include/llvm/Support/KnownBits.h

   1 //===- llvm/Support/KnownBits.h - Stores known zeros/ones -------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file contains a class for representing known zeros and ones used by
  10 // computeKnownBits.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #ifndef LLVM_SUPPORT_KNOWNBITS_H
  15 #define LLVM_SUPPORT_KNOWNBITS_H
  16
  17 #include "llvm/ADT/APInt.h"
  18
  19 namespace llvm {
  20
  21 // Struct for tracking the known zeros and ones of a value.
  22 struct KnownBits {
  23   APInt Zero;
  24   APInt One;
  25
  26 private:
  27   // Internal constructor for creating a KnownBits from two APInts.
  28   KnownBits(APInt Zero, APInt One)
  29       : Zero(std::move(Zero)), One(std::move(One)) {}
  30
  31 public:
  32   // Default construct Zero and One.
  33   KnownBits() {}
  34
  35   /// Create a known bits object of BitWidth bits initialized to unknown.
  36   KnownBits(unsigned BitWidth) : Zero(BitWidth, 0), One(BitWidth, 0) {}
  37
  38   /// Get the bit width of this value.
  39   unsigned getBitWidth() const {
  40     assert(Zero.getBitWidth() == One.getBitWidth() &&
  41            "Zero and One should have the same width!");
  42     return Zero.getBitWidth();
  43   }
  44
  45   /// Returns true if there is conflicting information.
  46   bool hasConflict() const { return Zero.intersects(One); }
  47
  48   /// Returns true if we know the value of all bits.
  49   bool isConstant() const {
  50     assert(!hasConflict() && "KnownBits conflict!");
  51     return Zero.countPopulation() + One.countPopulation() == getBitWidth();
  52   }
  53
  54   /// Returns the value when all bits have a known value. This just returns One
  55   /// with a protective assertion.
  56   const APInt &getConstant() const {
  57     assert(isConstant() && "Can only get value when all bits are known");
  58     return One;
  59   }
  60
  61   /// Returns true if we don't know any bits.
  62   bool isUnknown() const { return Zero.isNullValue() && One.isNullValue(); }
  63
  64   /// Resets the known state of all bits.
  65   void resetAll() {
  66     Zero.clearAllBits();
  67     One.clearAllBits();
  68   }
  69
  70   /// Returns true if value is all zero.
  71   bool isZero() const {
  72     assert(!hasConflict() && "KnownBits conflict!");
  73     return Zero.isAllOnesValue();
  74   }
  75
  76   /// Returns true if value is all one bits.
  77   bool isAllOnes() const {
  78     assert(!hasConflict() && "KnownBits conflict!");
  79     return One.isAllOnesValue();
  80   }
  81
  82   /// Make all bits known to be zero and discard any previous information.
  83   void setAllZero() {
  84     Zero.setAllBits();
  85     One.clearAllBits();
  86   }
  87
  88   /// Make all bits known to be one and discard any previous information.
  89   void setAllOnes() {
  90     Zero.clearAllBits();
  91     One.setAllBits();
  92   }
  93
  94   /// Returns true if this value is known to be negative.
  95   bool isNegative() const { return One.isSignBitSet(); }
  96
  97   /// Returns true if this value is known to be non-negative.
  98   bool isNonNegative() const { return Zero.isSignBitSet(); }
  99
 100   /// Make this value negative.
 101   void makeNegative() {
 102     One.setSignBit();
 103   }
 104
 105   /// Make this value non-negative.
 106   void makeNonNegative() {
 107     Zero.setSignBit();
 108   }
 109
 110   /// Truncate the underlying known Zero and One bits. This is equivalent
 111   /// to truncating the value we're tracking.
 112   KnownBits trunc(unsigned BitWidth) const {
 113     return KnownBits(Zero.trunc(BitWidth), One.trunc(BitWidth));
 114   }
 115
 116   /// Extends the underlying known Zero and One bits.
 117   /// By setting ExtendedBitsAreKnownZero=true this will be equivalent to
 118   /// zero extending the value we're tracking.
