lib/Support/KnownBits.cpp

   1 //===-- KnownBits.cpp - Stores known zeros/ones ---------------------------===//
   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 #include "llvm/Support/KnownBits.h"
  15
  16 using namespace llvm;
  17
  18 KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
  19                                       const KnownBits &LHS, KnownBits RHS) {
  20   // Carry in a 1 for a subtract, rather than 0.
  21   bool CarryIn = false;
  22   if (!Add) {
  23     // Sum = LHS + ~RHS + 1
  24     std::swap(RHS.Zero, RHS.One);
  25     CarryIn = true;
  26   }
  27
  28   APInt PossibleSumZero = ~LHS.Zero + ~RHS.Zero + CarryIn;
  29   APInt PossibleSumOne = LHS.One + RHS.One + CarryIn;
  30
  31   // Compute known bits of the carry.
  32   APInt CarryKnownZero = ~(PossibleSumZero ^ LHS.Zero ^ RHS.Zero);
  33   APInt CarryKnownOne = PossibleSumOne ^ LHS.One ^ RHS.One;
  34
  35   // Compute set of known bits (where all three relevant bits are known).
  36   APInt LHSKnownUnion = LHS.Zero | LHS.One;
  37   APInt RHSKnownUnion = RHS.Zero | RHS.One;
  38   APInt CarryKnownUnion = std::move(CarryKnownZero) | CarryKnownOne;
  39   APInt Known = std::move(LHSKnownUnion) & RHSKnownUnion & CarryKnownUnion;
  40
  41   assert((PossibleSumZero & Known) == (PossibleSumOne & Known) &&
  42          "known bits of sum differ");
  43
  44   // Compute known bits of the result.
  45   KnownBits KnownOut;
  46   KnownOut.Zero = ~std::move(PossibleSumZero) & Known;
  47   KnownOut.One = std::move(PossibleSumOne) & Known;
  48
  49   // Are we still trying to solve for the sign bit?
  50   if (!Known.isSignBitSet()) {
  51     if (NSW) {
  52       // Adding two non-negative numbers, or subtracting a negative number from
  53       // a non-negative one, can't wrap into negative.
  54       if (LHS.isNonNegative() && RHS.isNonNegative())
  55         KnownOut.makeNonNegative();
  56       // Adding two negative numbers, or subtracting a non-negative number from
  57       // a negative one, can't wrap into non-negative.
  58       else if (LHS.isNegative() && RHS.isNegative())
  59         KnownOut.makeNegative();
  60     }
  61   }
  62
  63   return KnownOut;
  64 }