//===- ValueTracking.cpp - Walk computations to compute properties --------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This file contains routines that help analyze properties that chains of // computations have. // //===----------------------------------------------------------------------===// #include "llvm/Analysis/ValueTracking.h" #include "llvm/Constants.h" #include "llvm/Instructions.h" #include "llvm/IntrinsicInst.h" #include "llvm/Target/TargetData.h" #include "llvm/Support/GetElementPtrTypeIterator.h" #include "llvm/Support/MathExtras.h" #include using namespace llvm; /// getOpcode - If this is an Instruction or a ConstantExpr, return the /// opcode value. Otherwise return UserOp1. static unsigned getOpcode(const Value *V) { if (const Instruction *I = dyn_cast(V)) return I->getOpcode(); if (const ConstantExpr *CE = dyn_cast(V)) return CE->getOpcode(); // Use UserOp1 to mean there's no opcode. return Instruction::UserOp1; } /// ComputeMaskedBits - Determine which of the bits specified in Mask are /// known to be either zero or one and return them in the KnownZero/KnownOne /// bit sets. This code only analyzes bits in Mask, in order to short-circuit /// processing. /// NOTE: we cannot consider 'undef' to be "IsZero" here. The problem is that /// we cannot optimize based on the assumption that it is zero without changing /// it to be an explicit zero. If we don't change it to zero, other code could /// optimized based on the contradictory assumption that it is non-zero. /// Because instcombine aggressively folds operations with undef args anyway, /// this won't lose us code quality. void llvm::ComputeMaskedBits(Value *V, const APInt &Mask, APInt &KnownZero, APInt &KnownOne, TargetData *TD, unsigned Depth) { assert(V && "No Value?"); assert(Depth <= 6 && "Limit Search Depth"); uint32_t BitWidth = Mask.getBitWidth(); assert((V->getType()->isInteger() || isa(V->getType())) && "Not integer or pointer type!"); assert((!TD || TD->getTypeSizeInBits(V->getType()) == BitWidth) && (!isa(V->getType()) || V->getType()->getPrimitiveSizeInBits() == BitWidth) && KnownZero.getBitWidth() == BitWidth && KnownOne.getBitWidth() == BitWidth && "V, Mask, KnownOne and KnownZero should have same BitWidth"); if (ConstantInt *CI = dyn_cast(V)) { // We know all of the bits for a constant! KnownOne = CI->getValue() & Mask; KnownZero = ~KnownOne & Mask; return; } // Null is all-zeros. if (isa(V)) { KnownOne.clear(); KnownZero = Mask; return; } // The address of an aligned GlobalValue has trailing zeros. if (GlobalValue *GV = dyn_cast(V)) { unsigned Align = GV->getAlignment(); if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) Align = TD->getPrefTypeAlignment(GV->getType()->getElementType()); if (Align > 0) KnownZero = Mask & APInt::getLowBitsSet(BitWidth, CountTrailingZeros_32(Align)); else KnownZero.clear(); KnownOne.clear(); return; } KnownZero.clear(); KnownOne.clear(); // Start out not knowing anything. if (Depth == 6 || Mask == 0) return; // Limit search depth. User *I = dyn_cast(V); if (!I) return; APInt KnownZero2(KnownZero), KnownOne2(KnownOne); switch (getOpcode(I)) { default: break; case Instruction::And: { // If either the LHS or the RHS are Zero, the result is zero. ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1); APInt Mask2(Mask & ~KnownZero); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, Depth+1); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); // Output known-1 bits are only known if set in both the LHS & RHS. KnownOne &= KnownOne2; // Output known-0 are known to be clear if zero in either the LHS | RHS. KnownZero |= KnownZero2; return; } case Instruction::Or: { ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1); APInt Mask2(Mask & ~KnownOne); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, Depth+1); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); // Output known-0 bits are only known if clear in both the LHS & RHS. KnownZero &= KnownZero2; // Output known-1 are known to be set if set in either the LHS | RHS. KnownOne |= KnownOne2; return; } case Instruction::Xor: { ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1); ComputeMaskedBits(I->getOperand(0), Mask, KnownZero2, KnownOne2, TD, Depth+1); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); // Output known-0 bits are known if clear or set in both the LHS & RHS. APInt KnownZeroOut = (KnownZero & KnownZero2) | (KnownOne & KnownOne2); // Output known-1 are known to be set if set in only one of the LHS, RHS. KnownOne = (KnownZero & KnownOne2) | (KnownOne & KnownZero2); KnownZero = KnownZeroOut; return; } case Instruction::Mul: { APInt Mask2 = APInt::getAllOnesValue(BitWidth); ComputeMaskedBits(I->getOperand(1), Mask2, KnownZero, KnownOne, TD,Depth+1); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, Depth+1); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); // If low bits are zero in either operand, output low known-0 bits. // Also compute a conserative estimate for high known-0 bits. // More trickiness is possible, but this is sufficient for the // interesting case of alignment computation. KnownOne.clear(); unsigned TrailZ = KnownZero.countTrailingOnes() + KnownZero2.countTrailingOnes(); unsigned LeadZ = std::max(KnownZero.countLeadingOnes() + KnownZero2.countLeadingOnes(), BitWidth) - BitWidth; TrailZ = std::min(TrailZ, BitWidth); LeadZ = std::min(LeadZ, BitWidth); KnownZero = APInt::getLowBitsSet(BitWidth, TrailZ) | APInt::getHighBitsSet(BitWidth, LeadZ); KnownZero &= Mask; return; } case Instruction::UDiv: { // For the purposes of computing leading zeros we can conservatively // treat a udiv as a logical right shift by the power of 2 known to // be less than the denominator. APInt AllOnes = APInt::getAllOnesValue(BitWidth); ComputeMaskedBits(I->getOperand(0), AllOnes, KnownZero2, KnownOne2, TD, Depth+1); unsigned LeadZ = KnownZero2.countLeadingOnes(); KnownOne2.clear(); KnownZero2.clear(); ComputeMaskedBits(I->getOperand(1), AllOnes, KnownZero2, KnownOne2, TD, Depth+1); unsigned RHSUnknownLeadingOnes = KnownOne2.countLeadingZeros(); if (RHSUnknownLeadingOnes != BitWidth) LeadZ = std::min(BitWidth, LeadZ + BitWidth - RHSUnknownLeadingOnes - 1); KnownZero = APInt::getHighBitsSet(BitWidth, LeadZ) & Mask; return; } case Instruction::Select: ComputeMaskedBits(I->getOperand(2), Mask, KnownZero, KnownOne, TD, Depth+1); ComputeMaskedBits(I->getOperand(1), Mask, KnownZero2, KnownOne2, TD, Depth+1); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); // Only known if known in both the LHS and RHS. KnownOne &= KnownOne2; KnownZero &= KnownZero2; return; case Instruction::FPTrunc: case Instruction::FPExt: case Instruction::FPToUI: case Instruction::FPToSI: case Instruction::SIToFP: case Instruction::UIToFP: return; // Can't work with floating point. case Instruction::PtrToInt: case Instruction::IntToPtr: // We can't handle these if we don't know the pointer size. if (!TD) return; // FALL THROUGH and handle them the same as zext/trunc. case Instruction::ZExt: case Instruction::Trunc: { // Note that we handle pointer operands here because of inttoptr/ptrtoint // which fall through here. const Type *SrcTy = I->getOperand(0)->getType(); uint32_t SrcBitWidth = TD ? TD->getTypeSizeInBits(SrcTy) : SrcTy->getPrimitiveSizeInBits(); APInt MaskIn(Mask); MaskIn.zextOrTrunc(SrcBitWidth); KnownZero.zextOrTrunc(SrcBitWidth); KnownOne.zextOrTrunc(SrcBitWidth); ComputeMaskedBits(I->getOperand(0), MaskIn, KnownZero, KnownOne, TD, Depth+1); KnownZero.zextOrTrunc(BitWidth); KnownOne.zextOrTrunc(BitWidth); // Any top bits are known to be zero. if (BitWidth > SrcBitWidth) KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth); return; } case Instruction::BitCast: { const Type *SrcTy = I->getOperand(0)->getType(); if (SrcTy->isInteger() || isa(SrcTy)) { ComputeMaskedBits(I->getOperand(0), Mask, KnownZero, KnownOne, TD, Depth+1); return; } break; } case Instruction::SExt: { // Compute the bits in the result that are not present in the input. const IntegerType *SrcTy = cast(I->getOperand(0)->getType()); uint32_t SrcBitWidth = SrcTy->getBitWidth(); APInt