 119   /// With ExtendedBitsAreKnownZero=false the extended bits are set to unknown.
 120   KnownBits zext(unsigned BitWidth, bool ExtendedBitsAreKnownZero) const {
 121     unsigned OldBitWidth = getBitWidth();
 122     APInt NewZero = Zero.zext(BitWidth);
 123     if (ExtendedBitsAreKnownZero)
 124       NewZero.setBitsFrom(OldBitWidth);
 125     return KnownBits(NewZero, One.zext(BitWidth));
 126   }
 127
 128   /// Sign extends the underlying known Zero and One bits. This is equivalent
 129   /// to sign extending the value we're tracking.
 130   KnownBits sext(unsigned BitWidth) const {
 131     return KnownBits(Zero.sext(BitWidth), One.sext(BitWidth));
 132   }
 133
 134   /// Extends or truncates the underlying known Zero and One bits. When
 135   /// extending the extended bits can either be set as known zero (if
 136   /// ExtendedBitsAreKnownZero=true) or as unknown (if
 137   /// ExtendedBitsAreKnownZero=false).
 138   KnownBits zextOrTrunc(unsigned BitWidth,
 139                         bool ExtendedBitsAreKnownZero) const {
 140     if (BitWidth > getBitWidth())
 141       return zext(BitWidth, ExtendedBitsAreKnownZero);
 142     return KnownBits(Zero.zextOrTrunc(BitWidth), One.zextOrTrunc(BitWidth));
 143   }
 144
 145   /// Returns the minimum number of trailing zero bits.
 146   unsigned countMinTrailingZeros() const {
 147     return Zero.countTrailingOnes();
 148   }
 149
 150   /// Returns the minimum number of trailing one bits.
 151   unsigned countMinTrailingOnes() const {
 152     return One.countTrailingOnes();
 153   }
 154
 155   /// Returns the minimum number of leading zero bits.
 156   unsigned countMinLeadingZeros() const {
 157     return Zero.countLeadingOnes();
 158   }
 159
 160   /// Returns the minimum number of leading one bits.
 161   unsigned countMinLeadingOnes() const {
 162     return One.countLeadingOnes();
 163   }
 164
 165   /// Returns the number of times the sign bit is replicated into the other
 166   /// bits.
 167   unsigned countMinSignBits() const {
 168     if (isNonNegative())
 169       return countMinLeadingZeros();
 170     if (isNegative())
 171       return countMinLeadingOnes();
 172     return 0;
 173   }
 174
 175   /// Returns the maximum number of trailing zero bits possible.
 176   unsigned countMaxTrailingZeros() const {
 177     return One.countTrailingZeros();
 178   }
 179
 180   /// Returns the maximum number of trailing one bits possible.
 181   unsigned countMaxTrailingOnes() const {
 182     return Zero.countTrailingZeros();
 183   }
 184
 185   /// Returns the maximum number of leading zero bits possible.
 186   unsigned countMaxLeadingZeros() const {
 187     return One.countLeadingZeros();
 188   }
 189
 190   /// Returns the maximum number of leading one bits possible.
 191   unsigned countMaxLeadingOnes() const {
 192     return Zero.countLeadingZeros();
 193   }
 194
 195   /// Returns the number of bits known to be one.
 196   unsigned countMinPopulation() const {
 197     return One.countPopulation();
 198   }
 199
 200   /// Returns the maximum number of bits that could be one.
 201   unsigned countMaxPopulation() const {
 202     return getBitWidth() - Zero.countPopulation();
 203   }
 204
 205   /// Compute known bits resulting from adding LHS, RHS and a 1-bit Carry.
 206   static KnownBits computeForAddCarry(
 207       const KnownBits &LHS, const KnownBits &RHS, const KnownBits &Carry);
 208
 209   /// Compute known bits resulting from adding LHS and RHS.
 210   static KnownBits computeForAddSub(bool Add, bool NSW, const KnownBits &LHS,
 211                                     KnownBits RHS);
 212 };
 213
 214 } // end namespace llvm
 215
 216 #endif