src/common/Math.cpp

// Copyright 2017 The Dawn 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 "common/Math.h"

#include "common/Assert.h"

#include <algorithm>
#include <cmath>

#if defined(DAWN_COMPILER_MSVC)
#    include <intrin.h>
#endif

uint32_t ScanForward(uint32_t bits) {
    ASSERT(bits != 0);
#if defined(DAWN_COMPILER_MSVC)
    unsigned long firstBitIndex = 0ul;
    unsigned char ret = _BitScanForward(&firstBitIndex, bits);
    ASSERT(ret != 0);
    return firstBitIndex;
#else
    return static_cast<uint32_t>(__builtin_ctz(bits));
#endif
}

uint32_t Log2(uint32_t value) {
    ASSERT(value != 0);
#if defined(DAWN_COMPILER_MSVC)
    unsigned long firstBitIndex = 0ul;
    unsigned char ret = _BitScanReverse(&firstBitIndex, value);
    ASSERT(ret != 0);
    return firstBitIndex;
#else
    return 31 - static_cast<uint32_t>(__builtin_clz(value));
#endif
}

uint32_t Log2(uint64_t value) {
    ASSERT(value != 0);
#if defined(DAWN_COMPILER_MSVC)
    unsigned long firstBitIndex = 0ul;
    unsigned char ret = _BitScanReverse64(&firstBitIndex, value);
    ASSERT(ret != 0);
    return firstBitIndex;
#else
    return 63 - static_cast<uint32_t>(__builtin_clzll(value));
#endif
}

uint64_t NextPowerOfTwo(uint64_t n) {
#if defined(DAWN_COMPILER_MSVC)
    if (n <= 1) {
        return 1;
    }

    unsigned long firstBitIndex = 0ul;
    unsigned char ret = _BitScanReverse64(&firstBitIndex, n - 1);
    ASSERT(ret != 0);
    return 1ull << (firstBitIndex + 1);
#else
    return n <= 1 ? 1 : 1ull << (64 - __builtin_clzll(n - 1));
#endif
}

uint16_t Float32ToFloat16(float fp32) {
    uint32_t fp32i = BitCast<uint32_t>(fp32);
    uint32_t sign16 = (fp32i & 0x80000000) >> 16;
    uint32_t mantissaAndExponent = fp32i & 0x7FFFFFFF;

    if (mantissaAndExponent > 0x7F800000) {  // NaN
        return 0x7FFF;
    } else if (mantissaAndExponent > 0x47FFEFFF) {  // Infinity
        return static_cast<uint16_t>(sign16 | 0x7C00);
    } else if (mantissaAndExponent < 0x38800000) {  // Denormal
        uint32_t mantissa = (mantissaAndExponent & 0x007FFFFF) | 0x00800000;
        int32_t exponent = 113 - (mantissaAndExponent >> 23);

        if (exponent < 24) {
            mantissaAndExponent = mantissa >> exponent;
        } else {
            mantissaAndExponent = 0;
        }

        return static_cast<uint16_t>(
            sign16 | (mantissaAndExponent + 0x00000FFF + ((mantissaAndExponent >> 13) & 1)) >> 13);
    } else {
        return static_cast<uint16_t>(sign16 | (mantissaAndExponent + 0xC8000000 + 0x00000FFF +
                                               ((mantissaAndExponent >> 13) & 1)) >>
                                                  13);
    }
}

bool IsFloat16NaN(uint16_t fp16) {
    return (fp16 & 0x7FFF) > 0x7C00;
}

// Based on the Khronos Data Format Specification 1.2 Section 13.3 sRGB transfer functions
float SRGBToLinear(float srgb) {
    // sRGB is always used in unsigned normalized formats so clamp to [0.0, 1.0]
    if (srgb <= 0.0f) {
        return 0.0f;
    } else if (srgb > 1.0f) {
        return 1.0f;
    }

    if (srgb < 0.04045f) {
        return srgb / 12.92f;
    } else {
        return std::pow((srgb + 0.055f) / 1.055f, 2.4f);
    }
}