move ComputeMaskedBits, MaskedValueIsZero, and ComputeNumSignBits
out of instcombine into a new file in libanalysis. This also teaches
ComputeNumSignBits about the number of sign bits in a constantint.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51863 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
new file mode 100644
index 0000000..a35d625
--- /dev/null
+++ b/lib/Analysis/ValueTracking.cpp
@@ -0,0 +1,709 @@
+//===- 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"
+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<Instruction>(V))
+ return I->getOpcode();
+ if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(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<PointerType>(V->getType())) &&
+ "Not integer or pointer type!");
+ assert((!TD || TD->getTypeSizeInBits(V->getType()) == BitWidth) &&
+ (!isa<IntegerType>(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<ConstantInt>(V)) {
+ // We know all of the bits for a constant!
+ KnownOne = CI->getValue() & Mask;
+ KnownZero = ~KnownOne & Mask;
+ return;
+ }
+ // Null is all-zeros.
+ if (isa<ConstantPointerNull>(V)) {
+ KnownOne.clear();
+ KnownZero = Mask;
+ return;
+ }
+ // The address of an aligned GlobalValue has trailing zeros.
+ if (GlobalValue *GV = dyn_cast<GlobalValue>(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<User>(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<PointerType>(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<IntegerType>(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<ConstantInt>(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<ConstantInt>(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<ConstantInt>(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<ConstantInt>(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<ConstantInt>(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<ConstantInt>(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<AllocationInst>(V);
+ unsigned Align = AI->getAlignment();
+ if (Align == 0 && TD) {
+ if (isa<AllocaInst>(AI))
+ Align = TD->getPrefTypeAlignment(AI->getType()->getElementType());
+ else if (isa<MallocInst>(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<StructType>(*GTI)) {
+ // Handle struct member offset arithmetic.
+ if (!TD) return;
+ const StructLayout *SL = TD->getStructLayout(STy);
+ unsigned Idx = cast<ConstantInt>(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<PHINode>(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<User>(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<IntrinsicInst>(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<IntegerType>(V->getType());
+ unsigned TyBits = Ty->getBitWidth();
+ unsigned Tmp, Tmp2;
+ unsigned FirstAnswer = 1;
+
+ if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
+ if (CI->getValue().isNegative())
+ return CI->getValue().countLeadingOnes();
+ return CI->getValue().countLeadingZeros();
+ }
+
+ if (Depth == 6)
+ return 1; // Limit search depth.
+
+ User *U = dyn_cast<User>(V);
+ switch (getOpcode(V)) {
+ default: break;
+ case Instruction::SExt:
+ Tmp = TyBits-cast<IntegerType>(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<ConstantInt>(U->getOperand(1))) {
+ Tmp += C->getZExtValue();
+ if (Tmp > TyBits) Tmp = TyBits;
+ }
+ return Tmp;
+ case Instruction::Shl:
+ if (ConstantInt *C = dyn_cast<ConstantInt>(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<ConstantInt>(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<ConstantInt>(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()));
+}