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/******************************************************************************
* Top contributors (to current version):
* Yancheng Ou, Michael Chang
*
* This file is part of the cvc5 project.
*
* Copyright (c) 2009-2021 by the authors listed in the file AUTHORS
* in the top-level source directory and their institutional affiliations.
* All rights reserved. See the file COPYING in the top-level source
* directory for licensing information.
* ****************************************************************************
*
* Optimizer for BitVector type.
*/
#include "omt/bitvector_optimizer.h"
#include "options/smt_options.h"
#include "smt/smt_engine.h"
#include "util/bitvector.h"
using namespace cvc5::smt;
namespace cvc5::omt {
OMTOptimizerBitVector::OMTOptimizerBitVector(bool isSigned)
: d_isSigned(isSigned)
{
}
BitVector OMTOptimizerBitVector::computeAverage(const BitVector& a,
const BitVector& b,
bool isSigned)
{
// computes (a + b) / 2 without overflow
// rounding towards -infinity: -1.5 --> -2, 1.5 --> 1
// average = (a / 2) + (b / 2) + (((a % 2) + (b % 2)) / 2)
uint32_t aMod2 = static_cast<uint32_t>(a.isBitSet(0));
uint32_t bMod2 = static_cast<uint32_t>(b.isBitSet(0));
BitVector aMod2PlusbMod2Div2(a.getSize(), (aMod2 + bMod2) / 2);
BitVector bv1 = BitVector::mkOne(a.getSize());
return (isSigned) ? ((a.arithRightShift(bv1) + b.arithRightShift(bv1)
+ aMod2PlusbMod2Div2))
: ((a.logicalRightShift(bv1) + b.logicalRightShift(bv1)
+ aMod2PlusbMod2Div2));
}
OptimizationResult OMTOptimizerBitVector::minimize(SmtEngine* optChecker,
TNode target)
{
// the smt engine to which we send intermediate queries
// for the binary search.
NodeManager* nm = optChecker->getNodeManager();
Result intermediateSatResult = optChecker->checkSat();
// Model-value of objective (used in optimization loop)
Node value;
if (intermediateSatResult.isUnknown())
{
return OptimizationResult(OptimizationResult::UNKNOWN, value);
}
if (intermediateSatResult.isSat() == Result::UNSAT)
{
return OptimizationResult(OptimizationResult::UNSAT, value);
}
// value equals to upperBound
value = optChecker->getValue(target);
// this gets the bitvector!
BitVector bvValue = value.getConst<BitVector>();
unsigned int bvSize = bvValue.getSize();
// lowerbound
BitVector lowerBound = ((this->d_isSigned) ? (BitVector::mkMinSigned(bvSize))
: (BitVector::mkZero(bvSize)));
// upperbound must be a satisfying value
// and value == upperbound
BitVector upperBound = bvValue;
Kind LTOperator =
((d_isSigned) ? (kind::BITVECTOR_SLT) : (kind::BITVECTOR_ULT));
Kind GEOperator =
((d_isSigned) ? (kind::BITVECTOR_SGE) : (kind::BITVECTOR_UGE));
// the pivot value for binary search,
// pivot = (lowerBound + upperBound) / 2
// rounded towards -infinity
BitVector pivot;
while ((d_isSigned && lowerBound.signedLessThan(upperBound))
|| (!d_isSigned && lowerBound.unsignedLessThan(upperBound)))
{
pivot = computeAverage(lowerBound, upperBound, d_isSigned);
optChecker->push();
if (lowerBound == pivot)
{
optChecker->assertFormula(
nm->mkNode(kind::EQUAL, target, nm->mkConst(lowerBound)));
}
else
{
// lowerBound <= target < pivot
optChecker->assertFormula(
nm->mkNode(kind::AND,
nm->mkNode(GEOperator, target, nm->mkConst(lowerBound)),
nm->mkNode(LTOperator, target, nm->mkConst(pivot))));
}
intermediateSatResult = optChecker->checkSat();
if (intermediateSatResult.isUnknown() || intermediateSatResult.isNull())
{
return OptimizationResult(OptimizationResult::UNKNOWN, value);
}
if (intermediateSatResult.isSat() == Result::SAT)
{
value = optChecker->getValue(target);
upperBound = value.getConst<BitVector>();
}
else if (intermediateSatResult.isSat() == Result::UNSAT)
{
if (lowerBound == pivot)
{
// lowerBound == pivot ==> upperbound = lowerbound + 1
// and lowerbound <= target < upperbound is UNSAT
// return the upperbound
return OptimizationResult(OptimizationResult::OPTIMAL, value);
}
else
{
lowerBound = pivot;
}
}
else
{
return OptimizationResult(OptimizationResult::UNKNOWN, value);
}
optChecker->pop();
}
return OptimizationResult(OptimizationResult::OPTIMAL, value);
}
OptimizationResult OMTOptimizerBitVector::maximize(SmtEngine* optChecker,
TNode target)
{
// the smt engine to which we send intermediate queries
// for the binary search.
