//===- InstCombineSimplifyDemanded.cpp ------------------------------------===// // // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. // See https://llvm.org/LICENSE.txt for license information. // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception // //===----------------------------------------------------------------------===// // // This file contains logic for simplifying instructions based on information // about how they are used. // //===----------------------------------------------------------------------===// #include "InstCombineInternal.h" #include "llvm/Analysis/ValueTracking.h" #include "llvm/IR/GetElementPtrTypeIterator.h" #include "llvm/IR/IntrinsicInst.h" #include "llvm/IR/PatternMatch.h" #include "llvm/Support/KnownBits.h" #include "llvm/Transforms/InstCombine/InstCombiner.h" using namespace llvm; using namespace llvm::PatternMatch; #define DEBUG_TYPE "instcombine" static cl::opt VerifyKnownBits("instcombine-verify-known-bits", cl::desc("Verify that computeKnownBits() and " "SimplifyDemandedBits() are consistent"), cl::Hidden, cl::init(false)); /// 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, const APInt &Demanded) { assert(I && "No instruction?"); assert(OpNo < I->getNumOperands() && "Operand index too large"); // The operand must be a constant integer or splat integer. Value *Op = I->getOperand(OpNo); const APInt *C; if (!match(Op, m_APInt(C))) return false; // If there are no bits set that aren't demanded, nothing to do. if (C->isSubsetOf(Demanded)) return false; // This instruction is producing bits that are not demanded. Shrink the RHS. I->setOperand(OpNo, ConstantInt::get(Op->getType(), *C & Demanded)); return true; } /// Returns the bitwidth of the given scalar or pointer type. For vector types, /// returns the element type's bitwidth. static unsigned getBitWidth(Type *Ty, const DataLayout &DL) { if (unsigned BitWidth = Ty->getScalarSizeInBits()) return BitWidth; return DL.getPointerTypeSizeInBits(Ty); } /// Inst is an integer instruction that SimplifyDemandedBits knows about. See if /// the instruction has any properties that allow us to simplify its operands. bool InstCombinerImpl::SimplifyDemandedInstructionBits(Instruction &Inst, KnownBits &Known) { APInt DemandedMask(APInt::getAllOnes(Known.getBitWidth())); Value *V = SimplifyDemandedUseBits(&Inst, DemandedMask, Known, 0, &Inst); if (!V) return false; if (V == &Inst) return true; replaceInstUsesWith(Inst, V); return true; } /// Inst is an integer instruction that SimplifyDemandedBits knows about. See if /// the instruction has any properties that allow us to simplify its operands. bool InstCombinerImpl::SimplifyDemandedInstructionBits(Instruction &Inst) { KnownBits Known(getBitWidth(Inst.getType(), DL)); return SimplifyDemandedInstructionBits(Inst, Known); } /// 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 InstCombinerImpl::SimplifyDemandedBits(Instruction *I, unsigned OpNo, const APInt &DemandedMask, KnownBits &Known, unsigned Depth) { Use &U = I->getOperandUse(OpNo); Value *NewVal = SimplifyDemandedUseBits(U.get(), DemandedMask, Known, Depth, I); if (!NewVal) return false; if (Instruction* OpInst = dyn_cast(U)) salvageDebugInfo(*OpInst); replaceUse(U, NewVal); return true; } /// 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. Known.Zero 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. /// Known.One and Known.Zero always follow the invariant that: /// Known.One & Known.Zero == 0. /// That is, a bit can't be both 1 and 0. The bits in Known.One and Known.Zero /// are accurate even for bits not in DemandedMask. Note /// also that the bitwidth of V, DemandedMask, Known.Zero and Known.One 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 *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask, KnownBits &Known, unsigned Depth, Instruction *CxtI) { assert(V != nullptr && "Null pointer of Value???"); assert(Depth <= MaxAnalysisRecursionDepth && "Limit Search Depth"); uint32_t BitWidth = DemandedMask.getBitWidth(); Type *VTy = V->getType(); assert( (!VTy->isIntOrIntVectorTy() || VTy->getScalarSizeInBits() == BitWidth) && Known.getBitWidth() == BitWidth && "Value *V, DemandedMask and Known must have same BitWidth"); if (isa(V)) { computeKnownBits(V, Known, Depth, CxtI); return nullptr; } Known.resetAll(); if (DemandedMask.isZero()) // Not demanding any bits from V. return UndefValue::get(VTy); if (Depth == MaxAnalysisRecursionDepth) return nullptr; Instruction *I = dyn_cast(V); if (!I) { computeKnownBits(V, Known, Depth, CxtI); return nullptr; // 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()) return SimplifyMultipleUseDemandedBits(I, DemandedMask, Known, Depth, CxtI); KnownBits LHSKnown(BitWidth), RHSKnown(BitWidth); // 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.setAllBits(); // Update flags after simplifying an operand based on the fact that some high // order bits are not demanded. auto disableWrapFlagsBasedOnUnusedHighBits = [](Instruction *I, unsigned NLZ) { if (NLZ > 0) { // Disable the nsw and nuw flags here: We can no longer guarantee that // we won't wrap after simplification. Removing the nsw/nuw flags is // legal here because the top bit is not demanded. I->setHasNoSignedWrap(false); I->setHasNoUnsignedWrap(false); } return I; }; // If the high-bits of an ADD/SUB/MUL are not demanded, then we do not care // about the high bits of the operands. auto simplifyOperandsBasedOnUnusedHighBits = [&](APInt &DemandedFromOps) { unsigned NLZ = DemandedMask.countl_zero(); // Right fill the mask of bits for the operands to demand the most // significant bit and all those below it. DemandedFromOps = APInt::getLowBitsSet(BitWidth, BitWidth - NLZ); if (ShrinkDemandedConstant(I, 0, DemandedFromOps) || SimplifyDemandedBits(I, 0, DemandedFromOps, LHSKnown, Depth + 1) || ShrinkDemandedConstant(I, 1, DemandedFromOps) || SimplifyDemandedBits(I, 1, DemandedFromOps, RHSKnown, Depth + 1)) { disableWrapFlagsBasedOnUnusedHighBits(I, NLZ); return true; } return false; }; switch (I->getOpcode()) { default: computeKnownBits(I, Known, Depth, CxtI); break; case Instruction::And: { // If either the LHS or the RHS are Zero, the result is zero. if (SimplifyDemandedBits(I, 1, DemandedMask, RHSKnown, Depth + 1) || SimplifyDemandedBits(I, 0, DemandedMask & ~RHSKnown.Zero, LHSKnown, Depth + 1)) return I; assert(!RHSKnown.hasConflict() && "Bits known to be one AND zero?"); assert(!LHSKnown.hasConflict() && "Bits known to be one AND zero?"); Known = analyzeKnownBitsFromAndXorOr(cast(I), LHSKnown, RHSKnown, Depth, SQ.getWithInstruction(CxtI)); // If the client is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(VTy, Known.One); // 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.isSubsetOf(LHSKnown.Zero | RHSKnown.One)) return I->getOperand(0); if (DemandedMask.isSubsetOf(RHSKnown.Zero | LHSKnown.One)) return I->getOperand(1); // If the RHS is a constant, see if we can simplify it. if (ShrinkDemandedConstant(I, 1, DemandedMask & ~LHSKnown.Zero)) return I; break; } case Instruction::Or: { // If either the LHS or the RHS are One, the result is One. if (SimplifyDemandedBits(I, 1, DemandedMask, RHSKnown, Depth + 1) || SimplifyDemandedBits(I, 0, DemandedMask & ~RHSKnown.One, LHSKnown, Depth + 1)) { // Disjoint flag may not longer hold. I->dropPoisonGeneratingFlags(); return I; } assert(!RHSKnown.hasConflict() && "Bits known to be one AND zero?"); assert(!LHSKnown.hasConflict() && "Bits