test/fuzzer_simd.cpp

#include <array>
#include <cstdint>
#include <cstring>
#include <limits>

#include "libdivide.h"

#if __cplusplus < 201703L
#error "Sorry, needs C++17 or later."
#endif

// How many bytes of data to use for numerators at most.
// Must be larger than the largest simd size.
constexpr const std::size_t NbytesOfInput = 512 / 8;

// how much data to consume for the divisor (at most).
constexpr const std::size_t NbytesForDivisor = sizeof(std::int64_t);

extern "C" int LLVMFuzzerTestOneInput(const uint8_t* Data, size_t Size) {
    // pick what types to operate on from the fuzz data
    if (Size < 2) return 0;
    const auto type_selector = Data[0];
    const auto branchfree_selector = Data[1];
    Data += 2;
    Size -= 2;

    // exit early to avoid reading outside bounds later
    if (Size < NbytesOfInput + NbytesForDivisor) {
        return 0;
    }

    // This generic lambda does the work for us, by conveniently deducing the type
    // It will return an int, just so the switch case looks prettier at the
    // call site.
    auto outer = [&](const auto integerdummy) -> int {
        auto inner = [&](const auto branchingtypedummy) -> int {
            using Integer = std::remove_const_t<decltype(integerdummy)>;
            static_assert(std::is_integral_v<Integer>, "input should be an integer");

            // pick the divisor from the fuzz data
            const Integer divisor = [&]() {
                static_assert(sizeof(Integer) <= NbytesForDivisor, "make NbytesForDivisor larger");
                Integer tmp;
                std::memcpy(&tmp, Data, sizeof(Integer));
                Data += NbytesForDivisor;
                Size -= NbytesForDivisor;
                return tmp;
            }();

            // don't let the universe explode
            if (divisor == 0) {
                return 0;
            }

            // respect the branchfree variant prohibiting 1
            if (branchingtypedummy.value == libdivide::BRANCHFREE && divisor == 1) {
                return 0;
            }

            // This array type is used for both input and output. It may be overly
            // large for the smaller simd types.
            using ArrayOfIntegers = std::array<Integer, NbytesOfInput / sizeof(Integer)>;

            // The numbers to later divide by divisor
            ArrayOfIntegers numerators;
            numerators.fill(0);
            if (Size < NbytesOfInput) return 0;
            std::memcpy(numerators.data(), Data, NbytesOfInput);

            // avoid problems with integer overflow from INT_MIN/-1
            if (std::is_signed_v<Integer> && divisor == -1) {
                for (auto& e : numerators) {
                    if (e == std::numeric_limits<Integer>::min()) {
                        e = 0;
                    }
                }
            }

            // get data into a simd register
#if defined(LIBDIVIDE_AVX512)
            // not tested!
            using Vector = __m512i;
            const Vector num_as_simdvector = _mm512_loadu_si512((const Vector*)numerators.data());
#endif

#if defined(LIBDIVIDE_AVX2)
            using Vector = __m256i;
            const Vector num_as_simdvector = _mm256_loadu_si256((const Vector*)numerators.data());
#endif

#if defined(LIBDIVIDE_SSE2)
            using Vector = __m128i;
            const Vector num_as_simdvector = _mm_loadu_si128((const Vector*)numerators.data());
#endif
            // carry out the division
            libdivide::divider<Integer, branchingtypedummy.value> divider(divisor);
            const Vector res = num_as_simdvector / divider;

            // this will eventually contain the result
            ArrayOfIntegers simdresult;
            simdresult.fill(0);

            // copy the results from the simd register
#if defined(LIBDIVIDE_AVX512)
            _mm512_storeu_si512((Vector*)simdresult.data(), res);
#endif

#if defined(LIBDIVIDE_AVX2)
            _mm256_storeu_si256((Vector*)simdresult.data(), res);
#endif

#if defined(LIBDIVIDE_SSE2)
            _mm_storeu_si128((Vector*)simdresult.data(), res);
#endif

            // how many elements will be assigned?
            constexpr const std::size_t Nelements = sizeof(Vector) / sizeof(Integer);

            // validate the result
            for (std::size_t i = 0; i < Nelements; ++i) {
                const Integer expected = numerators.at(i) / divisor;
                const Integer actual = simdresult.at(i);
                if (expected != actual) abort();
            }
            return 0;
        };

        if (branchfree_selector == libdivide::BRANCHFULL) {
            return inner(std::integral_constant<int, libdivide::BRANCHFULL>{});
        } else if (branchfree_selector == libdivide::BRANCHFREE) {
            return inner(std::integral_constant<int, libdivide::BRANCHFREE>{});
        }
        return 0;
    };

    switch (type_selector) {
        case 0:
            return outer(std::uint32_t{});
        case 1:
            return outer(std::int32_t{});
        case 2:
            return outer(std::uint64_t{});
        case 3:
            return outer(std::int64_t{});
        default:
            return 0;
    }
    return 0;
}