NodeManager* nm = optChecker->getNodeManager();
Result intermediateSatResult = optChecker->checkSat();
// Model-value of objective (used in optimization loop)
Node value;
if (intermediateSatResult.isUnknown())
{
return OptimizationResult(OptimizationResult::UNKNOWN, value);
}
if (intermediateSatResult.isSat() == Result::UNSAT)
{
return OptimizationResult(OptimizationResult::UNSAT, value);
}
// value equals to upperBound
value = optChecker->getValue(target);
// this gets the bitvector!
BitVector bvValue = value.getConst<BitVector>();
unsigned int bvSize = bvValue.getSize();
// lowerbound must be a satisfying value
// and value == lowerbound
BitVector lowerBound = bvValue;
// upperbound
BitVector upperBound = ((this->d_isSigned) ? (BitVector::mkMaxSigned(bvSize))
: (BitVector::mkOnes(bvSize)));
Kind LEOperator =
((d_isSigned) ? (kind::BITVECTOR_SLE) : (kind::BITVECTOR_ULE));
Kind GTOperator =
((d_isSigned) ? (kind::BITVECTOR_SGT) : (kind::BITVECTOR_UGT));
// the pivot value for binary search,
// pivot = (lowerBound + upperBound) / 2
// rounded towards -infinity
BitVector pivot;
while ((d_isSigned && lowerBound.signedLessThan(upperBound))
|| (!d_isSigned && lowerBound.unsignedLessThan(upperBound)))
{
pivot = computeAverage(lowerBound, upperBound, d_isSigned);
optChecker->push();
// notice that we don't have boundary condition here
// because lowerBound == pivot / lowerBound == upperBound + 1 is also
// covered
// pivot < target <= upperBound
optChecker->assertFormula(
nm->mkNode(kind::AND,
nm->mkNode(GTOperator, target, nm->mkConst(pivot)),
nm->mkNode(LEOperator, target, nm->mkConst(upperBound))));
intermediateSatResult = optChecker->checkSat();
if (intermediateSatResult.isUnknown() || intermediateSatResult.isNull())
{
return OptimizationResult(OptimizationResult::UNKNOWN, value);
}
if (intermediateSatResult.isSat() == Result::SAT)
{
value = optChecker->getValue(target);
lowerBound = value.getConst<BitVector>();
}
else if (intermediateSatResult.isSat() == Result::UNSAT)
{
if (lowerBound == pivot)
{
// upperbound = lowerbound + 1
// and lowerbound < target <= upperbound is UNSAT
// return the lowerbound
return OptimizationResult(OptimizationResult::OPTIMAL, value);
}
else
{
upperBound = pivot;
}
}
else
{
return OptimizationResult(OptimizationResult::UNKNOWN, value);
}
optChecker->pop();
}
return OptimizationResult(OptimizationResult::OPTIMAL, value);
}
} // namespace cvc5::omt
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