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Faster conversion from integer types to bfloat16

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Specialized `bfloat16_impl::float_to_bfloat16_rtne(float)` for normal floating point numbers, infinity and zero, in order to improve the performance of `bfloat16::bfloat16(const T&)` for integer argument types.

A reduction of more than 20% of the runtime duration of conversion from int to bfloat16 was observed, using Visual C++ 2019 on Windows 10.
This commit is contained in:
Niels Dekker 2020-07-14 23:22:34 +02:00
parent acab22c205
commit 0e1a33a461
2 changed files with 37 additions and 11 deletions
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44
Eigen/src/Core/arch/Default/BFloat16.h
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@ -58,7 +58,14 @@ struct __bfloat16_raw {
};
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw raw_uint16_to_bfloat16(unsigned short value);
template <bool AssumeArgumentIsNormalOrInfinityOrZero>
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne(float ff);
// Forward declarations of template specializations, to avoid Visual C++ 2019 errors, saying:
// > error C2908: explicit specialization; 'float_to_bfloat16_rtne' has already been instantiated
template <>
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne<false>(float ff);
template <>
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne<true>(float ff);
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC float bfloat16_to_float(__bfloat16_raw h);
struct bfloat16_base : public __bfloat16_raw {
@ -81,14 +88,14 @@ struct bfloat16 : public bfloat16_impl::bfloat16_base {
: bfloat16_impl::bfloat16_base(bfloat16_impl::raw_uint16_to_bfloat16(b ? 0x3f80 : 0)) {}
template<class T>
explicit EIGEN_DEVICE_FUNC bfloat16(const T& val)
: bfloat16_impl::bfloat16_base(bfloat16_impl::float_to_bfloat16_rtne(static_cast<float>(val))) {}
: bfloat16_impl::bfloat16_base(bfloat16_impl::float_to_bfloat16_rtne<internal::is_integral<T>::value>(static_cast<float>(val))) {}
explicit EIGEN_DEVICE_FUNC bfloat16(float f)
: bfloat16_impl::bfloat16_base(bfloat16_impl::float_to_bfloat16_rtne(f)) {}
: bfloat16_impl::bfloat16_base(bfloat16_impl::float_to_bfloat16_rtne<false>(f)) {}
// Following the convention of numpy, converting between complex and
// float will lead to loss of imag value.
template<typename RealScalar>
explicit EIGEN_DEVICE_FUNC bfloat16(const std::complex<RealScalar>& val)
: bfloat16_impl::bfloat16_base(bfloat16_impl::float_to_bfloat16_rtne(static_cast<float>(val.real()))) {}
: bfloat16_impl::bfloat16_base(bfloat16_impl::float_to_bfloat16_rtne<false>(static_cast<float>(val.real()))) {}
EIGEN_DEVICE_FUNC EIGEN_EXPLICIT_CAST(bool) const {
// +0.0 and -0.0 become false, everything else becomes true.
@ -326,11 +333,13 @@ EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw raw_uint16_to_bfloat16(unsi
return h;
}
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne(float ff) {
// float_to_bfloat16_rtne template specialization that does not make any
// assumption about the value of its function argument (ff).
template <>
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne<false>(float ff) {
#if (defined(EIGEN_HAS_CUDA_BF16) && defined(EIGEN_HAS_HIP_BF16))
// Nothing to do here
#else
unsigned int input = numext::as_uint(ff);
__bfloat16_raw output;
if (Eigen::numext::isnan EIGEN_NOT_A_MACRO(ff)) {
@ -491,15 +500,32 @@ EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne(floa
// Sign | Exp (8 bit) | Frac (first 7 bit)
// S E E E E E E E E F F F F F F L
// 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0
//
//
// At this point, ff must be either a normal float, or +/-infinity.
output = float_to_bfloat16_rtne<true>(ff);
}
return output;
#endif
}
// float_to_bfloat16_rtne template specialization that assumes that its function
// argument (ff) is either a normal floating point number, or +/-infinity, or
// zero. Used to improve the runtime performance of conversion from an integer
// type to bfloat16.
template <>
EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne<true>(float ff) {
#if (defined(EIGEN_HAS_CUDA_BF16) && defined(EIGEN_HAS_HIP_BF16))
// Nothing to do here
#else
unsigned int input = numext::as_uint(ff);
__bfloat16_raw output;
// Least significant bit of resulting bfloat.
unsigned int lsb = (input >> 16) & 1;
unsigned int rounding_bias = 0x7fff + lsb;
input += rounding_bias;
output.value = static_cast<unsigned short>(input >> 16);
}
return output;
return output;
#endif
}

4
test/bfloat16_float.cpp
View File

@ -31,7 +31,7 @@ float BinaryToFloat(uint32_t sign, uint32_t exponent, uint32_t high_mantissa,
void test_truncate(float input, float expected_truncation, float expected_rounding){
bfloat16 truncated = Eigen::bfloat16_impl::truncate_to_bfloat16(input);
bfloat16 rounded = Eigen::bfloat16_impl::float_to_bfloat16_rtne(input);
bfloat16 rounded = Eigen::bfloat16_impl::float_to_bfloat16_rtne<false>(input);
if ((numext::isnan)(input)){
VERIFY((numext::isnan)(static_cast<float>(truncated)) || (numext::isinf)(static_cast<float>(truncated)));
VERIFY((numext::isnan)(static_cast<float>(rounded)) || (numext::isinf)(static_cast<float>(rounded)));
@ -93,7 +93,7 @@ void test_conversion()
} else {
VERIFY_IS_EQUAL(bf_trunc.value, 0x0000);
}
bfloat16 bf_round = Eigen::bfloat16_impl::float_to_bfloat16_rtne(denorm);
bfloat16 bf_round = Eigen::bfloat16_impl::float_to_bfloat16_rtne<false>(denorm);
VERIFY_IS_EQUAL(static_cast<float>(bf_round), 0.0f);
if (std::signbit(denorm)) {
VERIFY_IS_EQUAL(bf_round.value, 0x8000);