CODEWITHSUNDEEP
clinuxperformance
CCoder
CCoder

Reputation: 2335

Structure initialization performance

I am trying to improve performance of my program (running on ARC platform, compiled with arc-gcc. Having said that, I am NOT expecting a platform specific answer).

I want to know which of the following methods is more optimal and why.

typedef struct _MY_STRUCT
{
    int my_height;
    int my_weight;
    char my_data_buffer[1024];
}MY_STRUCT;

int some_function(MY_STRUCT *px_my_struct)
{
    /*Many operations with the structure members done here*/
    return 0;
}

void poorly_performing_function_method_1()
{
    while(1)
    {
        MY_STRUCT x_struct_instance = {0}; /*x_struct_instance is automatic variable under WHILE LOOP SCOPE*/
        x_struct_instance.my_height = rand();
        x_struct_instance.my_weight = rand();
        if(x_struct_instance.my_weight > 100)
        {
            memcpy(&(x_struct_instance.my_data_buffer),"this is just an example string, there could be some binary data here.",sizeof(x_struct_instance.my_data_buffer));
        }
        some_function(&x_struct_instance);

        /******************************************************/
        /* No need for memset as it is initialized before use.*/
        /* memset(&x_struct_instance,0,sizeof(x_struct_instance));*/
        /******************************************************/
    }
}

void poorly_performing_function_method_2()
{
    MY_STRUCT x_struct_instance = {0}; /*x_struct_instance is automatic variable under FUNCTION SCOPE*/
    while(1)
    {
        x_struct_instance.my_height = rand();
        x_struct_instance.my_weight = rand();
        if(x_struct_instance.my_weight > 100)
        {
            memcpy(&(x_struct_instance.my_data_buffer),"this is just an example string, there could be some binary data here.",sizeof(x_struct_instance.my_data_buffer));
        }
        some_function(&x_struct_instance);
        memset(&x_struct_instance,0,sizeof(x_struct_instance));
    }
}

In the above code, will poorly_performing_function_method_1() perform better or will poorly_performing_function_method_2() perform better? Why?

Few things to think about..

I want to clarify that my question is more about WHICH method is more optimal and less about HOW to make this code more optimal. This code is just an example.

About making the above code more optimal, @Skizz has given the right answer.

Upvotes: 1

Views: 325

Answers (1)

Skizz
Skizz

Reputation: 71070

Generally, not doing something is going to be faster than doing something.

In your code, you're clearing a structure, and then initialising it with data. You're doing two memory writes, the second is just overwriting the first.

Try this:-

void function_to_try()
{
  MY_STRUCT x_struct_instance;
  while(1)
  {
    x_struct_instance.my_height = rand();
    x_struct_instance.my_weight = rand();
    x_struct_instance.my_name[0]='\0';
    if(x_struct_instance.my_weight > 100)
    {
        strlcpy(&(x_struct_instance.my_name),"Fatty",sizeof(x_struct_instance.my_name));
    }
    some_function(&x_struct_instance);
  }
}

Update

To answer the question, which is more optimal, I would suggest method #1, but it is probably marginal and dependent on the compiler and other factors. My reasoning is that there isn't any allocation / deallocation going on, the data is on the stack and the function preamble created by the compiler will allocate a big enough stack frame for the function such that it doesn't need to resize it. In any case, allocating on the stack is just moving the stack pointer so it's not a big overhead.

Also, memset is a general purpose method for setting memory and might have extra logic in it that copes with edge conditions such as unaligned memory. The compiler can implement an initialiser more intelligently than a general purpose algorithm (at least, one would hope so).

Upvotes: 3

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