getOperand(0), I->getOperand(1),
I->getName());
return InsertNewInstWith(Or, *I);
}
// We can say something about the output known-zero and known-one bits,
// depending on potential carries from the input constant and the
// unknowns. For example if the LHS is known to have at most the 0x0F0F0
// bits set and the RHS constant is 0x01001, then we know we have a known
// one mask of 0x00001 and a known zero mask of 0xE0F0E.
// To compute this, we first compute the potential carry bits. These are
// the bits which may be modified. I'm not aware of a better way to do
// this scan.
const APInt &RHSVal = RHS->getValue();
APInt CarryBits((~LHSKnownZero + RHSVal) ^ (~LHSKnownZero ^ RHSVal));
// Now that we know which bits have carries, compute the known-1/0 sets.
// Bits are known one if they are known zero in one operand and one in the
// other, and there is no input carry.
KnownOne = ((LHSKnownZero & RHSVal) |
(LHSKnownOne & ~RHSVal)) & ~CarryBits;
// Bits are known zero if they are known zero in both operands and there
// is no input carry.
KnownZero = LHSKnownZero & ~RHSVal & ~CarryBits;
} else {
// If the high-bits of this ADD are not demanded, then it does not demand
// the high bits of its LHS or RHS.
if (DemandedMask[BitWidth-1] == 0) {
// Right fill the mask of bits for this ADD to demand the most
// significant bit and all those below it.
APInt DemandedFromOps(APInt::getLowBitsSet(BitWidth, BitWidth-NLZ));
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedFromOps,
LHSKnownZero, LHSKnownOne, Depth+1) ||
SimplifyDemandedBits(I->getOperandUse(1), DemandedFromOps,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
}
}
break;
}
case Instruction::Sub:
// If the high-bits of this SUB are not demanded, then it does not demand
// the high bits of its LHS or RHS.
if (DemandedMask[BitWidth-1] == 0) {
// Right fill the mask of bits for this SUB to demand the most
// significant bit and all those below it.
uint32_t NLZ = DemandedMask.countLeadingZeros();
APInt DemandedFromOps(APInt::getLowBitsSet(BitWidth, BitWidth-NLZ));
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedFromOps,
LHSKnownZero, LHSKnownOne, Depth+1) ||
SimplifyDemandedBits(I->getOperandUse(1), DemandedFromOps,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
}
// Otherwise just hand the sub off to ComputeMaskedBits to fill in
// the known zeros and ones.
ComputeMaskedBits(V, KnownZero, KnownOne, Depth);
// Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
// zero.
if (ConstantInt *C0 = dyn_cast<ConstantInt>(I->getOperand(0))) {
APInt I0 = C0->getValue();
if ((I0 + 1).isPowerOf2() && (I0 | KnownZero).isAllOnesValue()) {
Instruction *Xor = BinaryOperator::CreateXor(I->getOperand(1), C0);
return InsertNewInstWith(Xor, *I);
}
}
break;
case Instruction::Shl:
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
uint64_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
APInt DemandedMaskIn(DemandedMask.lshr(ShiftAmt));
// If the shift is NUW/NSW, then it does demand the high bits.
ShlOperator *IOp = cast<ShlOperator>(I);
if (IOp->hasNoSignedWrap())
DemandedMaskIn |= APInt::getHighBitsSet(BitWidth, ShiftAmt+1);
else if (IOp->hasNoUnsignedWrap())
DemandedMaskIn |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedMaskIn,
KnownZero, KnownOne, Depth+1))
return I;
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
KnownZero <<= ShiftAmt;
KnownOne <<= ShiftAmt;
// low bits known zero.
if (ShiftAmt)
KnownZero |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::LShr:
// For a logical shift right
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
uint64_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
// Unsigned shift right.
