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