MaskIn(Mask); MaskIn.trunc(SrcBitWidth); KnownZero.trunc(SrcBitWidth); KnownOne.trunc(SrcBitWidth); ComputeMaskedBits(I->getOperand(0), MaskIn, KnownZero, KnownOne, TD, Depth+1); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); KnownZero.zext(BitWidth); KnownOne.zext(BitWidth); // If the sign bit of the input is known set or clear, then we know the // top bits of the result. if (KnownZero[SrcBitWidth-1]) // Input sign bit known zero KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth); else if (KnownOne[SrcBitWidth-1]) // Input sign bit known set KnownOne |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth); return; } case Instruction::Shl: // (shl X, C1) & C2 == 0 iff (X & C2 >>u C1) == 0 if (ConstantInt *SA = dyn_cast(I->getOperand(1))) { uint64_t ShiftAmt = SA->getLimitedValue(BitWidth); APInt Mask2(Mask.lshr(ShiftAmt)); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD, Depth+1); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); KnownZero <<= ShiftAmt; KnownOne <<= ShiftAmt; KnownZero |= APInt::getLowBitsSet(BitWidth, ShiftAmt); // low bits known 0 return; } break; case Instruction::LShr: // (ushr X, C1) & C2 == 0 iff (-1 >> C1) & C2 == 0 if (ConstantInt *SA = dyn_cast(I->getOperand(1))) { // Compute the new bits that are at the top now. uint64_t ShiftAmt = SA->getLimitedValue(BitWidth); // Unsigned shift right. APInt Mask2(Mask.shl(ShiftAmt)); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero,KnownOne, TD, Depth+1); assert((KnownZero & KnownOne) == 0&&"Bits known to be one AND zero?"); KnownZero = APIntOps::lshr(KnownZero, ShiftAmt); KnownOne = APIntOps::lshr(KnownOne, ShiftAmt); // high bits known zero. KnownZero |= APInt::getHighBitsSet(BitWidth, ShiftAmt); return; } break; case Instruction::AShr: // (ashr X, C1) & C2 == 0 iff (-1 >> C1) & C2 == 0 if (ConstantInt *SA = dyn_cast(I->getOperand(1))) { // Compute the new bits that are at the top now. uint64_t ShiftAmt = SA->getLimitedValue(BitWidth); // Signed shift right. APInt Mask2(Mask.shl(ShiftAmt)); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD, Depth+1); assert((KnownZero & KnownOne) == 0&&"Bits known to be one AND zero?"); KnownZero = APIntOps::lshr(KnownZero, ShiftAmt); KnownOne = APIntOps::lshr(KnownOne, ShiftAmt); APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt)); if (KnownZero[BitWidth-ShiftAmt-1]) // New bits are known zero. KnownZero |= HighBits; else if (KnownOne[BitWidth-ShiftAmt-1]) // New bits are known one. KnownOne |= HighBits; return; } break; case Instruction::Sub: { if (ConstantInt *CLHS = dyn_cast(I->getOperand(0))) { // We know that the top bits of C-X are clear if X contains less bits // than C (i.e. no wrap-around can happen). For example, 20-X is // positive if we can prove that X is >= 0 and < 16. if (!CLHS->getValue().isNegative()) { unsigned NLZ = (CLHS->getValue()+1).countLeadingZeros(); // NLZ can't be BitWidth with no sign bit APInt MaskV = APInt::getHighBitsSet(BitWidth, NLZ+1); ComputeMaskedBits(I->getOperand(1), MaskV, KnownZero2, KnownOne2, TD, Depth+1); // If all of the MaskV bits are known to be zero, then we know the // output top bits are zero, because we now know that the output is // from [0-C]. if ((KnownZero2 & MaskV) == MaskV) { unsigned NLZ2 = CLHS->getValue().countLeadingZeros(); // Top bits known zero. KnownZero = APInt::getHighBitsSet(BitWidth, NLZ2) & Mask; } } } } // fall through case Instruction::Add: { // Output known-0 bits are known if clear or set in both the low clear bits // common to both LHS & RHS. For example, 8+(X<<3) is known to have the // low 3 bits clear. APInt Mask2 = APInt::getLowBitsSet(BitWidth, Mask.countTrailingOnes()); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, Depth+1); assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); unsigned KnownZeroOut = KnownZero2.countTrailingOnes(); ComputeMaskedBits(I->getOperand(1), Mask2, KnownZero2, KnownOne2, TD, Depth+1); assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?"); KnownZeroOut = std::min(KnownZeroOut, KnownZero2.countTrailingOnes()); KnownZero |= APInt::getLowBitsSet(BitWidth, KnownZeroOut); return; } case Instruction::SRem: if (ConstantInt *Rem = dyn_cast(I->getOperand(1))) { APInt RA = Rem->getValue(); if (RA.isPowerOf2() || (-RA).isPowerOf2()) { APInt LowBits = RA.isStrictlyPositive() ? (RA - 1) : ~RA; APInt Mask2 = LowBits | APInt::getSignBit(BitWidth); ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, Depth+1); // The sign of a remainder is equal to the sign of the first // operand (zero being positive). if (KnownZero2[BitWidth-1] || ((KnownZero2 & LowBits) == LowBits)) KnownZero2 |= ~LowBits; else if (KnownOne2[BitWidth-1]) KnownOne2 |= ~LowBits; KnownZero |= KnownZero2 & Mask; KnownOne |= KnownOne2 & Mask; assert((KnownZero & KnownOne) == 0&&"Bits known to be one AND zero?"); } } break; case Instruction::URem: { if (ConstantInt *Rem = dyn_cast(I->getOperand(1))) { APInt RA = Rem->getValue(); if (RA.isPowerOf2()) { APInt LowBits = (RA - 1); APInt Mask2 = LowBits & Mask; KnownZero |= ~LowBits & Mask; ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD, Depth+1); assert((KnownZero & KnownOne) == 0&&"Bits known to be one AND zero?"); break; } } // Since the result is less than or equal to either operand, any leading // zero bits in either operand must also exist in the result. APInt AllOnes = APInt::getAllOnesValue(BitWidth); ComputeMaskedBits(I->getOperand(0), AllOnes, KnownZero, KnownOne, TD, Depth+1); ComputeMaskedBits(I->getOperand(1), AllOnes, KnownZero2, KnownOne2, TD, Depth+1); uint32_t Leaders = std::max(KnownZero.countLeadingOnes(), KnownZero2.countLeadingOnes()); KnownOne.clear(); KnownZero = APInt::getHighBitsSet(BitWidth, Leaders) & Mask; break; } case Instruction::Alloca: case Instruction::Malloc: { AllocationInst *AI = cast(V); unsigned Align = AI->getAlignment(); if (Align == 0 && TD) { if (isa(AI)) Align = TD->getPrefTypeAlignment(AI->getType()->getElementType()); else if (isa(AI)) { // Malloc returns maximally aligned memory. Align = TD->getABITypeAlignment(AI->getType()->getElementType()); Align = std::max(Align, (unsigned)TD->getABITypeAlignment(Type::DoubleTy)); Align = std::max(Align, (unsigned)TD->getABITypeAlignment(Type::Int64Ty)); } } if (Align > 0) KnownZero = Mask & APInt::getLowBitsSet(BitWidth, CountTrailingZeros_32(Align)); break; } case Instruction::GetElementPtr: { // Analyze all of the subscripts of this getelementptr instruction // to determine if we can prove known low zero bits. APInt LocalMask = APInt::getAllOnesValue(BitWidth); APInt LocalKnownZero(BitWidth, 0), LocalKnownOne(BitWidth, 0); ComputeMaskedBits(I->getOperand(0), LocalMask, LocalKnownZero, LocalKnownOne, TD, Depth+1); unsigned TrailZ = LocalKnownZero.countTrailingOnes(); gep_type_iterator GTI = gep_type_begin(I); for (unsigned i = 1, e = I->getNumOperands(); i != e; ++i, ++GTI) { Value *Index = I->getOperand(i); if (const StructType *STy = dyn_cast(*GTI)) { // Handle struct member offset arithmetic. if (!TD) return; const StructLayout *SL = TD->getStructLayout(STy); unsigned Idx = cast(Index)->getZExtValue(); uint64_t Offset = SL->getElementOffset(Idx); TrailZ = std::min(TrailZ, CountTrailingZeros_64(Offset)); } else { // Handle array index arithmetic. const Type *IndexedTy = GTI.getIndexedType(); if (!IndexedTy->isSized()) return; unsigned GEPOpiBits = Index->getType()->getPrimitiveSizeInBits(); uint64_t TypeSize = TD ? TD->getABITypeSize(IndexedTy) : 1; LocalMask = APInt::getAllOnesValue(GEPOpiBits); LocalKnownZero = LocalKnownOne = APInt(GEPOpiBits, 0); ComputeMaskedBits(Index, LocalMask, LocalKnownZero, LocalKnownOne, TD, Depth+1); TrailZ = std::min(TrailZ, CountTrailingZeros_64(TypeSize) + LocalKnownZero.countTrailingOnes()); } } KnownZero = APInt::getLowBitsSet(BitWidth, TrailZ) & Mask; break; } case Instruction::PHI: { PHINode *P = cast(I); // Handle the case of a simple two-predecessor recurrence PHI. // There's a lot more that could theoretically be done here, but // this is sufficient to catch some interesting cases. if (P->getNumIncomingValues() == 2) { for (unsigned i = 0; i != 2; ++i) { Value *L = P->getIncomingValue(i); Value *R = P->getIncomingValue(!i); User *LU = dyn_cast(L); if (!LU) continue; unsigned Opcode = getOpcode(LU); // Check for