APInt DemandedMaskIn(DemandedMask.shl(ShiftAmt));
// If the shift is exact, then it does demand the low bits (and knows that
// they are zero).
if (cast<LShrOperator>(I)->isExact())
DemandedMaskIn |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedMaskIn,
KnownZero, KnownOne, Depth+1))
return I;
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
KnownZero = APIntOps::lshr(KnownZero, ShiftAmt);
KnownOne = APIntOps::lshr(KnownOne, ShiftAmt);
if (ShiftAmt) {
// Compute the new bits that are at the top now.
APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
KnownZero |= HighBits; // high bits known zero.
}
}
break;
case Instruction::AShr:
// If this is an arithmetic shift right and only the low-bit is set, we can
// always convert this into a logical shr, even if the shift amount is
// variable. The low bit of the shift cannot be an input sign bit unless
// the shift amount is >= the size of the datatype, which is undefined.
if (DemandedMask == 1) {
// Perform the logical shift right.
Instruction *NewVal = BinaryOperator::CreateLShr(
I->getOperand(0), I->getOperand(1), I->getName());
return InsertNewInstWith(NewVal, *I);
}
// If the sign bit is the only bit demanded by this ashr, then there is no
// need to do it, the shift doesn't change the high bit.
if (DemandedMask.isSignBit())
return I->getOperand(0);
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
uint32_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
// Signed shift right.
APInt DemandedMaskIn(DemandedMask.shl(ShiftAmt));
// If any of the "high bits" are demanded, we should set the sign bit as
// demanded.
if (DemandedMask.countLeadingZeros() <= ShiftAmt)
DemandedMaskIn.setBit(BitWidth-1);
// If the shift is exact, then it does demand the low bits (and knows that
// they are zero).
if (cast<AShrOperator>(I)->isExact())
DemandedMaskIn |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
if (SimplifyDemandedBits(I->getOperandUse(0), DemandedMaskIn,
KnownZero, KnownOne, Depth+1))
return I;
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
// Compute the new bits that are at the top now.
APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
KnownZero = APIntOps::lshr(KnownZero, ShiftAmt);
KnownOne = APIntOps::lshr(KnownOne, ShiftAmt);
// Handle the sign bits.
APInt SignBit(APInt::getSignBit(BitWidth));
// Adjust to where it is now in the mask.
SignBit = APIntOps::lshr(SignBit, ShiftAmt);
// If the input sign bit is known to be zero, or if none of the top bits
// are demanded, turn this into an unsigned shift right.
if (BitWidth <= ShiftAmt || KnownZero[BitWidth-ShiftAmt-1] ||
(HighBits & ~DemandedMask) == HighBits) {
// Perform the logical shift right.
BinaryOperator *NewVal = BinaryOperator::CreateLShr(I->getOperand(0),
SA, I->getName());
NewVal->setIsExact(cast<BinaryOperator>(I)->isExact());
return InsertNewInstWith(NewVal, *I);
} else if ((KnownOne & SignBit) != 0) { // New bits are known one.
KnownOne |= HighBits;
}
}
break;
case Instruction::SRem:
if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
// X % -1 demands all the bits because we don't want to introduce
// INT_MIN % -1 (== undef) by accident.
if (Rem->isAllOnesValue())
break;
APInt RA = Rem->getValue().abs();
if (RA.isPowerOf2()) {
if (DemandedMask.ult(RA)) // srem won't affect demanded bits
return I->getOperand(0);
APInt LowBits = RA - 1;
APInt Mask2 = LowBits | APInt::getSignBit(BitWidth);
if (SimplifyDemandedBits(I->getOperandUse(0), Mask2,
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
// The low bits of LHS are unchanged by the srem.
