Will the compiler optimize the for loop to match the bus width?

user_11590274 0
Viewed 53

Let's say that I want to set a char array with different values, but for the sake of simplicity:

char buff[1024];
...
for (int i = 0; i < 1024; i++) buff[i] = NULL;

Is the compiler going to optimize this to match the bus width? Or should I manually do this:

char buff[1024];
...
size_t empty = NULL;
for (int i = 0; i < 1024 / sizeof(size_t); i++)
    memcpy(buff + i * sizeof(size_t), &empty, sizeof(size_t));

In the supposed case that sizeof(size_t) is the bus width.

I made some measurements, I think it just proves the points stated:

#define TIMES 512
#define SIZE 4194304

int main(void) {

    char *buff = new char[SIZE];

    int times = TIMES;

    clock_t begin = clock();

    void *pattern = (void*)0xffeeddcc;

    while (times--) {

        ... some for loop ...
    };

    clock_t end = clock();

    delete[] buff;

    std::cout << ((float)(end - begin) / CLOCKS_PER_SEC) << " s elapsed.\n";

    return 0;
};

Set char by char:

for (int i = 0; i < SIZE; i++) buff[i] = i % 0xff;

Average elapsed time: 13.6284 s

Set fixed size at a time (bus width):

for (int i = 0; i < SIZE / sizeof(void*); i++) {
    void* sub = (void*)(((i * sizeof(void*)) % 0xff) + (((i * sizeof(size_t) + 1) % 0xff) << 8) + (((i * sizeof(void*) + 2) % 0xff) << 16) + (((i * sizeof(void*) + 3) % 0xff) << 24));

    memcpy(buff + i * sizeof(void*), &sub, sizeof(void*));
};

Average elapsed time: 19.4352 s

Pattern char by char:

for (int i = 0; i < SIZE; i++) buff[i] = ((char*)&pattern)[i % sizeof(void*)];

Average elapsed time: 17.1696 s

Pattern fixed size (bus width):

for (int i = 0; i < SIZE / sizeof(void*); i++) memcpy(buff + i * sizeof(void*), &pattern, sizeof(void*));

Average elapsed time: 5.6976 s

I don't know if all this measurement was necessary XD

Done with a 2 GHz, 2 cores CPU (Intel Core i3-5005U CPU @ 2.00 GHz).

bus memory c++
1 Answers

If you're setting each char to the same value, just call memset.

Compilers (namely gcc and clang) do recognize loops like

for (int i = 0; i < 1024; i++) buff[i] = 0xff;

Clang turns it into a memset call; gcc uses instructions setting one word at a time: https://gcc.godbolt.org/z/ovRTPU. But you get the same assembly output from making a call to memset (it's generally a compiler-recognized function (like memcpy)).

If you're setting a buffer with static storage duration to 0, you don't need to do anything because it already will have been zeroed by the time the program is loaded.

(BTW, using NULL for 0 isn't a good idea. At least in C, NULL can be (void*)0 which won't get assigned to an integer type without warnings/errors.)

user_10847740
Related