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Implement IEEE-754R 2008 nextUp/nextDown functions in the guise of the function APFloat::next(bool nextDown).

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rdar://13852078

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182945 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Michael Gottesman
2013-05-30 18:07:13 +00:00
parent 26266a1ece
commit 964722ca40
3 changed files with 682 additions and 25 deletions
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254
lib/Support/APFloat.cpp
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@ -684,6 +684,67 @@ APFloat::isDenormal() const {
semantics->precision - 1) == 0);
}
bool
APFloat::isSmallest() const {
// The smallest number by magnitude in our format will be the smallest
// denormal, i.e. the floating point normal with exponent being minimum
// exponent and significand bitwise equal to 1 (i.e. with MSB equal to 0).
return isNormal() && exponent == semantics->minExponent &&
significandMSB() == 0;
}
bool APFloat::isSignificandAllOnes() const {
// Test if the significand excluding the integral bit is all ones. This allows
// us to test for binade boundaries.
const integerPart *Parts = significandParts();
const unsigned PartCount = partCount();
for (unsigned i = 0; i < PartCount - 1; i++)
if (~Parts[i])
return false;
// Set the unused high bits to all ones when we compare.
const unsigned NumHighBits =
PartCount*integerPartWidth - semantics->precision + 1;
assert(NumHighBits <= integerPartWidth && "Can not have more high bits to "
"fill than integerPartWidth");
const integerPart HighBitFill =
~integerPart(0) << (integerPartWidth - NumHighBits);
if (~(Parts[PartCount - 1] | HighBitFill))
return false;
return true;
}
bool APFloat::isSignificandAllZeros() const {
// Test if the significand excluding the integral bit is all zeros. This
// allows us to test for binade boundaries.
const integerPart *Parts = significandParts();
const unsigned PartCount = partCount();
for (unsigned i = 0; i < PartCount - 1; i++)
if (Parts[i])
return false;
const unsigned NumHighBits =
PartCount*integerPartWidth - semantics->precision + 1;
assert(NumHighBits <= integerPartWidth && "Can not have more high bits to "
"clear than integerPartWidth");
const integerPart HighBitMask = ~integerPart(0) >> NumHighBits;
if (Parts[PartCount - 1] & HighBitMask)
return false;
return true;
}
bool
APFloat::isLargest() const {
// The largest number by magnitude in our format will be the floating point
// number with maximum exponent and with significand that is all ones.
return isNormal() && exponent == semantics->maxExponent
&& isSignificandAllOnes();
}
bool
APFloat::bitwiseIsEqual(const APFloat &rhs) const {
if (this == &rhs)
@ -3236,42 +3297,60 @@ APFloat::getAllOnesValue(unsigned BitWidth, bool isIEEE)
}
}
APFloat APFloat::getLargest(const fltSemantics &Sem, bool Negative) {
APFloat Val(Sem, fcNormal, Negative);
/// Make this number the largest magnitude normal number in the given
/// semantics.
void APFloat::makeLargest(bool Negative) {
// We want (in interchange format):
// sign = {Negative}
// exponent = 1..10
// significand = 1..1
category = fcNormal;
sign = Negative;
exponent = semantics->maxExponent;
Val.exponent = Sem.maxExponent; // unbiased
// Use memset to set all but the highest integerPart to all ones.
integerPart *significand = significandParts();
unsigned PartCount = partCount();
memset(significand, 0xFF, sizeof(integerPart)*(PartCount - 1));
// 1-initialize all bits....
Val.zeroSignificand();
integerPart *significand = Val.significandParts();
unsigned N = partCountForBits(Sem.precision);
for (unsigned i = 0; i != N; ++i)
significand[i] = ~((integerPart) 0);
// ...and then clear the top bits for internal consistency.
if (Sem.precision % integerPartWidth != 0)
significand[N-1] &=
(((integerPart) 1) << (Sem.precision % integerPartWidth)) - 1;
return Val;
// Set the high integerPart especially setting all unused top bits for
// internal consistency.
const unsigned NumUnusedHighBits =
PartCount*integerPartWidth - semantics->precision;
significand[PartCount - 1] = ~integerPart(0) >> NumUnusedHighBits;
}
APFloat APFloat::getSmallest(const fltSemantics &Sem, bool Negative) {
APFloat Val(Sem, fcNormal, Negative);
/// Make this number the smallest magnitude denormal number in the given
/// semantics.