known to be one AND zero?"); Known = analyzeKnownBitsFromAndXorOr(cast(I), LHSKnown, RHSKnown, Depth, SQ.getWithInstruction(CxtI)); // If the client is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(VTy, Known.One); // 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.isSubsetOf(LHSKnown.One | RHSKnown.Zero)) return I->getOperand(0); if (DemandedMask.isSubsetOf(RHSKnown.One | LHSKnown.Zero)) return I->getOperand(1); // If the RHS is a constant, see if we can simplify it. if (ShrinkDemandedConstant(I, 1, DemandedMask)) return I; // Infer disjoint flag if no common bits are set. if (!cast(I)->isDisjoint()) { WithCache LHSCache(I->getOperand(0), LHSKnown), RHSCache(I->getOperand(1), RHSKnown); if (haveNoCommonBitsSet(LHSCache, RHSCache, SQ.getWithInstruction(I))) { cast(I)->setIsDisjoint(true); return I; } } break; } case Instruction::Xor: { if (SimplifyDemandedBits(I, 1, DemandedMask, RHSKnown, Depth + 1) || SimplifyDemandedBits(I, 0, DemandedMask, LHSKnown, Depth + 1)) return I; Value *LHS, *RHS; if (DemandedMask == 1 && match(I->getOperand(0), m_Intrinsic(m_Value(LHS))) && match(I->getOperand(1), m_Intrinsic(m_Value(RHS)))) { // (ctpop(X) ^ ctpop(Y)) & 1 --> ctpop(X^Y) & 1 IRBuilderBase::InsertPointGuard Guard(Builder); Builder.SetInsertPoint(I); auto *Xor = Builder.CreateXor(LHS, RHS); return Builder.CreateUnaryIntrinsic(Intrinsic::ctpop, Xor); } assert(!RHSKnown.hasConflict() && "Bits known to be one AND zero?"); assert(!LHSKnown.hasConflict() && "Bits known to be one AND zero?"); Known = analyzeKnownBitsFromAndXorOr(cast(I), LHSKnown, RHSKnown, Depth, SQ.getWithInstruction(CxtI)); // If the client is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(VTy, Known.One); // 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.isSubsetOf(RHSKnown.Zero)) return I->getOperand(0); if (DemandedMask.isSubsetOf(LHSKnown.Zero)) 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.isSubsetOf(RHSKnown.Zero | LHSKnown.Zero)) { Instruction *Or = BinaryOperator::CreateOr(I->getOperand(0), I->getOperand(1)); if (DemandedMask.isAllOnes()) cast(Or)->setIsDisjoint(true); Or->takeName(I); return InsertNewInstWith(Or, I->getIterator()); } // 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.isSubsetOf(RHSKnown.Zero|RHSKnown.One) && RHSKnown.One.isSubsetOf(LHSKnown.One)) { Constant *AndC = Constant::getIntegerValue(VTy, ~RHSKnown.One & DemandedMask); Instruction *And = BinaryOperator::CreateAnd(I->getOperand(0), AndC); return InsertNewInstWith(And, I->getIterator()); } // If the RHS is a constant, see if we can change it. Don't alter a -1 // constant because that's a canonical 'not' op, and that is better for // combining, SCEV, and codegen. const APInt *C; if (match(I->getOperand(1), m_APInt(C)) && !C->isAllOnes()) { if ((*C | ~DemandedMask).isAllOnes()) { // Force bits to 1 to create a 'not' op. I->setOperand(1, ConstantInt::getAllOnesValue(VTy)); return I; } // If we can't turn this into a 'not', try to shrink the constant. 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(I->getOperand(0))) { ConstantInt *AndRHS, *XorRHS; if (LHSInst->getOpcode() == Instruction::And && LHSInst->hasOneUse() && match(I->getOperand(1), m_ConstantInt(XorRHS)) && match(LHSInst->getOperand(1), m_ConstantInt(AndRHS)) && (LHSKnown.One & RHSKnown.One & DemandedMask) != 0) { APInt NewMask = ~(LHSKnown.One & RHSKnown.One & DemandedMask); Constant *AndC = ConstantInt::get(VTy, NewMask & AndRHS->getValue()); Instruction *NewAnd = BinaryOperator::CreateAnd(I->getOperand(0), AndC); InsertNewInstWith(NewAnd, I->getIterator()); Constant *XorC = ConstantInt::get(VTy, NewMask & XorRHS->getValue()); Instruction *NewXor = BinaryOperator::CreateXor(NewAnd, XorC); return InsertNewInstWith(NewXor, I->getIterator()); } } break; } case Instruction::Select: { if (SimplifyDemandedBits(I, 2, DemandedMask, RHSKnown, Depth + 1) || SimplifyDemandedBits(I, 1, DemandedMask, LHSKnown, Depth + 1)) return I; assert(!RHSKnown.hasConflict() && "Bits known to be one AND zero?"); assert(!LHSKnown.hasConflict() && "Bits known to be one AND zero?"); // If the operands are constants, see if we can simplify them. // This is similar to ShrinkDemandedConstant, but for a select we want to // try to keep the selected constants the same as icmp value constants, if // we can. This helps not break apart (or helps put back together) // canonical patterns like min and max. auto CanonicalizeSelectConstant = [](Instruction *I, unsigned OpNo, const APInt &DemandedMask) { const APInt *SelC; if (!match(I->getOperand(OpNo), m_APInt(SelC))) return false; // Get the constant out of the ICmp, if there is one. // Only try this when exactly 1 operand is a constant (if both operands // are constant, the icmp should eventually simplify). Otherwise, we may // invert the transform that reduces set bits and infinite-loop. Value *X; const APInt *CmpC; ICmpInst::Predicate Pred; if (!match(I->getOperand(0), m_ICmp(Pred, m_Value(X), m_APInt(CmpC))) || isa(X) || CmpC->getBitWidth() != SelC->getBitWidth()) return ShrinkDemandedConstant(I, OpNo, DemandedMask); // If the constant is already the same as the ICmp, leave it as-is. if (*CmpC == *SelC) return false; // If the constants are not already the same, but can be with the demand // mask, use the constant value from the ICmp. if ((*CmpC & DemandedMask) == (*SelC & DemandedMask)) { I->setOperand(OpNo, ConstantInt::get(I->getType(), *CmpC)); return true; } return ShrinkDemandedConstant(I, OpNo, DemandedMask); }; if (CanonicalizeSelectConstant(I, 1, DemandedMask) || CanonicalizeSelectConstant(I, 2, DemandedMask)) return I; // Only known if known in both the LHS and RHS. Known = LHSKnown.intersectWith(RHSKnown); break; } case Instruction::Trunc: { // If we do not demand the high bits of a right-shifted and truncated value, // then we may be able to truncate it before the shift. Value *X; const APInt *C; if (match(I->getOperand(0), m_OneUse(m_LShr(m_Value(X), m_APInt(C))))) { // The shift amount must be valid (not poison) in the narrow type, and // it must not be greater than the high bits demanded of the result. if (C->ult(VTy->getScalarSizeInBits()) && C->ule(DemandedMask.countl_zero())) { // trunc (lshr X, C) --> lshr (trunc X), C IRBuilderBase::InsertPointGuard Guard(Builder); Builder.SetInsertPoint(I); Value *Trunc = Builder.CreateTrunc(X, VTy); return Builder.CreateLShr(Trunc, C->getZExtValue()); } } } [[fallthrough]]; case Instruction::ZExt: { unsigned SrcBitWidth = I->getOperand(0)->getType()->getScalarSizeInBits(); APInt InputDemandedMask = DemandedMask.zextOrTrunc(SrcBitWidth); KnownBits InputKnown(SrcBitWidth); if (SimplifyDemandedBits(I, 0, InputDemandedMask, InputKnown, Depth + 1)) { // For zext nneg, we may have dropped the instruction which made the // input non-negative. I->dropPoisonGeneratingFlags(); return I; } assert(InputKnown.getBitWidth() == SrcBitWidth && "Src width changed?"); if (I->getOpcode() == Instruction::ZExt && I->hasNonNeg() && !InputKnown.isNegative()) InputKnown.makeNonNegative(); Known = InputKnown.zextOrTrunc(BitWidth); assert(!Known.hasConflict() && "Bits known to be one AND zero?"); 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.trunc(SrcBitWidth); // If any of the sign extended bits are demanded, we know that the sign // bit is demanded. if (DemandedMask.getActiveBits() > SrcBitWidth) InputDemandedBits.setBit(SrcBitWidth-1); KnownBits InputKnown(SrcBitWidth); if (SimplifyDemandedBits(I, 0, InputDemandedBits, InputKnown, Depth + 1)) return I; // If the input sign bit is known zero, or if the NewBits are not demanded // convert this into a zero extension. if (InputKnown.isNonNegative() || DemandedMask.getActiveBits() <= SrcBitWidth) { // Convert to ZExt cast. CastInst *NewCast = new ZExtInst(I->getOperand(0), VTy); NewCast->takeName(I); return InsertNewInstWith(NewCast, I->getIterator()); } // If the sign bit of the input is known set or clear, then we know the // top bits of the result. Known = InputKnown.sext(BitWidth); assert(!Known.hasConflict() && "Bits known to be one AND zero?"); break; } case Instruction::Add: { if ((DemandedMask & 1) == 0) { // If we do not need the low bit, try to convert bool math to logic: // add iN (zext i1 X), (sext i1 Y) --> sext (~X & Y) to iN Value *X, *Y; if (match(I, m_c_Add(m_OneUse(m_ZExt(m_Value(X))), m_OneUse(m_SExt(m_Value(Y))))) && X->getType()->isIntOrIntVectorTy(1) && X->getType() == Y->getType()) { // Truth table for inputs and output signbits: // X:0 | X:1 // ---------- // Y:0 | 0 | 0 | // Y:1 | -1 | 0 | // ---------- IRBuilderBase::InsertPointGuard Guard(Builder); Builder.SetInsertPoint(I); Value *AndNot = Builder.CreateAnd(Builder.CreateNot(X), Y); return Builder.CreateSExt(AndNot, VTy); } // add iN (sext i1 X), (sext i1 Y) --> sext (X | Y) to iN // TODO: Relax the one-use checks because we are removing an instruction? if (match(I, m_Add(m_OneUse(m_SExt(m_Value(X))), m_OneUse(m_SExt(m_Value(Y))))) && X->getType()->isIntOrIntVectorTy(1) && X->getType() == Y->getType()) { // Truth table for inputs and output signbits: // X:0 | X:1 // ----------- // Y:0 | -1 | -1 | // Y:1 | -1 | 0 | // ----------- IRBuilderBase::InsertPointGuard Guard(Builder); Builder.SetInsertPoint(I); Value *Or = Builder.CreateOr(X, Y); return Builder.CreateSExt(Or, VTy); } } // Right fill the mask of bits for the operands to demand the most // significant bit and all those below it. unsigned NLZ = DemandedMask.countl_zero(); APInt DemandedFromOps = APInt::getLowBitsSet(BitWidth, BitWidth - NLZ); if (ShrinkDemandedConstant(I, 1, DemandedFromOps) || SimplifyDemandedBits(I, 1, DemandedFromOps, RHSKnown, Depth + 1)) return disableWrapFlagsBasedOnUnusedHighBits(I, NLZ); // If low order bits are not demanded and known to be zero in one operand, // then we don't need to demand them from the other operand, since they // can't cause overflow into any bits that are demanded in the result. unsigned NTZ = (~DemandedMask & RHSKnown.Zero).countr_one(); APInt DemandedFromLHS = DemandedFromOps; DemandedFromLHS.clearLowBits(NTZ); if (ShrinkDemandedConstant(I, 0, DemandedFromLHS) || SimplifyDemandedBits(I, 0, DemandedFromLHS, LHSKnown, Depth + 1)) return disableWrapFlagsBasedOnUnusedHighBits(I, NLZ); // If we are known to be adding zeros to every bit below // the highest demanded bit, we just return the other side. if (DemandedFromOps.isSubsetOf(RHSKnown.Zero)) return I->getOperand(0); if (DemandedFromOps.isSubsetOf(LHSKnown.Zero)) return I->getOperand(1); // (add X, C) --> (xor X, C) IFF C is equal to the top bit of the DemandMask { const APInt *C; if (match(I->getOperand(1), m_APInt(C)) && C->isOneBitSet(DemandedMask.getActiveBits() - 1)) { IRBuilderBase::InsertPointGuard Guard(Builder); Builder.SetInsertPoint(I); return Builder.CreateXor(I->getOperand(0), ConstantInt::get(VTy, *C)); } } // Otherwise just compute the known bits of the result. bool NSW = cast(I)->hasNoSignedWrap(); Known = KnownBits::computeForAddSub(true, NSW, LHSKnown, RHSKnown); break; } case Instruction::Sub: { // Right fill the mask of bits for the operands to demand the most // significant bit and all those below it. unsigned NLZ = DemandedMask.countl_zero(); APInt DemandedFromOps = APInt::getLowBitsSet(BitWidth, BitWidth - NLZ); if (ShrinkDemandedConstant(I, 1, DemandedFromOps) || SimplifyDemandedBits(I, 1, DemandedFromOps, RHSKnown, Depth + 1)) return disableWrapFlagsBasedOnUnusedHighBits(I, NLZ); // If low order bits are not demanded and are known to be zero in RHS, // then we don't need to demand them from LHS, since they can't cause a // borrow from any bits that are demanded in the result. unsigned NTZ = (~DemandedMask & RHSKnown.Zero).countr_one(); APInt DemandedFromLHS = DemandedFromOps; DemandedFromLHS.clearLowBits(NTZ); if (ShrinkDemandedConstant(I, 0, DemandedFromLHS) || SimplifyDemandedBits(I, 0, DemandedFromLHS, LHSKnown, Depth + 1)) return disableWrapFlagsBasedOnUnusedHighBits(I, NLZ); // If we are known to be subtracting zeros from every bit below // the highest demanded bit, we just return the other side. if (DemandedFromOps.isSubsetOf(RHSKnown.Zero)) return I->getOperand(0); // We can't do this with the LHS for subtraction, unless we are only // demanding the LSB. if (DemandedFromOps.isOne() && DemandedFromOps.isSubsetOf(LHSKnown.Zero)) return I->getOperand(1); // Otherwise just compute the known bits of the result. bool NSW = cast(I)->hasNoSignedWrap(); Known = KnownBits::computeForAddSub(false, NSW, LHSKnown, RHSKnown); break; } case Instruction::Mul: { APInt DemandedFromOps; if (simplifyOperandsBasedOnUnusedHighBits(DemandedFromOps)) return I; if (DemandedMask.isPowerOf2()) { // The LSB of X*Y is set only if (X & 1) == 1 and (Y & 1) == 1. // If we demand exactly one bit N and we have "X * (C' << N)" where C' is // odd (has LSB set), then the left-shifted low bit of X is the answer. unsigned CTZ = DemandedMask.countr_zero(); const APInt *C; if (match(I->getOperand(1), m_APInt(C)) && C->countr_zero() == CTZ) { Constant *ShiftC = ConstantInt::get(VTy, CTZ); Instruction *Shl = BinaryOperator::CreateShl(I->getOperand(0), ShiftC); return InsertNewInstWith(Shl, I->getIterator()); } } // For a squared value "X * X", the bottom 2 bits are 0 and X[0] because: // X * X is odd iff X is odd. // 'Quadratic Reciprocity': X * X -> 0 for bit[1] if (I->getOperand(0) == I->getOperand(1) && DemandedMask.ult(4)) { Constant *One = ConstantInt::get(VTy, 1); Instruction *And1 = BinaryOperator::CreateAnd(I->getOperand(0), One); return InsertNewInstWith(And1, I->getIterator()); } computeKnownBits(I, Known, Depth, CxtI); break; } case Instruction::Shl: { const APInt *SA; if (match(I->getOperand(1), m_APInt(SA))) { const APInt *ShrAmt; if (match(I->getOperand(0), m_Shr(m_Value(), m_APInt(ShrAmt)))) if (Instruction *Shr = dyn_cast(I->getOperand(0))) if (Value *R = simplifyShrShlDemandedBits(Shr, *ShrAmt, I, *SA, DemandedMask, Known)) return R; // TODO: If we only want bits that already match the signbit then we don't // need to shift. // If we can pre-shift a right-shifted constant to the left without // losing any high bits amd we don't demand the low bits, then eliminate // the left-shift: // (C >> X) << LeftShiftAmtC --> (C << RightShiftAmtC) >> X uint64_t ShiftAmt = SA->getLimitedValue(BitWidth-1); Value *X; Constant *C; if (DemandedMask.countr_zero() >= ShiftAmt && match(I->getOperand(0), m_LShr(m_ImmConstant(C), m_Value(X)))) { Constant *LeftShiftAmtC = ConstantInt::get(VTy, ShiftAmt); Constant *NewC = ConstantFoldBinaryOpOperands(Instruction::Shl, C, LeftShiftAmtC, DL); if (ConstantFoldBinaryOpOperands(Instruction::LShr, NewC, LeftShiftAmtC, DL) == C) { Instruction *Lshr = BinaryOperator::CreateLShr(NewC, X); return InsertNewInstWith(Lshr, I->getIterator()); } } APInt DemandedMaskIn(DemandedMask.lshr(ShiftAmt)); // If the shift is NUW/NSW, then it does demand the high