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[APFloat] Add APFloat support for E8M0 type
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This patch adds an APFloat type for unsigned E8M0 format.
This format is used for representing the "scale-format"
in the MX specification:
https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf

This format does not support {Inf, denorms, zeroes}.
Like FP32, this format's exponents are 8-bits (all bits here)
and the bias value is 127. However, it differs from IEEE-FP32
in that the minExponent is -127 (instead of -126).
There are updates done in the APFloat utility functions
to handle these constraints for this format.

* The bias calculation is different and convertIEEE* APIs
  are updated to handle this.
* Since there are no significand bits, the
  isSignificandAll{Zeroes/Ones} methods are updated accordingly.
* Although the format does not have any precision, the precision
  bit in the fltSemantics is set to 1 for consistency with
  APFloat's internal representation.
* Many utility functions are updated to handle the fact that this
  format does not support Zero.
* Provide a separate initFromAPInt() implementation to
  handle the quirks of the format.
* Add specific tests to verify the range of values for this format.

Signed-off-by: Durgadoss R <durgadossr@nvidia.com>
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@durga4github
durga4github committed Sep 3, 2024
1 parent a8e1c6f commit cbfa6fd
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Showing 3 changed files with 374 additions and 22 deletions.
29 changes: 29 additions & 0 deletions llvm/include/llvm/ADT/APFloat.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -195,6 +195,12 @@ struct APFloatBase {
// improved range compared to half (16-bit) formats, at (potentially)
// greater throughput than single precision (32-bit) formats.
S_FloatTF32,
// 8-bit floating point number with (all the) 8 bits for the exponent
// like in FP32. There are no zeroes, no infinities, and no denormal values.
// NaN is represented with all bits set to 1. Bias is 127.
// This represents the scale data type in the MX specification from
// https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf
S_Float8E8M0FN,
// 6-bit floating point number with bit layout S1E3M2. Unlike IEEE-754
// types, there are no infinity or NaN values. The format is detailed in
// https://www.opencompute.org/documents/ocp-microscaling-formats-mx-v1-0-spec-final-pdf
Expand Down Expand Up @@ -229,6 +235,7 @@ struct APFloatBase {
static const fltSemantics &Float8E4M3B11FNUZ() LLVM_READNONE;
static const fltSemantics &Float8E3M4() LLVM_READNONE;
static const fltSemantics &FloatTF32() LLVM_READNONE;
static const fltSemantics &Float8E8M0FN() LLVM_READNONE;
static const fltSemantics &Float6E3M2FN() LLVM_READNONE;
static const fltSemantics &Float6E2M3FN() LLVM_READNONE;
static const fltSemantics &Float4E2M1FN() LLVM_READNONE;
Expand Down Expand Up @@ -652,6 +659,7 @@ class IEEEFloat final : public APFloatBase {
APInt convertFloat8E4M3B11FNUZAPFloatToAPInt() const;
APInt convertFloat8E3M4APFloatToAPInt() const;
APInt convertFloatTF32APFloatToAPInt() const;
APInt convertFloat8E8M0FNAPFloatToAPInt() const;
APInt convertFloat6E3M2FNAPFloatToAPInt() const;
APInt convertFloat6E2M3FNAPFloatToAPInt() const;
APInt convertFloat4E2M1FNAPFloatToAPInt() const;
Expand All @@ -672,6 +680,7 @@ class IEEEFloat final : public APFloatBase {
void initFromFloat8E4M3B11FNUZAPInt(const APInt &api);
void initFromFloat8E3M4APInt(const APInt &api);
void initFromFloatTF32APInt(const APInt &api);
void initFromFloat8E8M0FNAPInt(const APInt &api);
void initFromFloat6E3M2FNAPInt(const APInt &api);
void initFromFloat6E2M3FNAPInt(const APInt &api);
void initFromFloat4E2M1FNAPInt(const APInt &api);
Expand Down Expand Up @@ -1091,6 +1100,26 @@ class APFloat : public APFloatBase {
}
}

static bool hasZero(const fltSemantics &Sem) {
switch (SemanticsToEnum(Sem)) {
default:
return true;
// The Float8E8M0FN does not have an encoding for Zeroes.
case APFloat::S_Float8E8M0FN:
return false;
}
}

static bool hasExponentOnly(const fltSemantics &Sem) {
switch (SemanticsToEnum(Sem)) {
default:
return false;
// The Float8E8M0FN has exponent only and no significand.
case APFloat::S_Float8E8M0FN:
return true;
}
}