KnownZero = LHSKnownZero & LowBits;
KnownOne = LHSKnownOne & LowBits;
// If LHS is non-negative or has all low bits zero, then the upper bits
// are all zero.
if (LHSKnownZero[BitWidth-1] || ((LHSKnownZero & LowBits) == LowBits))
KnownZero |= ~LowBits;
// If LHS is negative and not all low bits are zero, then the upper bits
// are all one.
if (LHSKnownOne[BitWidth-1] && ((LHSKnownOne & LowBits) != 0))
KnownOne |= ~LowBits;
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
}
}
// The sign bit is the LHS's sign bit, except when the result of the
// remainder is zero.
if (DemandedMask.isNegative() && KnownZero.isNonNegative()) {
APInt LHSKnownZero(BitWidth, 0), LHSKnownOne(BitWidth, 0);
ComputeMaskedBits(I->getOperand(0), LHSKnownZero, LHSKnownOne, Depth+1);
// If it's known zero, our sign bit is also zero.
if (LHSKnownZero.isNegative())
KnownZero |= LHSKnownZero;
}
break;
case Instruction::URem: {
APInt KnownZero2(BitWidth, 0), KnownOne2(BitWidth, 0);
APInt AllOnes = APInt::getAllOnesValue(BitWidth);
if (SimplifyDemandedBits(I->getOperandUse(0), AllOnes,
KnownZero2, KnownOne2, Depth+1) ||
SimplifyDemandedBits(I->getOperandUse(1), AllOnes,
KnownZero2, KnownOne2, Depth+1))
return I;
unsigned Leaders = KnownZero2.countLeadingOnes();
Leaders = std::max(Leaders,
KnownZero2.countLeadingOnes());
KnownZero = APInt::getHighBitsSet(BitWidth, Leaders) & DemandedMask;
break;
}
case Instruction::Call:
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::bswap: {
// If the only bits demanded come from one byte of the bswap result,
// just shift the input byte into position to eliminate the bswap.
unsigned NLZ = DemandedMask.countLeadingZeros();
unsigned NTZ = DemandedMask.countTrailingZeros();
// Round NTZ down to the next byte. If we have 11 trailing zeros, then
// we need all the bits down to bit 8. Likewise, round NLZ. If we
// have 14 leading zeros, round to 8.
NLZ &= ~7;
NTZ &= ~7;
// If we need exactly one byte, we can do this transformation.
if (BitWidth-NLZ-NTZ == 8) {
unsigned ResultBit = NTZ;
unsigned InputBit = BitWidth-NTZ-8;
// Replace this with either a left or right shift to get the byte into
// the right place.
Instruction *NewVal;
if (InputBit > ResultBit)
NewVal = BinaryOperator::CreateLShr(II->getArgOperand(0),
ConstantInt::get(I->getType(), InputBit-ResultBit));
else
NewVal = BinaryOperator::CreateShl(II->getArgOperand(0),
ConstantInt::get(I->getType(), ResultBit-InputBit));
NewVal->takeName(I);
return InsertNewInstWith(NewVal, *I);
}
// TODO: Could compute known zero/one bits based on the input.
break;
}
case Intrinsic::x86_sse42_crc32_64_8:
case Intrinsic::x86_sse42_crc32_64_64:
KnownZero = APInt::getHighBitsSet(64, 32);
return 0;
}
}
ComputeMaskedBits(V, KnownZero, KnownOne, Depth);
break;
}
// If the client is only demanding bits that we know, return the known
// constant.
if ((DemandedMask & (KnownZero|KnownOne)) == DemandedMask)
return Constant::getIntegerValue(VTy, KnownOne);
return 0;
}
/// SimplifyDemandedVectorElts - The specified value produces a vector with
/// any number of elements. DemandedElts contains the set of elements that are
/// actually used by the caller. This method analyzes which elements of the
/// operand are undef and returns that information in UndefElts.
///
/// If the information about demanded elements can be used to simplify the
/// operation, the operation is simplified, then the resultant value is
/// returned. This returns null if no change was made.
Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts,
APInt &UndefElts,
unsigned Depth) {
unsigned VWidth = cast<VectorType>(V->getType())->getNumElements();
APInt EltMask(APInt::getAllOnesValue(VWidth));
assert((DemandedElts & ~EltMask) == 0 && "Invalid DemandedElts!");
if (isa<UndefValue>(V)) {
// If the entire vector is undefined, just return this info.
UndefElts = EltMask;
return 0;
}
if (DemandedElts == 0) { // If nothing is demanded, provide undef.