bits. ShlOperator *IOp = cast(I); if (IOp->hasNoSignedWrap()) DemandedMaskIn.setHighBits(ShiftAmt+1); else if (IOp->hasNoUnsignedWrap()) DemandedMaskIn.setHighBits(ShiftAmt); if (SimplifyDemandedBits(I, 0, DemandedMaskIn, Known, Depth + 1)) return I; assert(!Known.hasConflict() && "Bits known to be one AND zero?"); Known = KnownBits::shl(Known, KnownBits::makeConstant(APInt(BitWidth, ShiftAmt)), /* NUW */ IOp->hasNoUnsignedWrap(), /* NSW */ IOp->hasNoSignedWrap()); } else { // This is a variable shift, so we can't shift the demand mask by a known // amount. But if we are not demanding high bits, then we are not // demanding those bits from the pre-shifted operand either. if (unsigned CTLZ = DemandedMask.countl_zero()) { APInt DemandedFromOp(APInt::getLowBitsSet(BitWidth, BitWidth - CTLZ)); if (SimplifyDemandedBits(I, 0, DemandedFromOp, Known, Depth + 1)) { // We can't guarantee that nsw/nuw hold after simplifying the operand. I->dropPoisonGeneratingFlags(); return I; } } computeKnownBits(I, Known, Depth, CxtI); } break; } case Instruction::LShr: { const APInt *SA; if (match(I->getOperand(1), m_APInt(SA))) { uint64_t ShiftAmt = SA->getLimitedValue(BitWidth-1); // If we are just demanding the shifted sign bit and below, then this can // be treated as an ASHR in disguise. if (DemandedMask.countl_zero() >= ShiftAmt) { // If we only want bits that already match the signbit then we don't // need to shift. unsigned NumHiDemandedBits = BitWidth - DemandedMask.countr_zero(); unsigned SignBits = ComputeNumSignBits(I->getOperand(0), Depth + 1, CxtI); if (SignBits >= NumHiDemandedBits) return I->getOperand(0); // If we can pre-shift a left-shifted constant to the right without // losing any low bits (we already know we don't demand the high bits), // then eliminate the right-shift: // (C << X) >> RightShiftAmtC --> (C >> RightShiftAmtC) << X Value *X; Constant *C; if (match(I->getOperand(0), m_Shl(m_ImmConstant(C), m_Value(X)))) { Constant *RightShiftAmtC = ConstantInt::get(VTy, ShiftAmt); Constant *NewC = ConstantFoldBinaryOpOperands(Instruction::LShr, C, RightShiftAmtC, DL); if (ConstantFoldBinaryOpOperands(Instruction::Shl, NewC, RightShiftAmtC, DL) == C) { Instruction *Shl = BinaryOperator::CreateShl(NewC, X); return InsertNewInstWith(Shl, I->getIterator()); } } } // Unsigned shift right. APInt DemandedMaskIn(DemandedMask.shl(ShiftAmt)); if (SimplifyDemandedBits(I, 0, DemandedMaskIn, Known, Depth + 1)) { // exact flag may not longer hold. I->dropPoisonGeneratingFlags(); return I; } assert(!Known.hasConflict() && "Bits known to be one AND zero?"); Known.Zero.lshrInPlace(ShiftAmt); Known.One.lshrInPlace(ShiftAmt); if (ShiftAmt) Known.Zero.setHighBits(ShiftAmt); // high bits known zero. } else { computeKnownBits(I, Known, Depth, CxtI); } break; } case Instruction::AShr: { unsigned SignBits = ComputeNumSignBits(I->getOperand(0), Depth + 1, CxtI); // If we only want bits that already match the signbit then we don't need // to shift. unsigned NumHiDemandedBits = BitWidth - DemandedMask.countr_zero(); if (SignBits >= NumHiDemandedBits) return I->getOperand(0); // 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.isOne()) { // Perform the logical shift right. Instruction *NewVal = BinaryOperator::CreateLShr( I->getOperand(0), I->getOperand(1), I->getName()); return InsertNewInstWith(NewVal, I->getIterator()); } const APInt *SA; if (match(I->getOperand(1), m_APInt(SA))) { 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.countl_zero() <= ShiftAmt) DemandedMaskIn.setSignBit(); if (SimplifyDemandedBits(I, 0, DemandedMaskIn, Known, Depth + 1)) { // exact flag may not longer hold. I->dropPoisonGeneratingFlags(); return I; } assert(!Known.hasConflict() && "Bits known to be one AND zero?"); // Compute the new bits that are at the top now plus sign bits. APInt HighBits(APInt::getHighBitsSet( BitWidth, std::min(SignBits + ShiftAmt - 1, BitWidth))); Known.Zero.lshrInPlace(ShiftAmt); Known.One.lshrInPlace(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. assert(BitWidth > ShiftAmt && "Shift amount not saturated?"); if (Known.Zero[BitWidth-ShiftAmt-1] || !DemandedMask.intersects(HighBits)) { BinaryOperator *LShr = BinaryOperator::CreateLShr(I->getOperand(0), I->getOperand(1)); LShr->setIsExact(cast(I)->isExact()); LShr->takeName(I); return InsertNewInstWith(LShr, I->getIterator()); } else if (Known.One[BitWidth-ShiftAmt-1]) { // New bits are known one. Known.One |= HighBits; } } else { computeKnownBits(I, Known, Depth, CxtI); } break; } case Instruction::UDiv: { // UDiv doesn't demand low bits that are zero in the divisor. const APInt *SA; if (match(I->getOperand(1), m_APInt(SA))) { // TODO: Take the demanded mask of the result into account. unsigned RHSTrailingZeros = SA->countr_zero(); APInt DemandedMaskIn = APInt::getHighBitsSet(BitWidth, BitWidth - RHSTrailingZeros); if (SimplifyDemandedBits(I, 0, DemandedMaskIn, LHSKnown, Depth + 1)) { // We can't guarantee that "exact" is still true after changing the // the dividend. I->dropPoisonGeneratingFlags(); return I; } Known = KnownBits::udiv(LHSKnown, KnownBits::makeConstant(*SA), cast(I)->isExact()); } else { computeKnownBits(I, Known, Depth, CxtI); } break; } case Instruction::SRem: { const APInt *Rem; if (match(I->getOperand(1), m_APInt(Rem))) { // X % -1 demands all the bits because we don't want to introduce // INT_MIN % -1 (== undef) by accident. if (Rem->isAllOnes()) break; APInt RA = Rem->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::getSignMask(BitWidth); if (SimplifyDemandedBits(I, 0, Mask2, LHSKnown, Depth + 1)) return I; // The low bits of LHS are unchanged by the srem. Known.Zero = LHSKnown.Zero & LowBits; Known.One = LHSKnown.One & LowBits; // If LHS is non-negative or has all low bits zero, then the upper bits // are all zero. if (LHSKnown.isNonNegative() || LowBits.isSubsetOf(LHSKnown.Zero)) Known.Zero |= ~LowBits; // If LHS is negative and not all low bits are zero, then the upper bits // are all one. if (LHSKnown.isNegative() && LowBits.intersects(LHSKnown.One)) Known.One |= ~LowBits; assert(!Known.hasConflict() && "Bits known to be one AND zero?"); break; } } computeKnownBits(I, Known, Depth, CxtI); break; } case Instruction::URem: { APInt AllOnes = APInt::getAllOnes(BitWidth); if (SimplifyDemandedBits(I, 0, AllOnes, LHSKnown, Depth + 1) || SimplifyDemandedBits(I, 1, AllOnes, RHSKnown, Depth + 1)) return I; Known = KnownBits::urem(LHSKnown, RHSKnown); break; } case Instruction::Call: { bool KnownBitsComputed = false; if (IntrinsicInst *II = dyn_cast(I)) { switch (II->getIntrinsicID()) { case Intrinsic::abs: { if (DemandedMask == 1) return II->getArgOperand(0); break; } case Intrinsic::ctpop: { // Checking if the number of clear bits is odd (parity)? If the type has // an even number of bits, that's the same as checking if the number of // set bits is odd, so we can eliminate the 'not' op. Value *X; if (DemandedMask == 1 && VTy->getScalarSizeInBits() % 2 == 0 && match(II->getArgOperand(0), m_Not(m_Value(X)))) { Function *Ctpop = Intrinsic::getDeclaration( II->getModule(), Intrinsic::ctpop, VTy); return InsertNewInstWith(CallInst::Create(Ctpop, {X}), I->getIterator()); } 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.countl_zero(); unsigned NTZ = DemandedMask.countr_zero(); // 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 = alignDown(NLZ, 8); NTZ = alignDown(NTZ, 8); // If we need exactly one byte, we can do this