using memset or loop

The first call to set_float_zero() will invoke float_zero_is_all_bits_zero() to determine how 0.0 is actually represented; it then uses memset() if it's safe, and falls back to an explicit loop if it isn't.  On calls after the first, it merely checks a single flag.

This is useful if the memset() is significantly faster than the explicit loop, something that is not at all obvious.

The code: -

 

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

 

static int float_zero_is_all_bits_zero(void)

{

    float f = 0.0;

    unsigned char zero_bytes[sizeof f] = { 0 };

    return memcmp(&f, zero_bytes, sizeof f) == 0;

}

void set_float_zero(float *dest, size_t count)

{

    static int initialized = 0;

    int may_use_memset;

    if (!initialized) {

        may_use_memset = float_zero_is_all_bits_zero();

        initialized = 1;

    }

    if (may_use_memset) {

        memset(dest, 0, count * sizeof *dest);

    }

    else {

        size_t i;

        for (i = 0; i < count; i ++) {

            dest[i] = 0.0;

        }

    }

}

#define ARR_LEN 1000

int main(void)

{

    float *arr = (float*)malloc(ARR_LEN * sizeof *arr);

    set_float_zero(arr, ARR_LEN);

    return 0;

}

 

 

 

Pradyut
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