operations that have the property that if // both their operands have low zero bits, the result // will have low zero bits. if (Opcode == Instruction::Add || Opcode == Instruction::Sub || Opcode == Instruction::And || Opcode == Instruction::Or || Opcode == Instruction::Mul) { Value *LL = LU->getOperand(0); Value *LR = LU->getOperand(1); // Find a recurrence. if (LL == I) L = LR; else if (LR == I) L = LL; else break; // Ok, we have a PHI of the form L op= R. Check for low // zero bits. APInt Mask2 = APInt::getAllOnesValue(BitWidth); ComputeMaskedBits(R, Mask2, KnownZero2, KnownOne2, TD, Depth+1); Mask2 = APInt::getLowBitsSet(BitWidth, KnownZero2.countTrailingOnes()); KnownOne2.clear(); KnownZero2.clear(); ComputeMaskedBits(L, Mask2, KnownZero2, KnownOne2, TD, Depth+1); KnownZero = Mask & APInt::getLowBitsSet(BitWidth, KnownZero2.countTrailingOnes()); break; } } } break; } case Instruction::Call: if (IntrinsicInst *II = dyn_cast(I)) { switch (II->getIntrinsicID()) { default: break; case Intrinsic::ctpop: case Intrinsic::ctlz: case Intrinsic::cttz: { unsigned LowBits = Log2_32(BitWidth)+1; KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits); break; } } } break; } } /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero. We use /// this predicate to simplify operations downstream. Mask is known to be zero /// for bits that V cannot have. bool llvm::MaskedValueIsZero(Value *V, const APInt &Mask, TargetData *TD, unsigned Depth) { APInt KnownZero(Mask.getBitWidth(), 0), KnownOne(Mask.getBitWidth(), 0); ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD, Depth); assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); return (KnownZero & Mask) == Mask; } /// ComputeNumSignBits - Return the number of times the sign bit of the /// register is replicated into the other bits. We know that at least 1 bit /// is always equal to the sign bit (itself), but other cases can give us /// information. For example, immediately after an "ashr X, 2", we know that /// the top 3 bits are all equal to each other, so we return 3. /// /// 'Op' must have a scalar integer type. /// unsigned llvm::ComputeNumSignBits(Value *V, TargetData *TD, unsigned Depth) { const IntegerType *Ty = cast(V->getType()); unsigned TyBits = Ty->getBitWidth(); unsigned Tmp, Tmp2; unsigned FirstAnswer = 1; // Note that ConstantInt is handled by the general ComputeMaskedBits case // below. if (Depth == 6) return 1; // Limit search depth. User *U = dyn_cast(V); switch (getOpcode(V)) { default: break; case Instruction::SExt: Tmp = TyBits-cast(U->getOperand(0)->getType())->getBitWidth(); return ComputeNumSignBits(U->getOperand(0), TD, Depth+1) + Tmp; case Instruction::AShr: Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1); // ashr X, C -> adds C sign bits. if (ConstantInt *C = dyn_cast(U->getOperand(1))) { Tmp += C->getZExtValue(); if (Tmp > TyBits) Tmp = TyBits; } return Tmp; case Instruction::Shl: if (ConstantInt *C = dyn_cast(U->getOperand(1))) { // shl destroys sign bits. Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1); if (C->getZExtValue() >= TyBits || // Bad shift. C->getZExtValue() >= Tmp) break; // Shifted all sign bits out. return Tmp - C->getZExtValue(); } break; case Instruction::And: case Instruction::Or: case Instruction::Xor: // NOT is handled here. // Logical binary ops preserve the number of sign bits at the worst. Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1); if (Tmp != 1) { Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1); FirstAnswer = std::min(Tmp, Tmp2); // We computed what we know about the sign bits as our first // answer. Now proceed to the generic code that uses // ComputeMaskedBits, and pick whichever answer is better. } break; case Instruction::Select: Tmp = ComputeNumSignBits(U->getOperand(1), TD, Depth+1); if (Tmp == 1) return 1; // Early out. Tmp2 = ComputeNumSignBits(U->getOperand(2), TD, Depth+1); return std::min(Tmp, Tmp2); case Instruction::Add: // Add can have at most one carry bit. Thus we know that the output // is, at worst, one more bit than the inputs. Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1); if (Tmp == 1) return 1; // Early out. // Special case decrementing a value (ADD X, -1): if (ConstantInt *CRHS = dyn_cast(U->getOperand(0))) if (CRHS->isAllOnesValue()) { APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0); APInt Mask = APInt::getAllOnesValue(TyBits); ComputeMaskedBits(U->getOperand(0), Mask, KnownZero, KnownOne, TD, Depth+1); // If the input is known to be 0 or 1, the output is 0/-1, which is all // sign bits set. if ((KnownZero | APInt(TyBits, 1)) == Mask) return TyBits; // If we are subtracting one from a positive number, there is no carry // out of the result. if (KnownZero.isNegative()) return Tmp; } Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1); if (Tmp2 == 1) return 1; return std::min(Tmp, Tmp2)-1; break; case Instruction::Sub: Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1); if (Tmp2 == 1) return 1; // Handle NEG. if (ConstantInt *CLHS = dyn_cast(U->getOperand(0))) if (CLHS->isNullValue()) { APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0); APInt Mask = APInt::getAllOnesValue(TyBits); ComputeMaskedBits(U->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1); // If the input is known to be 0 or 1, the output is 0/-1, which is all // sign bits set. if ((KnownZero | APInt(TyBits, 1)) == Mask) return TyBits; // If the input is known to be positive (the sign bit is known clear), // the output of the NEG has the same number of sign bits as the input. if (KnownZero.isNegative()) return Tmp2; // Otherwise, we treat this like a SUB. } // Sub can have at most one carry bit. Thus we know that the output // is, at worst, one more bit than the inputs. Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1); if (Tmp == 1) return 1; // Early out. return std::min(Tmp, Tmp2)-1; break; case Instruction::Trunc: // FIXME: it's tricky to do anything useful for this, but it is an important // case for targets like X86. break; } // Finally, if we can prove that the top bits of the result are 0's or 1's, // use this information. APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0); APInt Mask = APInt::getAllOnesValue(TyBits); ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD, Depth); if (KnownZero.isNegative()) { // sign bit is 0 Mask = KnownZero; } else if (KnownOne.isNegative()) { // sign bit is 1; Mask = KnownOne; } else { // Nothing known. return FirstAnswer; } // Okay, we know that the sign bit in Mask is set. Use CLZ to determine // the number of identical bits in the top of the input value. Mask = ~Mask; Mask <<= Mask.getBitWidth()-TyBits; // Return # leading zeros. We use 'min' here in case Val was zero before // shifting. We don't want to return '64' as for an i32 "0". return std::max(FirstAnswer, std::min(TyBits, Mask.countLeadingZeros())); } /// CannotBeNegativeZero - Return true if we can prove that the specified FP /// value is never equal to -0.0. /// /// NOTE: this function will need to be revisited when we support non-default /// rounding modes! /// bool llvm::CannotBeNegativeZero(const Value *V, unsigned Depth) { if (const ConstantFP *CFP = dyn_cast(V)) return !CFP->getValueAPF().isNegZero(); if (Depth == 6) return 1; // Limit search depth. const Instruction *I = dyn_cast(V); if (I == 0) return false; // (add x, 0.0) is guaranteed to return +0.0, not -0.0. if (I->getOpcode() == Instruction::Add && isa(I->getOperand(1)) && cast(I->getOperand(1))->isNullValue()) return true; // sitofp and uitofp turn into +0.0 for zero. if (isa(I) || isa(I)) return true; if (const IntrinsicInst *II = dyn_cast(I)) // sqrt(-0.0) = -0.0, no other negative results are possible. if (II->getIntrinsicID() == Intrinsic::sqrt) return CannotBeNegativeZero(II->getOperand(1), Depth+1); if (const CallInst *CI = dyn_cast(I)) if (const Function *F = CI->getCalledFunction()) { if (F->isDeclaration()) { switch (F->getNameLen()) { case 3: // abs(x) != -0.0 if (!strcmp(F->getNameStart(), "abs")) return true; break; case 4: // abs[lf](x) != -0.0 if (!strcmp(F->getNameStart(), "absf")) return true; if (!strcmp(F->getNameStart(), "absl")) return true; break; } } } return false; }