transformation. if (BitWidth - NLZ - NTZ == 8) { // Replace this with either a left or right shift to get the byte into // the right place. Instruction *NewVal; if (NLZ > NTZ) NewVal = BinaryOperator::CreateLShr( II->getArgOperand(0), ConstantInt::get(VTy, NLZ - NTZ)); else NewVal = BinaryOperator::CreateShl( II->getArgOperand(0), ConstantInt::get(VTy, NTZ - NLZ)); NewVal->takeName(I); return InsertNewInstWith(NewVal, I->getIterator()); } break; } case Intrinsic::ptrmask: { unsigned MaskWidth = I->getOperand(1)->getType()->getScalarSizeInBits(); RHSKnown = KnownBits(MaskWidth); // If either the LHS or the RHS are Zero, the result is zero. if (SimplifyDemandedBits(I, 0, DemandedMask, LHSKnown, Depth + 1) || SimplifyDemandedBits( I, 1, (DemandedMask & ~LHSKnown.Zero).zextOrTrunc(MaskWidth), RHSKnown, Depth + 1)) return I; // TODO: Should be 1-extend RHSKnown = RHSKnown.anyextOrTrunc(BitWidth); assert(!RHSKnown.hasConflict() && "Bits known to be one AND zero?"); assert(!LHSKnown.hasConflict() && "Bits known to be one AND zero?"); Known = LHSKnown & RHSKnown; KnownBitsComputed = true; // If the client is only demanding bits we know to be zero, return // `llvm.ptrmask(p, 0)`. We can't return `null` here due to pointer // provenance, but making the mask zero will be easily optimizable in // the backend. if (DemandedMask.isSubsetOf(Known.Zero) && !match(I->getOperand(1), m_Zero())) return replaceOperand( *I, 1, Constant::getNullValue(I->getOperand(1)->getType())); // Mask in demanded space does nothing. // NOTE: We may have attributes associated with the return value of the // llvm.ptrmask intrinsic that will be lost when we just return the // operand. We should try to preserve them. if (DemandedMask.isSubsetOf(RHSKnown.One | LHSKnown.Zero)) return I->getOperand(0); // If the RHS is a constant, see if we can simplify it. if (ShrinkDemandedConstant( I, 1, (DemandedMask & ~LHSKnown.Zero).zextOrTrunc(MaskWidth))) return I; break; } case Intrinsic::fshr: case Intrinsic::fshl: { const APInt *SA; if (!match(I->getOperand(2), m_APInt(SA))) break; // Normalize to funnel shift left. APInt shifts of BitWidth are well- // defined, so no need to special-case zero shifts here. uint64_t ShiftAmt = SA->urem(BitWidth); if (II->getIntrinsicID() == Intrinsic::fshr) ShiftAmt = BitWidth - ShiftAmt; APInt DemandedMaskLHS(DemandedMask.lshr(ShiftAmt)); APInt DemandedMaskRHS(DemandedMask.shl(BitWidth - ShiftAmt)); if (I->getOperand(0) != I->getOperand(1)) { if (SimplifyDemandedBits(I, 0, DemandedMaskLHS, LHSKnown, Depth + 1) || SimplifyDemandedBits(I, 1, DemandedMaskRHS, RHSKnown, Depth + 1)) return I; } else { // fshl is a rotate // Avoid converting rotate into funnel shift. // Only simplify if one operand is constant. LHSKnown = computeKnownBits(I->getOperand(0), Depth + 1, I); if (DemandedMaskLHS.isSubsetOf(LHSKnown.Zero | LHSKnown.One) && !match(I->getOperand(0), m_SpecificInt(LHSKnown.One))) { replaceOperand(*I, 0, Constant::getIntegerValue(VTy, LHSKnown.One)); return I; } RHSKnown = computeKnownBits(I->getOperand(1), Depth + 1, I); if (DemandedMaskRHS.isSubsetOf(RHSKnown.Zero | RHSKnown.One) && !match(I->getOperand(1), m_SpecificInt(RHSKnown.One))) { replaceOperand(*I, 1, Constant::getIntegerValue(VTy, RHSKnown.One)); return I; } } Known.Zero = LHSKnown.Zero.shl(ShiftAmt) | RHSKnown.Zero.lshr(BitWidth - ShiftAmt); Known.One = LHSKnown.One.shl(ShiftAmt) | RHSKnown.One.lshr(BitWidth - ShiftAmt); KnownBitsComputed = true; break; } case Intrinsic::umax: { // UMax(A, C) == A if ... // The lowest non-zero bit of DemandMask is higher than the highest // non-zero bit of C. const APInt *C; unsigned CTZ = DemandedMask.countr_zero(); if (match(II->getArgOperand(1), m_APInt(C)) && CTZ >= C->getActiveBits()) return II->getArgOperand(0); break; } case Intrinsic::umin: { // UMin(A, C) == A if ... // The lowest non-zero bit of DemandMask is higher than the highest // non-one bit of C. // This comes from using DeMorgans on the above umax example. const APInt *C; unsigned CTZ = DemandedMask.countr_zero(); if (match(II->getArgOperand(1), m_APInt(C)) && CTZ >= C->getBitWidth() - C->countl_one()) return II->getArgOperand(0); break; } default: { // Handle target specific intrinsics std::optional V = targetSimplifyDemandedUseBitsIntrinsic( *II, DemandedMask, Known, KnownBitsComputed); if (V) return *V; break; } } } if (!KnownBitsComputed) computeKnownBits(V, Known, Depth, CxtI); break; } } if (V->getType()->isPointerTy()) { Align Alignment = V->getPointerAlignment(DL); Known.Zero.setLowBits(Log2(Alignment)); } // If the client is only demanding bits that we know, return the known // constant. We can't directly simplify pointers as a constant because of // pointer provenance. // TODO: We could return `(inttoptr const)` for pointers. if (!V->getType()->isPointerTy() && DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(VTy, Known.One); if (VerifyKnownBits) { KnownBits ReferenceKnown = computeKnownBits(V, Depth, CxtI); if (Known != ReferenceKnown) { errs() << "Mismatched known bits for " << *V << " in " << I->getFunction()->getName() << "\n"; errs() << "computeKnownBits(): " << ReferenceKnown << "\n"; errs() << "SimplifyDemandedBits(): " << Known << "\n"; std::abort(); } } return nullptr; } /// Helper routine of SimplifyDemandedUseBits. It computes Known /// bits. It also tries to handle simplifications that can be done based on /// DemandedMask, but without modifying the Instruction. Value *InstCombinerImpl::SimplifyMultipleUseDemandedBits( Instruction *I, const APInt &DemandedMask, KnownBits &Known, unsigned Depth, Instruction *CxtI) { unsigned BitWidth = DemandedMask.getBitWidth(); Type *ITy = I->getType(); KnownBits LHSKnown(BitWidth); KnownBits RHSKnown(BitWidth); // Despite the fact that we can't simplify this instruction in all User's // context, we can at least compute the known 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. switch (I->getOpcode()) { case Instruction::And: { computeKnownBits(I->getOperand(1), RHSKnown, Depth + 1, CxtI); computeKnownBits(I->getOperand(0), LHSKnown, Depth + 1, CxtI); Known = analyzeKnownBitsFromAndXorOr(cast(I), LHSKnown, RHSKnown, Depth, SQ.getWithInstruction(CxtI)); computeKnownBitsFromContext(I, Known, Depth, SQ.getWithInstruction(CxtI)); // If the client is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(ITy, Known.One); // 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.isSubsetOf(LHSKnown.Zero | RHSKnown.One)) return I->getOperand(0); if (DemandedMask.isSubsetOf(RHSKnown.Zero | LHSKnown.One)) return I->getOperand(1); break; } case Instruction::Or: { computeKnownBits(I->getOperand(1), RHSKnown, Depth + 1, CxtI); computeKnownBits(I->getOperand(0), LHSKnown, Depth + 1, CxtI); Known = analyzeKnownBitsFromAndXorOr(cast(I), LHSKnown, RHSKnown, Depth, SQ.getWithInstruction(CxtI)); computeKnownBitsFromContext(I, Known, Depth, SQ.getWithInstruction(CxtI)); // If the client is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(ITy, Known.One); // 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 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.isSubsetOf(LHSKnown.One | RHSKnown.Zero)) return I->getOperand(0); if (DemandedMask.isSubsetOf(RHSKnown.One | LHSKnown.Zero)) return I->getOperand(1); break; } case Instruction::Xor: { computeKnownBits(I->getOperand(1), RHSKnown, Depth + 1, CxtI); computeKnownBits(I->getOperand(0), LHSKnown, Depth + 1, CxtI); Known = analyzeKnownBitsFromAndXorOr(cast(I), LHSKnown, RHSKnown, Depth, SQ.getWithInstruction(CxtI)); computeKnownBitsFromContext(I, Known, Depth, SQ.getWithInstruction(CxtI)); // If the client is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(ITy, Known.One); // 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 all of the demanded bits are known zero on one side, return the other. if (DemandedMask.isSubsetOf(RHSKnown.Zero)) return I->getOperand(0); if (DemandedMask.isSubsetOf(LHSKnown.Zero)) return I->getOperand(1); break; } case Instruction::Add: { unsigned NLZ = DemandedMask.countl_zero(); APInt DemandedFromOps = APInt::getLowBitsSet(BitWidth, BitWidth - NLZ); // If an operand adds zeros to every bit below the highest demanded bit, // that operand doesn't change the result. Return the other side. computeKnownBits(I->getOperand(1), RHSKnown, Depth + 1, CxtI); if (DemandedFromOps.isSubsetOf(RHSKnown.Zero)) return I->getOperand(0); computeKnownBits(I->getOperand(0), LHSKnown, Depth + 1, CxtI); if (DemandedFromOps.isSubsetOf(LHSKnown.Zero)) return I->getOperand(1); bool NSW = cast(I)->hasNoSignedWrap(); Known = KnownBits::computeForAddSub(/*Add*/ true, NSW, LHSKnown, RHSKnown); computeKnownBitsFromContext(I, Known, Depth, SQ.getWithInstruction(CxtI)); break; } case Instruction::Sub: { unsigned NLZ = DemandedMask.countl_zero(); APInt DemandedFromOps = APInt::getLowBitsSet(BitWidth, BitWidth - NLZ); // If an operand subtracts zeros from every bit below the highest demanded // bit, that operand doesn't change the result. Return the other side. computeKnownBits(I->getOperand(1), RHSKnown, Depth + 1, CxtI); if (DemandedFromOps.isSubsetOf(RHSKnown.Zero)) return I->getOperand(0); bool NSW = cast(I)->hasNoSignedWrap(); computeKnownBits(I->getOperand(0), LHSKnown, Depth + 1, CxtI); Known = KnownBits::computeForAddSub(/*Add*/ false, NSW, LHSKnown, RHSKnown); computeKnownBitsFromContext(I, Known, Depth, SQ.getWithInstruction(CxtI)); break; } case Instruction::AShr: { // Compute the Known bits to simplify things downstream. computeKnownBits(I, Known, Depth, CxtI); // If this user is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero | Known.One)) return Constant::getIntegerValue(ITy, Known.One); // If the right shift operand 0 is a result of a left shift by the same // amount, this is probably a zero/sign extension, which may be unnecessary, // if we do not demand any of the new sign bits. So, return the original // operand instead. const APInt *ShiftRC; const APInt *ShiftLC; Value *X; unsigned BitWidth = DemandedMask.getBitWidth(); if (match(I, m_AShr(m_Shl(m_Value(X), m_APInt(ShiftLC)), m_APInt(ShiftRC))) && ShiftLC == ShiftRC && ShiftLC->ult(BitWidth) && DemandedMask.isSubsetOf(APInt::getLowBitsSet( BitWidth, BitWidth - ShiftRC->getZExtValue()))) { return X; } break; } default: // Compute the Known bits to simplify things downstream. computeKnownBits(I, Known, Depth, CxtI); // If this user is only demanding bits that we know, return the known // constant. if (DemandedMask.isSubsetOf(Known.Zero|Known.One)) return Constant::getIntegerValue(ITy, Known.One); break; } return nullptr; } /// Helper routine of SimplifyDemandedUseBits. It tries to simplify /// "E1 = (X lsr C1) << C2", where the C1 and C2 are constant, into /// "E2 = X << (C2 - C1)" or "E2 = X >> (C1 - C2)", depending on the sign /// of "C2-C1". /// /// Suppose E1 and E2 are generally different in bits S={bm, bm+1, /// ..., bn}, without considering the specific value X is holding. /// This transformation is legal iff one of following conditions is hold: /// 1) All the bit in S are 0, in this case E1 == E2. /// 2) We don't care those bits in S, per the input DemandedMask. /// 3) Combination of 1) and 2). Some bits in S are 0, and we don't care the /// rest bits. /// /// Currently we only test condition 2). /// /// As with SimplifyDemandedUseBits, it returns NULL if the simplification was /// not successful. Value *InstCombinerImpl::simplifyShrShlDemandedBits( Instruction *Shr, const APInt &ShrOp1, Instruction *Shl, const APInt &ShlOp1, const APInt &DemandedMask, KnownBits &Known) { if (!ShlOp1 || !ShrOp1) return nullptr; // No-op. Value *VarX = Shr->getOperand(0); Type *Ty = VarX->getType(); unsigned BitWidth = Ty->getScalarSizeInBits(); if (ShlOp1.uge(BitWidth) || ShrOp1.uge(BitWidth)) return nullptr; // Undef. unsigned ShlAmt = ShlOp1.getZExtValue(); unsigned ShrAmt = ShrOp1.getZExtValue(); Known.One.clearAllBits(); Known.Zero.setLowBits(ShlAmt - 1); Known.Zero &= DemandedMask; APInt BitMask1(APInt::getAllOnes(BitWidth)); APInt BitMask2(APInt::getAllOnes(BitWidth)); bool isLshr = (Shr->getOpcode() == Instruction::LShr); BitMask1 = isLshr ? (BitMask1.lshr(ShrAmt) << ShlAmt) : (BitMask1.ashr(ShrAmt) << ShlAmt); if (ShrAmt <= ShlAmt) { BitMask2 <<= (ShlAmt - ShrAmt); } else { BitMask2 = isLshr ? BitMask2.lshr(ShrAmt - ShlAmt): BitMask2.ashr(ShrAmt - ShlAmt); } // Check if condition-2 (see the comment to this function) is satified. if ((BitMask1 & DemandedMask) == (BitMask2 & DemandedMask)) { if (ShrAmt == ShlAmt) return VarX; if (!Shr->hasOneUse()) return nullptr; BinaryOperator *New; if (ShrAmt < ShlAmt) { Constant *Amt = ConstantInt::get(VarX->getType(), ShlAmt - ShrAmt); New = BinaryOperator::CreateShl(VarX, Amt); BinaryOperator *Orig = cast(Shl); New->setHasNoSignedWrap(Orig->hasNoSignedWrap()); New->setHasNoUnsignedWrap(Orig->hasNoUnsignedWrap()); } else { Constant *Amt = ConstantInt::get(VarX->getType(), ShrAmt - ShlAmt); New = isLshr ? BinaryOperator::CreateLShr(VarX, Amt) : BinaryOperator::CreateAShr(VarX, Amt); if (cast(Shr)->isExact()) New->setIsExact(true); } return InsertNewInstWith(New, Shl->getIterator()); } return nullptr; } /// The specified value produces a vector with any number of elements. /// This method analyzes which elements of the operand are poison and /// returns that information in PoisonElts. /// /// DemandedElts contains the set of elements that are actually used by the /// caller, and by default (AllowMultipleUsers equals false) the value is /// simplified only if it has a single caller. If AllowMultipleUsers is set /// to true, DemandedElts refers to the union of sets of elements that are /// used by all callers. /// /// 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 *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts, APInt &PoisonElts, unsigned Depth, bool AllowMultipleUsers) { // Cannot analyze scalable type. The number of vector elements is not a // compile-time constant. if (isa(V->getType())) return nullptr; unsigned VWidth = cast(V->getType())->getNumElements(); APInt EltMask(APInt::getAllOnes(VWidth)); assert((DemandedElts & ~EltMask) == 0 && "Invalid DemandedElts!"); if (match(V, m_Poison())) { // If the entire vector is poison, just return this info. PoisonElts = EltMask; return nullptr; } if (DemandedElts.isZero()) { // If nothing is demanded, provide poison. PoisonElts = EltMask; return PoisonValue::get(V->getType()); } PoisonElts = 0; if (auto *C = dyn_cast(V)) { // Check if this is identity. If so, return 0 since we are not simplifying // anything. if (DemandedElts.isAllOnes()) return nullptr; Type *EltTy = cast(V->getType())->getElementType(); Constant *Poison = PoisonValue::get(EltTy); SmallVector Elts; for (unsigned i = 0; i != VWidth; ++i) { if (!DemandedElts[i]) { // If not demanded, set to poison. Elts.push_back(Poison); PoisonElts.setBit(i); continue; } Constant *Elt = C->getAggregateElement(i); if (!Elt) return nullptr; Elts.push_back(Elt); if (isa(Elt)) // Already poison. PoisonElts.setBit(i); } // If we changed the constant, return it. Constant *NewCV = ConstantVector::get(Elts); return NewCV != C ? NewCV : nullptr; } // Limit search depth. if (Depth == 10) return nullptr; if (!AllowMultipleUsers) { // 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 PoisonElts information recursively. return nullptr; // Conservatively assume that all elements are needed. DemandedElts = EltMask; } } Instruction *I = dyn_cast(V); if (!I) return nullptr; // Only analyze instructions. bool MadeChange = false; auto simplifyAndSetOp = [&](Instruction *Inst, unsigned OpNum, APInt Demanded, APInt &Undef) { auto *II = dyn_cast(Inst); Value *Op = II ? II->getArgOperand(OpNum) : Inst->getOperand(OpNum); if (Value *V = SimplifyDemandedVectorElts(Op, Demanded, Undef, Depth + 1)) { replaceOperand(*Inst, OpNum, V); MadeChange = true; } }; APInt PoisonElts2(VWidth, 0); APInt PoisonElts3(VWidth, 0); switch (I->getOpcode()) { default: break; case Instruction::GetElementPtr: { // The LangRef requires that struct geps have all constant indices. As // such, we can't convert any operand to partial undef. auto mayIndexStructType = [](GetElementPtrInst &GEP) { for (auto I = gep_type_begin(GEP), E = gep_type_end(GEP); I != E; I++) if (I.isStruct()) return true; return false; }; if (mayIndexStructType(cast(*I))) break; // Conservatively track the demanded elements back through any vector // operands we may have. We know there must be at least one, or we // wouldn't have a vector result to get here. Note that we intentionally // merge the undef bits here since gepping with either an poison base or // index results in poison. for (unsigned i = 0; i < I->getNumOperands(); i++) { if (i == 0 ? match(I->getOperand(i), m_Undef()) : match(I->getOperand(i), m_Poison())) { // If the entire vector is undefined, just return this info. PoisonElts = EltMask; return nullptr; } if (I->getOperand(i)->getType()->isVectorTy()) { APInt PoisonEltsOp(VWidth, 0); simplifyAndSetOp(I, i, DemandedElts, PoisonEltsOp); // gep(x, undef) is not undef, so skip considering idx ops here // Note that we could propagate poison, but we can't distinguish between // undef & poison bits ATM if (i == 0) PoisonElts |= PoisonEltsOp; } } 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(I->getOperand(2)); if (!Idx) { // Note that we can't propagate undef elt info, because we don't know // which elt is getting updated. simplifyAndSetOp(I, 0, DemandedElts, PoisonElts2); break; } // The element inserted overwrites whatever was there, so the input demanded // set is simpler than the output set. unsigned IdxNo = Idx->getZExtValue(); APInt PreInsertDemandedElts = DemandedElts; if (IdxNo < VWidth) PreInsertDemandedElts.clearBit(IdxNo); // If we only demand the element that is being inserted and that element // was extracted from the same index in another vector with the same type, // replace this insert with that other vector. // Note: This is attempted before the call to simplifyAndSetOp because that // may change PoisonElts to a value that does not match with Vec. Value *Vec; if (PreInsertDemandedElts == 0 && match(I->getOperand(1), m_ExtractElt(m_Value(Vec), m_SpecificInt(IdxNo))) && Vec->getType() == I->getType()) { return Vec; } simplifyAndSetOp(I, 0, PreInsertDemandedElts, PoisonElts); // If this is inserting an element that isn't demanded, remove this // insertelement. if (IdxNo >= VWidth || !DemandedElts[IdxNo]) { Worklist.push(I); return I->getOperand(0); } // The inserted element is defined. PoisonElts.clearBit(IdxNo); break; } case Instruction::ShuffleVector: { auto *Shuffle = cast(I); assert(Shuffle->getOperand(0)->getType() == Shuffle->getOperand(1)->getType() && "Expected shuffle operands to have same type"); unsigned OpWidth = cast(Shuffle->getOperand(0)->getType()) ->getNumElements(); // Handle trivial case of a splat. Only check the first element of LHS // operand. if (all_of(Shuffle->getShuffleMask(), [](int Elt) { return Elt == 0; }) && DemandedElts.isAllOnes()) { if (!isa(I->getOperand(1))) { I->setOperand(1, PoisonValue::get(I->getOperand(1)->getType())); MadeChange = true; } APInt LeftDemanded(OpWidth, 1); APInt LHSPoisonElts(OpWidth, 0); simplifyAndSetOp(I, 0, LeftDemanded, LHSPoisonElts); if (LHSPoisonElts[0]) PoisonElts = EltMask; else PoisonElts.clearAllBits(); break; } APInt LeftDemanded(OpWidth, 0), RightDemanded(OpWidth, 0); for (unsigned i = 0; i < VWidth; i++) { if (DemandedElts[i]) { unsigned MaskVal = Shuffle->getMaskValue(i); if (MaskVal != -1u) { assert(MaskVal < OpWidth * 2 && "shufflevector mask index out of range!"); if (MaskVal < OpWidth) LeftDemanded.setBit(MaskVal); else RightDemanded.setBit(MaskVal - OpWidth); } } } APInt LHSPoisonElts(OpWidth, 0); simplifyAndSetOp(I, 0, LeftDemanded, LHSPoisonElts); APInt RHSPoisonElts(OpWidth, 0); simplifyAndSetOp(I, 1, RightDemanded, RHSPoisonElts); // If this shuffle does not change the vector length and the elements // demanded by this shuffle are an identity mask, then this shuffle is // unnecessary. // // We are assuming canonical form for the mask, so the source vector is // operand 0 and operand 1 is not used. // // Note that if an element is demanded and this shuffle mask is undefined // for that element, then the shuffle is not considered an identity // operation. The shuffle prevents poison from the operand vector from // leaking to the result by replacing poison with an undefined value. if (VWidth == OpWidth) { bool IsIdentityShuffle = true; for (unsigned i = 0; i < VWidth; i++) { unsigned MaskVal = Shuffle->getMaskValue(i); if (DemandedElts[i] && i != MaskVal) { IsIdentityShuffle = false; break; } } if (IsIdentityShuffle) return Shuffle->getOperand(0); } bool NewPoisonElts = false; unsigned LHSIdx = -1u, LHSValIdx = -1u; unsigned RHSIdx = -1u, RHSValIdx = -1u; bool LHSUniform = true; bool RHSUniform = true; for (unsigned i = 0; i < VWidth; i++) { unsigned MaskVal = Shuffle->getMaskValue(i); if (MaskVal == -1u) { PoisonElts.setBit(i); } else if (!DemandedElts[i]) { NewPoisonElts = true; PoisonElts.setBit(i); } else if (MaskVal < OpWidth) { if (LHSPoisonElts[MaskVal]) { NewPoisonElts = true; PoisonElts.setBit(i); } else { LHSIdx = LHSIdx == -1u ? i : OpWidth; LHSValIdx = LHSValIdx == -1u ? MaskVal : OpWidth; LHSUniform = LHSUniform && (MaskVal == i); } } else { if (RHSPoisonElts[MaskVal - OpWidth]) { NewPoisonElts = true; PoisonElts.setBit(i); } else { RHSIdx = RHSIdx == -1u ? i : OpWidth; RHSValIdx = RHSValIdx == -1u ? MaskVal - OpWidth : OpWidth; RHSUniform = RHSUniform && (MaskVal - OpWidth == i); } } } // Try to transform shuffle with constant vector and single element from // this constant vector to single insertelement instruction. // shufflevector V, C, -> // insertelement V, C[ci], ci-n if (OpWidth == cast(Shuffle->getType())->getNumElements()) { Value *Op = nullptr; Constant *Value = nullptr; unsigned Idx = -1u; // Find constant vector with the single element in shuffle (LHS or RHS). if (LHSIdx < OpWidth && RHSUniform) { if (auto *CV = dyn_cast(Shuffle->getOperand(0))) { Op = Shuffle->getOperand(1); Value = CV->getOperand(LHSValIdx); Idx = LHSIdx; } } if (RHSIdx < OpWidth && LHSUniform) { if (auto *CV = dyn_cast(Shuffle->getOperand(1))) { Op = Shuffle->getOperand(0); Value = CV->getOperand(RHSValIdx); Idx = RHSIdx; } } // Found constant vector with