UndefElts = EltMask;
return UndefValue::get(V->getType());
}
UndefElts = 0;
// Handle ConstantAggregateZero, ConstantVector, ConstantDataSequential.
if (Constant *C = dyn_cast<Constant>(V)) {
// Check if this is identity. If so, return 0 since we are not simplifying
// anything.
if (DemandedElts.isAllOnesValue())
return 0;
Type *EltTy = cast<VectorType>(V->getType())->getElementType();
Constant *Undef = UndefValue::get(EltTy);
SmallVector<Constant*, 16> Elts;
for (unsigned i = 0; i != VWidth; ++i) {
if (!DemandedElts[i]) { // If not demanded, set to undef.
Elts.push_back(Undef);
UndefElts.setBit(i);
continue;
}
Constant *Elt = C->getAggregateElement(i);
if (Elt == 0) return 0;
if (isa<UndefValue>(Elt)) { // Already undef.
Elts.push_back(Undef);
UndefElts.setBit(i);
} else { // Otherwise, defined.
Elts.push_back(Elt);
}
}
// If we changed the constant, return it.
Constant *NewCV = ConstantVector::get(Elts);
return NewCV != C ? NewCV : 0;
}
// Limit search depth.
if (Depth == 10)
return 0;
// If multiple users are using the root value, proceed with
// simplification conservatively assuming that all elements
// are needed.
if (!V->hasOneUse()) {
// Quit if we find multiple users of a non-root value though.
// They'll be handled when it's their turn to be visited by
// the main instcombine process.
if (Depth != 0)
// TODO: Just compute the UndefElts information recursively.
return 0;
// Conservatively assume that all elements are needed.
DemandedElts = EltMask;
}
Instruction *I = dyn_cast<Instruction>(V);
if (!I) return 0; // Only analyze instructions.
bool MadeChange = false;
APInt UndefElts2(VWidth, 0);
Value *TmpV;
switch (I->getOpcode()) {
default: break;
case Instruction::InsertElement: {
// If this is a variable index, we don't know which element it overwrites.
// demand exactly the same input as we produce.
ConstantInt *Idx = dyn_cast<ConstantInt>(I->getOperand(2));
if (Idx == 0) {
// Note that we can't propagate undef elt info, because we don't know
// which elt is getting updated.
TmpV = SimplifyDemandedVectorElts(I->getOperand(0), DemandedElts,
UndefElts2, Depth+1);
if (TmpV) { I->setOperand(0, TmpV); MadeChange = true; }
break;
}
// If this is inserting an element that isn't demanded, remove this
// insertelement.
unsigned IdxNo = Idx->getZExtValue();
if (IdxNo >= VWidth || !DemandedElts[IdxNo]) {
Worklist.Add(I);
return I->getOperand(0);
}
// Otherwise, the element inserted overwrites whatever was there, so the
// input demanded set is simpler than the output set.
APInt DemandedElts2 = DemandedElts;
DemandedElts2.clearBit(IdxNo);
TmpV = SimplifyDemandedVectorElts(I->getOperand(0), DemandedElts2,
UndefElts, Depth+1);
if (TmpV) { I->setOperand(0, TmpV); MadeChange = true; }
// The inserted element is defined.
