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Fix precondition of conditionalNegate() #6

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Fix precondition of conditionalNegate() #6

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26 changes: 16 additions & 10 deletions core/convert.h
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Original file line number Diff line number Diff line change
Expand Up @@ -483,17 +483,23 @@ template <class t>

// Put the result together
bwt roundSigWidth(rounded.significand.getWidth());
prop msbSet(rounded.significand.extract(roundSigWidth - 1,
roundSigWidth - 1).isAllOnes());
// -2^{n-1} is the only safe "overflow" case
prop safeOverflow(input.getSign() &&
msbSet &&
rounded.significand.extract(roundSigWidth - 2,
0).isAllZeros());
prop undefinedResult(earlyUndefinedResult ||
rounded.incrementExponent || // Definite Overflow
(rounded.significand.extract(roundSigWidth - 1,
roundSigWidth - 1).isAllOnes() &&
!(input.getSign() && rounded.significand.extract(roundSigWidth - 2, 0).isAllZeros()))); // -2^{n-1} is the only safe "overflow" case


sbv result(ITE(undefinedResult,
undefValue,
conditionalNegate<t,sbv,prop>(input.getSign(), rounded.significand.toSigned())));

rounded.incrementExponent || // Definite Overflow
(msbSet && !safeOverflow));

sbv definedResult(
ITE(safeOverflow,
rounded.significand.toSigned(),
conditionalNegate<t,sbv,prop>(input.getSign(),
rounded.significand.toSigned())));
sbv result(ITE(undefinedResult, undefValue, definedResult));
return result;
}

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8 changes: 6 additions & 2 deletions core/operations.h
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Original file line number Diff line number Diff line change
Expand Up @@ -134,8 +134,12 @@ namespace symfpu {
bv conditionalNegate (const prop &p, const bv &b) {
typename t::bwt w(b.getWidth());
PRECONDITION(w >= 2);
PRECONDITION(IMPLIES(p, !(b.extract(w - 1, w - 1).isAllOnes() &&
b.extract(w - 2, 0).isAllZeros())));
// We may be tempted to check that we are not trying to negate the smallest
// value that can be represented using two's complement (i.e., `100...0`).
// This is problematic, however, in cases where we apply
// `conditionalNegate()` conditionally (e.g., in `convertFloatToSBV()`),
// because the arguments to the branches are evaluated eagerly, so the
// assertion fails, even though the result is not used.

return bv(ITE(p, -b, b));
}
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