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lionaneesh 5Jun2012 16:00

Macros v/s Functions
 
In this article we'll be learning how we can improve our C code by using Macros instead of functions. How simple functions can be converted into macros to save on some CPU instructions. Some basic knowledge of C and a little bit Assembly is considered as pre-requisites.

The Code



We'll be testing a basic C code, that Squares a number '10' and displays it on the screen.

Using Functions

Function.c
Code: c

#include<stdio.h>

int SQUARE(int x)
{
    return (x)*(x);
}
int main()
{
    printf("%d\n", SQUARE(10));
}


Disassembly :-

Code:

Dump of assembler code for function main:
  0x080483d0 <+0>:        push  ebp
  0x080483d1 <+1>:        mov    ebp,esp
  0x080483d3 <+3>:        and    esp,0xfffffff0
  0x080483d6 <+6>:        sub    esp,0x10
  0x080483d9 <+9>:        mov    DWORD PTR [esp],0xa
  0x080483e0 <+16>:        call  0x80483c4 <SQUARE>
  0x080483e5 <+21>:        mov    edx,0x80484c0
  0x080483ea <+26>:        mov    DWORD PTR [esp+0x4],eax
  0x080483ee <+30>:        mov    DWORD PTR [esp],edx
  0x080483f1 <+33>:        call  0x80482f4 <printf@plt>
  0x080483f6 <+38>:        leave 
  0x080483f7 <+39>:        ret   
End of assembler dump.

Using Macros

Macros.c
Code: c

#include<stdio.h>

#define SQUARE(x) (x)*(x)

int main()
{
    printf("%d\n", SQUARE(10));
}


Disassembly:-

Code:

Dump of assembler code for function main:
  0x080483c4 <+0>:        push  ebp
  0x080483c5 <+1>:        mov    ebp,esp
  0x080483c7 <+3>:        and    esp,0xfffffff0
  0x080483ca <+6>:        sub    esp,0x10
  0x080483cd <+9>:        mov    eax,0x80484b0
  0x080483d2 <+14>:        mov    DWORD PTR [esp+0x4],0x64
  0x080483da <+22>:        mov    DWORD PTR [esp],eax
  0x080483dd <+25>:        call  0x80482f4 <printf@plt>
  0x080483e2 <+30>:        leave 
  0x080483e3 <+31>:        ret   
End of assembler dump.

Differences



From the Comparison above its quite trivial to note the differences :-

1. Size
The size of the “Function.c” bytecode is 9 bytes more than that of “Macros.c” bytecode.
2. Instructions
We are 12 instructions in Function.c:main, while we are using 10 instructions in Macros.c:main.
The differences is mainly because we are actually wasting 2 instructions, 1st to load the 'esp' with the input and 2nd to Call the function. Why? Because in Macros.c, the Preprocessor actually hardcodes "10*10", instead of calling a function.

That's all for this article stay tuned for more.


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