Subtract or add constant to large array

You can just write a function with a plain loop:

void add(uint8_t* a, size_t a_len, uint8_t b) {
    for(uint8_t* ae = a + a_len; a < ae; ++a)
        *a += b;
}

And hope that the compiler vectorizes that for you, which it does, see assembly.

Solutions with std::for_each and std::transform such as:

void add(uint8_t* a, size_t a_len, uint8_t b) {
    std::transform(a, a + a_len, a, [b](auto value) { return value + b; });
}

Should generate exactly the same code, but sometimes they don't.

[Updated]

Out of curiosity, I benchmarked the following solutions:

#include <benchmark/benchmark.h>

#include <cstdint>
#include <array>
#include <algorithm>

#include <immintrin.h>

constexpr size_t SIZE = 1824 * 942;
alignas(32) std::array<uint8_t, SIZE> A;

__attribute__((noinline)) void add_loop(uint8_t* a, size_t a_len, uint8_t b) {
    for(uint8_t* ae = a + a_len; a < ae; ++a)
        *a += b;
}

__attribute__((noinline)) void add_loop_4way(uint8_t* a, size_t a_len, uint8_t b) {
    a_len /= 4;
    for(uint8_t* ae = a + a_len; a < ae; ++a) {
        a[a_len * 0] += b;
        a[a_len * 1] += b;
        a[a_len * 2] += b;
        a[a_len * 3] += b;
    }
}

__attribute__((noinline)) void add_transform(uint8_t* a, size_t a_len, uint8_t b) {
    std::transform(a, a + a_len, a, [b](auto value) { return value + b; });
}

inline void add_sse_(__m128i* sse_a, size_t a_len, uint8_t b) {
    __m128i sse_b = _mm_set1_epi8(b);
    for(__m128i* ae = sse_a + a_len / (sizeof *sse_a / sizeof b); sse_a < ae; ++sse_a)
        *sse_a = _mm_add_epi8(*sse_a, sse_b);
}

__attribute__((noinline)) void add_sse(uint8_t* a, size_t a_len, uint8_t b) {
    add_sse_(reinterpret_cast<__m128i*>(a), a_len, b);
}

inline void add_avx_(__m256i* avx_a, size_t a_len, uint8_t b) {
    __m256i avx_b = _mm256_set1_epi8(b);
    for(__m256i* ae = avx_a + a_len / (sizeof *avx_a / sizeof b); avx_a < ae; ++avx_a)
        *avx_a = _mm256_add_epi8(*avx_a, avx_b);
}

__attribute__((noinline)) void add_avx(uint8_t* a, size_t a_len, uint8_t b) {
    add_avx_(reinterpret_cast<__m256i*>(a), a_len, b);
}

template<decltype(&add_loop) F>
void B(benchmark::State& state) {
    for(auto _ : state)
        F(A.data(), A.size(), 15);
}

BENCHMARK_TEMPLATE(B, add_loop);
BENCHMARK_TEMPLATE(B, add_loop_4way);
BENCHMARK_TEMPLATE(B, add_transform);
BENCHMARK_TEMPLATE(B, add_sse);
BENCHMARK_TEMPLATE(B, add_avx);

BENCHMARK_MAIN();

Results on i7-7700k CPU and g++-8.3 -DNDEBUG -O3 -march=native -mtune=native:

------------------------------------------------------------------
Benchmark                        Time             CPU   Iterations
------------------------------------------------------------------
B<add_loop>                  31589 ns        31589 ns        21981
B<add_loop_4way>             30030 ns        30030 ns        23265
B<add_transform>             31590 ns        31589 ns        22159
B<add_sse>                   39993 ns        39992 ns        17403
B<add_avx>                   31588 ns        31587 ns        22161

Times for loop, transform and AVX2 versions are pretty much identical.

SSE version is slower because the compiler generates faster AVX2 code.

perf report reports ~50% L1d-cache miss rate which indicates that the algorithm is bottlenecked by memory access. Modern CPUs can handle multiple memory accesses simultaneously, so that you can squeeze an extra ~5% of performance here by accessing 4 regions of memory in parallel, which is what the 4-way loop does (for your particular array size 4 ways is the fastest). See Memory-level parallelism: Intel Skylake versus Intel Cannonlake for more details.

You could use std::for_each:

uint8_t value = 15;
std::for_each(std::begin(nums), std::end(nums), [value](uint8_t& num) { num -= value; });

where nums is an array of uint8_t.

This should be the fastest way to do it:

#include <iostream>
#include <cstdint>
#include <array>
#include <algorithm>
#include <execution>


int main() {
    constexpr size_t  size = 1824 * 942;
    uint16_t input{};
    std::cout << "Initialize with: ";
    std::cin >> input;
    std::array<uint8_t, size> array{};
    std::fill(std::execution::par_unseq, array.begin(), array.end(), input);

    std::transform(std::execution::par_unseq,array.begin(), array.end(), array.begin(), [] (const auto& value) { return value + 15; });

    std::for_each(array.begin(),array.end(), [] (auto value) {
        std::cout << static_cast<uint16_t>(value) << ",";
    });
    std::cout << "\n";
}

Note the significant line std::transform(std::execution::par_unseq,array.begin(), array.end(), array.begin(), [] (const auto& value) { return value + 15; }); the rest is for examples sake.

Also note that, since you did not specify which array type, you can transform built in arrays like uint8_t array[1924*924]; to an std::array with std::to_array.

You can create a struct (or class) that will hold that parameter that is common to all of the elements in your array.

struct nameIt
{
    uint8_t* arr;
    uint8_t delta;
}