Re: [2.4] gcc3 warns about type-punned pointers ?

["J.A. Magallon" <jamagallon@able.es>]

On 08.29, J.A. Magallon wrote:
> Hi all...
> 
> gcc3 gives this warning when using the __set_64bit_var function:
> 
> /usr/src/linux/include/asm/system.h:190: warning: dereferencing type-punned pointer will break strict-aliasing rules
> 
> Is it a potential problem ?
> 
> This seems to cure it:
> 
> --- linux-2.4.22-jam1m/include/asm-i386/system.h.orig	2003-08-29 00:26:41.000000000 +0200
> +++ linux-2.4.22-jam1m/include/asm-i386/system.h	2003-08-29 00:26:55.000000000 +0200
> @@ -181,8 +181,8 @@
>  {
>  	__set_64bit(ptr,(unsigned int)(value), (unsigned int)((value)>>32ULL));
>  }
> -#define ll_low(x)	*(((unsigned int*)&(x))+0)
> -#define ll_high(x)	*(((unsigned int*)&(x))+1)
> +#define ll_low(x)	*(((unsigned int*)(void*)&(x))+0)
> +#define ll_high(x)	*(((unsigned int*)(void*)&(x))+1)
>  

How about something like this:

typedef unsigned long long	u64;
typedef unsigned long		u32;
typedef unsigned short		u16;
typedef unsigned char		u8;

union u64_split {
	u64	ll;
	u32 l[2];
	u16 w[4];
	u8  b[8];
};

void f()
{
	u64 a;

	int l = ((union u64_split)a).l[0];
	int h = ((union u64_split)a).l[1];
}

The clean way todo it without pointer arithmetic...

And with a modern gcc (anonymous structs):

union u64_split {
	u64	x;
	struct {
		u32 	l32,h32;
	};
	struct {
		u16 ll16,lh16,hl16,hh16;
	};
};

So you just write:

int l = ((union u64_split)a).l32;
u16 word = ((union u64_split)a).ll16;

I think it is better with structs, to take care of endianness if needed.

-- 
J.A. Magallon <jamagallon@able.es>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.22-jam1m (gcc 3.3.1 (Mandrake Linux 9.2 3.3.1-1mdk))