/// Used to insert APFloat objects, or objects that contain APFloat objects,
/// into FoldingSets.
void Profile(FoldingSetNodeID &NID) const;
Expand Down
128 changes: 124 additions & 4 deletions llvm/lib/Support/APFloat.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -145,6 +145,9 @@ static constexpr fltSemantics semFloat8E4M3B11FNUZ = {
4, -10, 4, 8, fltNonfiniteBehavior::NanOnly, fltNanEncoding::NegativeZero};
static constexpr fltSemantics semFloat8E3M4 = {3, -2, 5, 8};
static constexpr fltSemantics semFloatTF32 = {127, -126, 11, 19};
static constexpr fltSemantics semFloat8E8M0FN = {
127, -127, 1, 8, fltNonfiniteBehavior::NanOnly, fltNanEncoding::AllOnes};

static constexpr fltSemantics semFloat6E3M2FN = {
4, -2, 3, 6, fltNonfiniteBehavior::FiniteOnly};
static constexpr fltSemantics semFloat6E2M3FN = {
Expand Down Expand Up @@ -222,6 +225,8 @@ const llvm::fltSemantics &APFloatBase::EnumToSemantics(Semantics S) {
return Float8E3M4();
case S_FloatTF32:
return FloatTF32();
case S_Float8E8M0FN:
return Float8E8M0FN();
case S_Float6E3M2FN:
return Float6E3M2FN();
case S_Float6E2M3FN:
Expand Down Expand Up @@ -264,6 +269,8 @@ APFloatBase::SemanticsToEnum(const llvm::fltSemantics &Sem) {
return S_Float8E3M4;
else if (&Sem == &llvm::APFloat::FloatTF32())
return S_FloatTF32;
else if (&Sem == &llvm::APFloat::Float8E8M0FN())
return S_Float8E8M0FN;
else if (&Sem == &llvm::APFloat::Float6E3M2FN())
return S_Float6E3M2FN;
else if (&Sem == &llvm::APFloat::Float6E2M3FN())
Expand Down Expand Up @@ -294,6 +301,7 @@ const fltSemantics &APFloatBase::Float8E4M3B11FNUZ() {
}
const fltSemantics &APFloatBase::Float8E3M4() { return semFloat8E3M4; }
const fltSemantics &APFloatBase::FloatTF32() { return semFloatTF32; }
const fltSemantics &APFloatBase::Float8E8M0FN() { return semFloat8E8M0FN; }
const fltSemantics &APFloatBase::Float6E3M2FN() { return semFloat6E3M2FN; }
const fltSemantics &APFloatBase::Float6E2M3FN() { return semFloat6E2M3FN; }
const fltSemantics &APFloatBase::Float4E2M1FN() { return semFloat4E2M1FN; }
Expand Down Expand Up @@ -396,6 +404,8 @@ static inline Error createError(const Twine &Err) {
}

static constexpr inline unsigned int partCountForBits(unsigned int bits) {
if (bits == 0)
return 1;
return ((bits) + APFloatBase::integerPartWidth - 1) / APFloatBase::integerPartWidth;
}

Expand Down Expand Up @@ -955,6 +965,12 @@ void IEEEFloat::makeNaN(bool SNaN, bool Negative, const APInt *fill) {
significand[part] = 0;
}

// For the E8M0 types, precision is just 1 and the
// the NaNBit handling below is not relevant.
// So, exit early.
if (semantics == &semFloat8E8M0FN)
return;

unsigned QNaNBit = semantics->precision - 2;

if (SNaN) {
Expand Down Expand Up @@ -1007,6 +1023,10 @@ IEEEFloat &IEEEFloat::operator=(IEEEFloat &&rhs) {
}

bool IEEEFloat::isDenormal() const {
// No denormals in Float8E8M0FN
if (semantics == &semFloat8E8M0FN)
return false;

return isFiniteNonZero() && (exponent == semantics->minExponent) &&
(APInt::tcExtractBit(significandParts(),
semantics->precision - 1) == 0);
Expand All @@ -1028,6 +1048,10 @@ bool IEEEFloat::isSmallestNormalized() const {
bool IEEEFloat::isSignificandAllOnes() const {
// Test if the significand excluding the integral bit is all ones. This allows
// us to test for binade boundaries.
// For the E8M0 format, this is always false since there are no
// actual significand bits.
if (semantics == &semFloat8E8M0FN)
return false;
const integerPart *Parts = significandParts();
const unsigned PartCount = partCountForBits(semantics->precision);
for (unsigned i = 0; i < PartCount - 1; i++)
Expand Down Expand Up @@ -1075,6 +1099,11 @@ bool IEEEFloat::isSignificandAllOnesExceptLSB() const {
}

bool IEEEFloat::isSignificandAllZeros() const {
// For the E8M0 format, this is always true since there are no
// actual significand bits.
if (semantics == &semFloat8E8M0FN)
return true;

