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// Copyright 2020 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "src/tint/writer/float_to_string.h"
#include <cmath>
#include <cstring>
#include <functional>
#include <iomanip>
#include <limits>
#include <sstream>
#include "src/tint/debug.h"
namespace tint::writer {
namespace {
template <typename T>
struct Traits;
template <>
struct Traits<float> {
using uint_t = uint32_t;
static constexpr int kExponentBias = 127;
static constexpr uint_t kExponentMask = 0x7f800000;
static constexpr uint_t kMantissaMask = 0x007fffff;
static constexpr uint_t kSignMask = 0x80000000;
static constexpr int kMantissaBits = 23;
};
template <>
struct Traits<double> {
using uint_t = uint64_t;
static constexpr int kExponentBias = 1023;
static constexpr uint_t kExponentMask = 0x7ff0000000000000;
static constexpr uint_t kMantissaMask = 0x000fffffffffffff;
static constexpr uint_t kSignMask = 0x8000000000000000;
static constexpr int kMantissaBits = 52;
};
template <typename F>
std::string ToString(F f) {
// Try printing the float in fixed point, with a smallish limit on the precision
std::stringstream fixed;
fixed.flags(fixed.flags() | std::ios_base::showpoint | std::ios_base::fixed);
fixed.imbue(std::locale::classic());
fixed.precision(9);
fixed << f;
std::string str = fixed.str();
// If this string can be parsed without loss of information, use it.
// (Use double here to dodge a bug in older libc++ versions which would incorrectly read back
// FLT_MAX as INF.)
double roundtripped;
fixed >> roundtripped;
auto float_equal_no_warning = std::equal_to<F>();
if (float_equal_no_warning(f, static_cast<F>(roundtripped))) {
while (str.length() >= 2 && str[str.size() - 1] == '0' && str[str.size() - 2] != '.') {
str.pop_back();
}
return str;
}
// Resort to scientific, with the minimum precision needed to preserve the whole float
std::stringstream sci;
sci.imbue(std::locale::classic());
sci.precision(std::numeric_limits<F>::max_digits10);
sci << f;
return sci.str();
}
template <typename F>
std::string ToBitPreservingString(F f) {
using T = Traits<F>;
using uint_t = typename T::uint_t;
// For the NaN case, avoid handling the number as a floating point value.
// Some machines will modify the top bit in the mantissa of a NaN.
std::stringstream ss;
typename T::uint_t float_bits = 0u;
static_assert(sizeof(float_bits) == sizeof(f));
std::memcpy(&float_bits, &f, sizeof(float_bits));
// Handle the sign.
if (float_bits & T::kSignMask) {
// If `f` is -0.0 print -0.0.
ss << '-';
// Strip sign bit.
float_bits = float_bits & (~T::kSignMask);
}
switch (std::fpclassify(f)) {
case FP_ZERO:
case FP_NORMAL:
std::memcpy(&f, &float_bits, sizeof(float_bits));
ss << ToString(f);
break;
default: {
// Infinity, NaN, and Subnormal
// TODO(dneto): It's unclear how Infinity and NaN should be handled.
// See https://github.com/gpuweb/gpuweb/issues/1769
// std::hexfloat prints 'nan' and 'inf' instead of an explicit representation like we
// want. Split it out manually.
int mantissa_nibbles = (T::kMantissaBits + 3) / 4;
const int biased_exponent =
static_cast<int>((float_bits & T::kExponentMask) >> T::kMantissaBits);
int exponent = biased_exponent - T::kExponentBias;
uint_t mantissa = float_bits & T::kMantissaMask;
ss << "0x";
if (exponent == T::kExponentBias + 1) {
if (mantissa == 0) {
// Infinity case.
ss << "1p+" << exponent;
} else {
// NaN case.
// Emit the mantissa bits as if they are left-justified after the binary point.
// This is what SPIRV-Tools hex float emitter does, and it's a justifiable
// choice independent of the bit width of the mantissa.
mantissa <<= (4 - (T::kMantissaBits % 4));
// Remove trailing zeroes, for tidiness.
while (0 == (0xf & mantissa)) {
mantissa >>= 4;
mantissa_nibbles--;
}
ss << "1." << std::hex << std::setfill('0') << std::setw(mantissa_nibbles)
<< mantissa << "p+" << std::dec << exponent;
}
} else {
// Subnormal, and not zero.
TINT_ASSERT(Writer, mantissa != 0);
const auto kTopBit = static_cast<uint_t>(1u) << T::kMantissaBits;
// Shift left until we get 1.x
while (0 == (kTopBit & mantissa)) {
mantissa <<= 1;
exponent--;
}
// Emit the leading 1, and remove it from the mantissa.
ss << "1";
mantissa = mantissa ^ kTopBit;
exponent++;
// Left-justify mantissa to whole nibble.
mantissa <<= (4 - (T::kMantissaBits % 4));
// Emit the fractional part.
if (mantissa) {
// Remove trailing zeroes, for tidiness
while (0 == (0xf & mantissa)) {
mantissa >>= 4;
mantissa_nibbles--;
}
ss << "." << std::hex << std::setfill('0') << std::setw(mantissa_nibbles)
<< mantissa;
}
// Emit the exponent
ss << "p" << std::showpos << std::dec << exponent;
}
}
}
return ss.str();
}
} // namespace
std::string FloatToString(float f) {
return ToString(f);
}
std::string FloatToBitPreservingString(float f) {
return ToBitPreservingString(f);
}
std::string DoubleToString(double f) {
return ToString(f);
}
std::string DoubleToBitPreservingString(double f) {
return ToBitPreservingString(f);
}
} // namespace tint::writer