Do not optimise the contents of a function

__attribute__((optimize(0))) void MyFunction()
{
}

Remove the in and out code for a function

The compiler does not generate prologue and epilogue sequences for functions with:

__attribute__((naked)) void MyFunction()
{
}

Avoiding Padding

You can used the packed variable attribute. For example in the following code, type c_t will have a size of 3 bytes and there will be no wasted space between a and b:

typedef struct
{
  unsigned char a[1];
} __attribute__ ((packed)) a_t;

typedef struct
{
  unsigned char b[2];
} __attribute__ ((packed)) b_t;

typedef struct
{
  a_t a;
  b_t b;
} __attribute__ ((packed)) c_t;