// Test if the significand excluding the integral bit is all zeros. This
// allows us to test for binade boundaries.
const integerPart *Parts = significandParts();
Expand Down Expand Up @@ -1113,6 +1142,8 @@ bool IEEEFloat::isSignificandAllZerosExceptMSB() const {
}

bool IEEEFloat::isLargest() const {
if (semantics == &semFloat8E8M0FN)
return isFiniteNonZero() && exponent == semantics->maxExponent;
if (semantics->nonFiniteBehavior == fltNonfiniteBehavior::NanOnly &&
semantics->nanEncoding == fltNanEncoding::AllOnes) {
// The largest number by magnitude in our format will be the floating point
Expand Down Expand Up @@ -1165,6 +1196,12 @@ IEEEFloat::IEEEFloat(const fltSemantics &ourSemantics, integerPart value) {

IEEEFloat::IEEEFloat(const fltSemantics &ourSemantics) {
initialize(&ourSemantics);
// The E8M0 type cannot represent the value zero.
// So, initialize with the closest representation instead.
if (semantics == &semFloat8E8M0FN) {
makeSmallestNormalized(false);
return;
}
makeZero(false);
}

Expand Down Expand Up @@ -1727,6 +1764,11 @@ IEEEFloat::opStatus IEEEFloat::normalize(roundingMode rounding_mode,
/* Canonicalize zeroes. */
if (omsb == 0) {
category = fcZero;
// The E8M0 type cannot represent the value zero and
// thus the category cannot be fcZero. So, get the
// closest representation to fcZero instead.
if (semantics == &semFloat8E8M0FN)
makeSmallestNormalized(false);
if (semantics->nanEncoding == fltNanEncoding::NegativeZero)
sign = false;
}
Expand Down Expand Up @@ -2606,6 +2648,11 @@ IEEEFloat::opStatus IEEEFloat::convert(const fltSemantics &toSemantics,
fs = opOK;
}

// The E8M0 type cannot represent the value zero and
// thus the category cannot be fcZero. So, get the
// closest representation to fcZero instead.
if (category == fcZero && semantics == &semFloat8E8M0FN)
makeSmallestNormalized(false);
return fs;
}

Expand Down Expand Up @@ -3070,6 +3117,11 @@ IEEEFloat::convertFromDecimalString(StringRef str, roundingMode rounding_mode) {
fs = opOK;
if (semantics->nanEncoding == fltNanEncoding::NegativeZero)
sign = false;
// The E8M0 type cannot represent the value zero and
// thus the category cannot be fcZero. So, get the
// closest representation to fcZero instead.
if (semantics == &semFloat8E8M0FN)
makeSmallestNormalized(false);

/* Check whether the normalized exponent is high enough to overflow
max during the log-rebasing in the max-exponent check below. */
Expand Down Expand Up @@ -3533,15 +3585,15 @@ APInt IEEEFloat::convertPPCDoubleDoubleAPFloatToAPInt() const {
template <const fltSemantics &S>
APInt IEEEFloat::convertIEEEFloatToAPInt() const {
assert(semantics == &S);

constexpr int bias = -(S.minExponent - 1);
const int bias = (semantics == &semFloat8E8M0FN) ?
-S.minExponent : -(S.minExponent - 1);
constexpr unsigned int trailing_significand_bits = S.precision - 1;
constexpr int integer_bit_part = trailing_significand_bits / integerPartWidth;
constexpr integerPart integer_bit =
integerPart{1} << (trailing_significand_bits % integerPartWidth);
constexpr uint64_t significand_mask = integer_bit - 1;
constexpr unsigned int exponent_bits =
S.sizeInBits - 1 - trailing_significand_bits;
constexpr unsigned int exponent_bits = trailing_significand_bits ?
(S.sizeInBits - 1 - trailing_significand_bits) : S.sizeInBits;
static_assert(exponent_bits < 64);
constexpr uint64_t exponent_mask = (uint64_t{1} << exponent_bits) - 1;

Expand All @@ -3557,6 +3609,8 @@ APInt IEEEFloat::convertIEEEFloatToAPInt() const {
!(significandParts()[integer_bit_part] & integer_bit))
myexponent = 0; // denormal
} else if (category == fcZero) {
if (semantics == &semFloat8E8M0FN)
llvm_unreachable("semantics does not support zero!");
myexponent = ::exponentZero(S) + bias;
mysignificand.fill(0);
} else if (category == fcInfinity) {
Expand Down Expand Up @@ -3659,6 +3713,11 @@ APInt IEEEFloat::convertFloatTF32APFloatToAPInt() const {
return convertIEEEFloatToAPInt<semFloatTF32>();
}

APInt IEEEFloat::convertFloat8E8M0FNAPFloatToAPInt() const {
assert(partCount() == 1);
return convertIEEEFloatToAPInt<semFloat8E8M0FN>();
}