UndefElts.clearBit(IdxNo);
break;
}
case Instruction::ShuffleVector: {
ShuffleVectorInst *Shuffle = cast<ShuffleVectorInst>(I);
uint64_t LHSVWidth =
cast<VectorType>(Shuffle->getOperand(0)->getType())->getNumElements();
APInt LeftDemanded(LHSVWidth, 0), RightDemanded(LHSVWidth, 0);
for (unsigned i = 0; i < VWidth; i++) {
if (DemandedElts[i]) {
unsigned MaskVal = Shuffle->getMaskValue(i);
if (MaskVal != -1u) {
assert(MaskVal < LHSVWidth * 2 &&
"shufflevector mask index out of range!");
if (MaskVal < LHSVWidth)
LeftDemanded.setBit(MaskVal);
else
RightDemanded.setBit(MaskVal - LHSVWidth);
}
}
}
APInt UndefElts4(LHSVWidth, 0);
TmpV = SimplifyDemandedVectorElts(I->getOperand(0), LeftDemanded,
UndefElts4, Depth+1);
if (TmpV) { I->setOperand(0, TmpV); MadeChange = true; }
APInt UndefElts3(LHSVWidth, 0);
TmpV = SimplifyDemandedVectorElts(I->getOperand(1), RightDemanded,
UndefElts3, Depth+1);
if (TmpV) { I->setOperand(1, TmpV); MadeChange = true; }
bool NewUndefElts = false;
for (unsigned i = 0; i < VWidth; i++) {
unsigned MaskVal = Shuffle->getMaskValue(i);
if (MaskVal == -1u) {
UndefElts.setBit(i);
} else if (!DemandedElts[i]) {
NewUndefElts = true;
UndefElts.setBit(i);
} else if (MaskVal < LHSVWidth) {
if (UndefElts4[MaskVal]) {
NewUndefElts = true;
UndefElts.setBit(i);
}
} else {
if (UndefElts3[MaskVal - LHSVWidth]) {
NewUndefElts = true;
UndefElts.setBit(i);
}
}
}
if (NewUndefElts) {
// Add additional discovered undefs.
SmallVector<Constant*, 16> Elts;
for (unsigned i = 0; i < VWidth; ++i) {
if (UndefElts[i])
Elts.push_back(UndefValue::get(Type::getInt32Ty(I->getContext())));
else
Elts.push_back(ConstantInt::get(Type::getInt32Ty(I->getContext()),
Shuffle->getMaskValue(i)));
}
I->setOperand(2, ConstantVector::get(Elts));
MadeChange = true;
}
break;
}
case Instruction::BitCast: {
// Vector->vector casts only.
VectorType *VTy = dyn_cast<VectorType>(I->getOperand(0)->getType());
if (!VTy) break;
unsigned InVWidth = VTy->getNumElements();
APInt InputDemandedElts(InVWidth, 0);
unsigned Ratio;
if (VWidth == InVWidth) {
// If we are converting from <4 x i32> -> <4 x f32>, we demand the same
// elements as are demanded of us.
Ratio = 1;
InputDemandedElts = DemandedElts;
} else if (VWidth > InVWidth) {
// Untested so far.
break;
// If there are more elements in the result than there are in the source,
// then an input element is live if any of the corresponding output
// elements are live.
Ratio = VWidth/InVWidth;
for (unsigned OutIdx = 0; OutIdx != VWidth; ++OutIdx) {
if (DemandedElts[OutIdx])
InputDemandedElts.setBit(OutIdx/Ratio);
}
} else {
// Untested so far.
break;
// If there are more elements in the source than there are in the result,
// then an input element is live if the corresponding output element is
// live.
Ratio = InVWidth/VWidth;
for (unsigned InIdx = 0; InIdx != InVWidth; ++InIdx)
if (DemandedElts[InIdx/Ratio])
InputDemandedElts.setBit(InIdx);
}
// div/rem demand all inputs, because they don't want divide by zero.
TmpV = SimplifyDemandedVectorElts(I->getOperand(0), InputDemandedElts,
UndefElts2, Depth+1);
if (TmpV) {
I->setOperand(0, TmpV);
MadeChange = true;
}
UndefElts = UndefElts2;
if (VWidth > InVWidth) {
llvm_unreachable("Unimp");
// If there are more elements in the result than there are in the source,
// then an output element is undef if the corresponding input element is
// undef.
for (unsigned OutIdx = 0; OutIdx != VWidth; ++OutIdx)
if (UndefElts2[OutIdx/Ratio])
UndefElts.setBit(OutIdx);
} else if (VWidth < InVWidth) {
llvm_unreachable("Unimp");
// If there are more elements in the source than there are in the result,
// then a result element is undef if all of the corresponding input
// elements are undef.
