Macros v/s Functions

Discussion in 'C' started by lionaneesh, Jun 5, 2012.

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1. lionaneeshActive Member

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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:
```#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:
```#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.