APInt IEEEFloat::convertFloat6E3M2FNAPFloatToAPInt() const {
assert(partCount() == 1);
return convertIEEEFloatToAPInt<semFloat6E3M2FN>();
Expand Down Expand Up @@ -3721,6 +3780,9 @@ APInt IEEEFloat::bitcastToAPInt() const {
if (semantics == (const llvm::fltSemantics *)&semFloatTF32)
return convertFloatTF32APFloatToAPInt();

if (semantics == (const llvm::fltSemantics *)&semFloat8E8M0FN)
return convertFloat8E8M0FNAPFloatToAPInt();

if (semantics == (const llvm::fltSemantics *)&semFloat6E3M2FN)
return convertFloat6E3M2FNAPFloatToAPInt();

Expand Down Expand Up @@ -3819,6 +3881,40 @@ void IEEEFloat::initFromPPCDoubleDoubleAPInt(const APInt &api) {
}
}

// The E8M0 format has the following characteristics:
// It is an 8-bit unsigned format with only exponents (no actual significand)
// No encodings for {zero, infinities or denorms}
// NaN is represented by all 1's
// Bias is 127
void IEEEFloat::initFromFloat8E8M0FNAPInt(const APInt &api) {
const uint64_t exponent_mask = 0xff;
uint64_t val = api.getRawData()[0];
uint64_t myexponent = (val & exponent_mask);

initialize(&semFloat8E8M0FN);
assert(partCount() == 1);

// This format has unsigned representation only
sign = 0;

// Set the significand
// This format does not have any significand but the 'Pth' precision bit is
// always set to 1 for consistency in APFloat's internal representation.
uint64_t mysignificand = 1;
significandParts()[0] = mysignificand;

// This format can either have a NaN or fcNormal
// All 1's i.e. 255 is a NaN
if (val == exponent_mask) {
category = fcNaN;
exponent = exponentNaN();
return;
}
// Handle fcNormal...
category = fcNormal;
exponent = myexponent - 127; // 127 is bias
return;
}
template <const fltSemantics &S>
void IEEEFloat::initFromIEEEAPInt(const APInt &api) {
assert(api.getBitWidth() == S.sizeInBits);
Expand Down Expand Up @@ -3999,6 +4095,8 @@ void IEEEFloat::initFromAPInt(const fltSemantics *Sem, const APInt &api) {
return initFromFloat8E3M4APInt(api);
if (Sem == &semFloatTF32)
return initFromFloatTF32APInt(api);
if (Sem == &semFloat8E8M0FN)
return initFromFloat8E8M0FNAPInt(api);
if (Sem == &semFloat6E3M2FN)
return initFromFloat6E3M2FNAPInt(api);
if (Sem == &semFloat6E2M3FN)
Expand Down Expand Up @@ -4032,6 +4130,13 @@ void IEEEFloat::makeLargest(bool Negative) {
significand[PartCount - 1] = (NumUnusedHighBits < integerPartWidth)
? (~integerPart(0) >> NumUnusedHighBits)
: 0;
// For E8M0 format, we only have the 'internal' precision bit
// (aka 'P' the precision bit) which is always set to 1.
// Hence, the below logic of setting the LSB to 0 does not apply.
// For other cases, the LSB is meant to be any bit other than
// the Pth precision bit.
if (semantics == &semFloat8E8M0FN)
return;

if (semantics->nonFiniteBehavior == fltNonfiniteBehavior::NanOnly &&
semantics->nanEncoding == fltNanEncoding::AllOnes)
Expand Down Expand Up @@ -4509,6 +4614,11 @@ IEEEFloat::opStatus IEEEFloat::next(bool nextDown) {
exponent = 0;
if (semantics->nanEncoding == fltNanEncoding::NegativeZero)
sign = false;
// The E8M0 type cannot represent the value zero and
// thus the category cannot be fcZero. So, get the
// closest representation to fcZero instead.
if (semantics == &semFloat8E8M0FN)
makeSmallestNormalized(false);
break;
}

Expand Down Expand Up @@ -4575,6 +4685,11 @@ IEEEFloat::opStatus IEEEFloat::next(bool nextDown) {
// 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();
// The E8M0 format does not support Denorms.
// Since there are only exponents, any increment always crosses the
// 'BinadeBoundary'. So, make this true always.
if (semantics == &semFloat8E8M0FN)
WillCrossBinadeBoundary = true;

if (WillCrossBinadeBoundary) {
integerPart *Parts = significandParts();
Expand Down Expand Up @@ -4626,6 +4741,11 @@ void IEEEFloat::makeInf(bool Negative) {
}

void IEEEFloat::makeZero(bool Negative) {
// The E8M0 type cannot represent the value zero.
if (semantics == &semFloat8E8M0FN) {
assert(false && "This floating point format does not support Zero\n");
return;
}
category = fcZero;
sign = Negative;
if (semantics->nanEncoding == fltNanEncoding::NegativeZero) {
Expand Down
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