UndefElts = ~0ULL >> (64-VWidth); // Start out all undef.
for (unsigned InIdx = 0; InIdx != InVWidth; ++InIdx)
if (!UndefElts2[InIdx]) // Not undef?
UndefElts.clearBit(InIdx/Ratio); // Clear undef bit.
}
break;
}
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Add:
case Instruction::Sub:
case Instruction::Mul:
// div/rem demand all inputs, because they don't want divide by zero.
TmpV = SimplifyDemandedVectorElts(I->getOperand(0), DemandedElts,
UndefElts, Depth+1);
if (TmpV) { I->setOperand(0, TmpV); MadeChange = true; }
TmpV = SimplifyDemandedVectorElts(I->getOperand(1), DemandedElts,
UndefElts2, Depth+1);
if (TmpV) { I->setOperand(1, TmpV); MadeChange = true; }
// Output elements are undefined if both are undefined. Consider things
// like undef&0. The result is known zero, not undef.
UndefElts &= UndefElts2;
break;
case Instruction::Call: {
IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
if (!II) break;
switch (II->getIntrinsicID()) {
default: break;
// Binary vector operations that work column-wise. A dest element is a
// function of the corresponding input elements from the two inputs.
case Intrinsic::x86_sse_sub_ss:
case Intrinsic::x86_sse_mul_ss:
case Intrinsic::x86_sse_min_ss:
case Intrinsic::x86_sse_max_ss:
case Intrinsic::x86_sse2_sub_sd:
case Intrinsic::x86_sse2_mul_sd:
case Intrinsic::x86_sse2_min_sd:
case Intrinsic::x86_sse2_max_sd:
TmpV = SimplifyDemandedVectorElts(II->getArgOperand(0), DemandedElts,
UndefElts, Depth+1);
if (TmpV) { II->setArgOperand(0, TmpV); MadeChange = true; }
TmpV = SimplifyDemandedVectorElts(II->getArgOperand(1), DemandedElts,
UndefElts2, Depth+1);
if (TmpV) { II->setArgOperand(1, TmpV); MadeChange = true; }
// If only the low elt is demanded and this is a scalarizable intrinsic,
// scalarize it now.
if (DemandedElts == 1) {
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::x86_sse_sub_ss:
case Intrinsic::x86_sse_mul_ss:
case Intrinsic::x86_sse2_sub_sd:
case Intrinsic::x86_sse2_mul_sd:
// TODO: Lower MIN/MAX/ABS/etc
Value *LHS = II->getArgOperand(0);
Value *RHS = II->getArgOperand(1);
// Extract the element as scalars.
LHS = InsertNewInstWith(ExtractElementInst::Create(LHS,
ConstantInt::get(Type::getInt32Ty(I->getContext()), 0U)), *II);
RHS = InsertNewInstWith(ExtractElementInst::Create(RHS,
ConstantInt::get(Type::getInt32Ty(I->getContext()), 0U)), *II);
switch (II->getIntrinsicID()) {
default: llvm_unreachable("Case stmts out of sync!");
case Intrinsic::x86_sse_sub_ss:
case Intrinsic::x86_sse2_sub_sd:
TmpV = InsertNewInstWith(BinaryOperator::CreateFSub(LHS, RHS,
II->getName()), *II);
break;
case Intrinsic::x86_sse_mul_ss:
case Intrinsic::x86_sse2_mul_sd:
TmpV = InsertNewInstWith(BinaryOperator::CreateFMul(LHS, RHS,
II->getName()), *II);
break;
}
Instruction *New =
InsertElementInst::Create(
UndefValue::get(II->getType()), TmpV,
ConstantInt::get(Type::getInt32Ty(I->getContext()), 0U, false),
II->getName());
InsertNewInstWith(New, *II);
return New;
}
}
// Output elements are undefined if both are undefined. Consider things
// like undef&0. The result is known zero, not undef.
UndefElts &= UndefElts2;
break;
}
break;
}
}
return MadeChange ? I : 0;
}
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