single element - convert to insertelement. if (Op && Value) { Instruction *New = InsertElementInst::Create( Op, Value, ConstantInt::get(Type::getInt64Ty(I->getContext()), Idx), Shuffle->getName()); InsertNewInstWith(New, Shuffle->getIterator()); return New; } } if (NewPoisonElts) { // Add additional discovered undefs. SmallVector Elts; for (unsigned i = 0; i < VWidth; ++i) { if (PoisonElts[i]) Elts.push_back(PoisonMaskElem); else Elts.push_back(Shuffle->getMaskValue(i)); } Shuffle->setShuffleMask(Elts); MadeChange = true; } break; } case Instruction::Select: { // If this is a vector select, try to transform the select condition based // on the current demanded elements. SelectInst *Sel = cast(I); if (Sel->getCondition()->getType()->isVectorTy()) { // TODO: We are not doing anything with PoisonElts based on this call. // It is overwritten below based on the other select operands. If an // element of the select condition is known undef, then we are free to // choose the output value from either arm of the select. If we know that // one of those values is undef, then the output can be undef. simplifyAndSetOp(I, 0, DemandedElts, PoisonElts); } // Next, see if we can transform the arms of the select. APInt DemandedLHS(DemandedElts), DemandedRHS(DemandedElts); if (auto *CV = dyn_cast(Sel->getCondition())) { for (unsigned i = 0; i < VWidth; i++) { // isNullValue() always returns false when called on a ConstantExpr. // Skip constant expressions to avoid propagating incorrect information. Constant *CElt = CV->getAggregateElement(i); if (isa(CElt)) continue; // TODO: If a select condition element is undef, we can demand from // either side. If one side is known undef, choosing that side would // propagate undef. if (CElt->isNullValue()) DemandedLHS.clearBit(i); else DemandedRHS.clearBit(i); } } simplifyAndSetOp(I, 1, DemandedLHS, PoisonElts2); simplifyAndSetOp(I, 2, DemandedRHS, PoisonElts3); // Output elements are undefined if the element from each arm is undefined. // TODO: This can be improved. See comment in select condition handling. PoisonElts = PoisonElts2 & PoisonElts3; break; } case Instruction::BitCast: { // Vector->vector casts only. VectorType *VTy = dyn_cast(I->getOperand(0)->getType()); if (!VTy) break; unsigned InVWidth = cast(VTy)->getNumElements(); APInt InputDemandedElts(InVWidth, 0); PoisonElts2 = APInt(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) == 0) { // If the number of elements in the output is a multiple of the number of // elements in the input 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 if ((InVWidth % VWidth) == 0) { // If the number of elements in the input is a multiple of the number of // elements in the output 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); } else { // Unsupported so far. break; } simplifyAndSetOp(I, 0, InputDemandedElts, PoisonElts2); if (VWidth == InVWidth) { PoisonElts = PoisonElts2; } else if ((VWidth % InVWidth) == 0) { // If the number of elements in the output is a multiple of the number of // elements in the input then an output element is undef if the // corresponding input element is undef. for (unsigned OutIdx = 0; OutIdx != VWidth; ++OutIdx) if (PoisonElts2[OutIdx / Ratio]) PoisonElts.setBit(OutIdx); } else if ((InVWidth % VWidth) == 0) { // If the number of elements in the input is a multiple of the number of // elements in the output then an output element is undef if all of the // corresponding input elements are undef. for (unsigned OutIdx = 0; OutIdx != VWidth; ++OutIdx) { APInt SubUndef = PoisonElts2.lshr(OutIdx * Ratio).zextOrTrunc(Ratio); if (SubUndef.popcount() == Ratio) PoisonElts.setBit(OutIdx); } } else { llvm_unreachable("Unimp"); } break; } case Instruction::FPTrunc: case Instruction::FPExt: simplifyAndSetOp(I, 0, DemandedElts, PoisonElts); break; case Instruction::Call: { IntrinsicInst *II = dyn_cast(I); if (!II) break; switch (II->getIntrinsicID()) { case Intrinsic::masked_gather: // fallthrough case Intrinsic::masked_load: { // Subtlety: If we load from a pointer, the pointer must be valid // regardless of whether the element is demanded. Doing otherwise risks // segfaults which didn't exist in the original program. APInt DemandedPtrs(APInt::getAllOnes(VWidth)), DemandedPassThrough(DemandedElts); if (auto *CV = dyn_cast(II->getOperand(2))) for (unsigned i = 0; i < VWidth; i++) { Constant *CElt = CV->getAggregateElement(i); if (CElt->isNullValue()) DemandedPtrs.clearBit(i); else if (CElt->isAllOnesValue()) DemandedPassThrough.clearBit(i); } if (II->getIntrinsicID() == Intrinsic::masked_gather) simplifyAndSetOp(II, 0, DemandedPtrs, PoisonElts2); simplifyAndSetOp(II, 3, DemandedPassThrough, PoisonElts3); // Output elements are undefined if the element from both sources are. // TODO: can strengthen via mask as well. PoisonElts = PoisonElts2 & PoisonElts3; break; } default: { // Handle target specific intrinsics std::optional V = targetSimplifyDemandedVectorEltsIntrinsic( *II, DemandedElts, PoisonElts, PoisonElts2, PoisonElts3, simplifyAndSetOp); if (V) return *V; break; } } // switch on IntrinsicID break; } // case Call } // switch on Opcode // TODO: We bail completely on integer div/rem and shifts because they have // UB/poison potential, but that should be refined. BinaryOperator *BO; if (match(I, m_BinOp(BO)) && !BO->isIntDivRem() && !BO->isShift()) { Value *X = BO->getOperand(0); Value *Y = BO->getOperand(1); // Look for an equivalent binop except that one operand has been shuffled. // If the demand for this binop only includes elements that are the same as // the other binop, then we may be able to replace this binop with a use of // the earlier one. // // Example: // %other_bo = bo (shuf X, {0}), Y // %this_extracted_bo = extelt (bo X, Y), 0 // --> // %other_bo = bo (shuf X, {0}), Y // %this_extracted_bo = extelt %other_bo, 0 // // TODO: Handle demand of an arbitrary single element or more than one // element instead of just element 0. // TODO: Unlike general demanded elements transforms, this should be safe // for any (div/rem/shift) opcode too. if (DemandedElts == 1 && !X->hasOneUse() && !Y->hasOneUse() && BO->hasOneUse() ) { auto findShufBO = [&](bool MatchShufAsOp0) -> User * { // Try to use shuffle-of-operand in place of an operand: // bo X, Y --> bo (shuf X), Y // bo X, Y --> bo X, (shuf Y) BinaryOperator::BinaryOps Opcode = BO->getOpcode(); Value *ShufOp = MatchShufAsOp0 ? X : Y; Value *OtherOp = MatchShufAsOp0 ? Y : X; for (User *U : OtherOp->users()) { auto Shuf = m_Shuffle(m_Specific(ShufOp), m_Value(), m_ZeroMask()); if (BO->isCommutative() ? match(U, m_c_BinOp(Opcode, Shuf, m_Specific(OtherOp))) : MatchShufAsOp0 ? match(U, m_BinOp(Opcode, Shuf, m_Specific(OtherOp))) : match(U, m_BinOp(Opcode, m_Specific(OtherOp), Shuf))) if (DT.dominates(U, I)) return U; } return nullptr; }; if (User *ShufBO = findShufBO(/* MatchShufAsOp0 */ true)) return ShufBO; if (User *ShufBO = findShufBO(/* MatchShufAsOp0 */ false)) return ShufBO; } simplifyAndSetOp(I, 0, DemandedElts, PoisonElts); simplifyAndSetOp(I, 1, DemandedElts, PoisonElts2); // Output elements are undefined if both are undefined. Consider things // like undef & 0. The result is known zero, not undef. PoisonElts &= PoisonElts2; } // If we've proven all of the lanes poison, return a poison value. // TODO: Intersect w/demanded lanes if (PoisonElts.isAllOnes()) return PoisonValue::get(I->getType()); return MadeChange ? I : nullptr; }