void APFloat::makeSmallest(bool Negative) {
// We want (in interchange format):
// sign = {Negative}
// exponent = 0..0
// significand = 0..01
category = fcNormal;
sign = Negative;
exponent = semantics->minExponent;
APInt::tcSet(significandParts(), 1, partCount());
}
Val.exponent = Sem.minExponent; // unbiased
Val.zeroSignificand();
Val.significandParts()[0] = 1;
APFloat APFloat::getLargest(const fltSemantics &Sem, bool Negative) {
// We want (in interchange format):
// sign = {Negative}
// exponent = 1..10
// significand = 1..1
APFloat Val(Sem, uninitialized);
Val.makeLargest(Negative);
return Val;
}
APFloat APFloat::getSmallest(const fltSemantics &Sem, bool Negative) {
// We want (in interchange format):
// sign = {Negative}
// exponent = 0..0
// significand = 0..01
APFloat Val(Sem, uninitialized);
Val.makeSmallest(Negative);
return Val;
}
@ -3615,3 +3694,132 @@ bool APFloat::getExactInverse(APFloat *inv) const {
return true;
}
bool APFloat::isSignaling() const {
if (!isNaN())
return false;
// IEEE-754R 2008 6.2.1: A signaling NaN bit string should be encoded with the
// first bit of the trailing significand being 0.
return !APInt::tcExtractBit(significandParts(), semantics->precision - 2);
}
/// IEEE-754R 2008 5.3.1: nextUp/nextDown.
///
/// *NOTE* since nextDown(x) = -nextUp(-x), we only implement nextUp with
/// appropriate sign switching before/after the computation.
APFloat::opStatus APFloat::next(bool nextDown) {
// If we are performing nextDown, swap sign so we have -x.
if (nextDown)
changeSign();
// Compute nextUp(x)
opStatus result = opOK;
// Handle each float category separately.
switch (category) {
case fcInfinity:
// nextUp(+inf) = +inf
if (!isNegative())
break;
// nextUp(-inf) = -getLargest()
makeLargest(true);
break;
case fcNaN:
// IEEE-754R 2008 6.2 Par 2: nextUp(sNaN) = qNaN. Set Invalid flag.
// IEEE-754R 2008 6.2: nextUp(qNaN) = qNaN. Must be identity so we do not
// change the payload.
if (isSignaling()) {
result = opInvalidOp;
// For consistency, propogate the sign of the sNaN to the qNaN.
makeNaN(false, isNegative(), 0);
}
break;
case fcZero:
// nextUp(pm 0) = +getSmallest()
makeSmallest(false);
break;
case fcNormal:
// nextUp(-getSmallest()) = -0
if (isSmallest() && isNegative()) {
APInt::tcSet(significandParts(), 0, partCount());
category = fcZero;
exponent = 0;
break;
}
// nextUp(getLargest()) == INFINITY
if (isLargest() && !isNegative()) {
APInt::tcSet(significandParts(), 0, partCount());
category = fcInfinity;
exponent = semantics->maxExponent + 1;
break;
}
// nextUp(normal) == normal + inc.
if (isNegative()) {
// If we are negative, we need to decrement the significand.
// We only cross a binade boundary that requires adjusting the exponent
// if:
// 1. exponent != semantics->minExponent. This implies we are not in the
// smallest binade or are dealing with denormals.
// 2. Our significand excluding the integral bit is all zeros.
bool WillCrossBinadeBoundary =
exponent != semantics->minExponent && isSignificandAllZeros();
// Decrement the significand.
//
// We always do this since:
// 1. If we are dealing with a non binade decrement, by definition we
// just decrement the significand.
// 2. If we are dealing with a normal -> normal binade decrement, since
// we have an explicit integral bit the fact that all bits but the
// integral bit are zero implies that subtracting one will yield a
// significand with 0 integral bit and 1 in all other spots. Thus we
// must just adjust the exponent and set the integral bit to 1.
// 3. If we are dealing with a normal -> denormal binade decrement,
// since we set the integral bit to 0 when we represent denormals, we
// just decrement the significand.
integerPart *Parts = significandParts();
APInt::tcDecrement(Parts, partCount());
if (WillCrossBinadeBoundary) {
// Our result is a normal number. Do the following:
// 1. Set the integral bit to 1.
// 2. Decrement the exponent.
APInt::tcSetBit(Parts, semantics->precision - 1);
exponent--;
}
} else {
// If we are positive, we need to increment the significand.
// We only cross a binade boundary that requires adjusting the exponent if
// the input is not a denormal and all of said input's significand bits
// are set. If all of said conditions are true: clear the significand, set
// the integral bit to 1, and increment the exponent. If we have a
// denormal always increment since moving denormals and the numbers in the
// smallest normal binade have the same exponent in our representation.
bool WillCrossBinadeBoundary = !isDenormal() && isSignificandAllOnes();
if (WillCrossBinadeBoundary) {
integerPart *Parts = significandParts();
APInt::tcSet(Parts, 0, partCount());
APInt::tcSetBit(Parts, semantics->precision - 1);
assert(exponent != semantics->maxExponent &&
"We can not increment an exponent beyond the maxExponent allowed"
" by the given floating point semantics.");
exponent++;
} else {
incrementSignificand();
}
}
break;
}
// If we are performing nextDown, swap sign so we have -nextUp(-x)
if (nextDown)
changeSign();
return result;
}