remove reverse_die_lists

[Geoff Keating]

On 24/04/2006, at 12:12 AM, Paolo Bonzini wrote:

>> +/* Evaluate 'expr' while 'c' is set to each child of DIE in  
>> order.  */
>> +#define FOR_EACH_CHILD(die, c, expr) do {	\
>> +  c = die->die_child;				\
>> +  if (c) do {					\
>> +    c = c->die_sib;				\
>> +    expr;					\
>> +  } while (c != die->die_child);		\
>> +} while (0)
>> +
>
> If you want to write this in a "iterator" style, it can be helpful  
> to know that GCC optimizes this completely:
>
>   #define NULL 0
>   #define TRUE 1
>
>   struct s {
>     int x;
>     struct s *next;
>   };
>
>   int s(struct s *last) {
>     struct s *p;
>     int n = 0;
>     for (p = NULL;
>          p != last && (p || (p = last)) && (p = p->next, 1); )
>       n += p->x;
>
>     return n;
>   }
>
> The NULL pointer checking optimization detects that p is  
> dereferenced in the loop, and deletes the "p || (p == last)" part  
> after the first iteration on the loop.  Dually, when entering the  
> loop jump threading simplifies the code to "p = last->next".
>
> The result is this (massaged from the .optimized dump):
>
>   if (last == 0)
>     return 0;
>
>   p = last;
>   n = 0;
>   do {
>     p = p->next;
>     n = p->x + n;
>   } while (p != last);
>   return n;
>
> In your case that would be:
>
>   #define FOR_EACH_CHILD(die, c)		\
>     for (c = NULL;				\
>          c != die->die_child			\
>          && (c || (c = die->die_child))		\
>          && (c = c->die_sib, 1); )

That's clever!  However, I did think of trying to do something like  
this (although I wasn't thinking of anything this nice), but I  
noticed that every single instance where I wanted this the body of  
the loop was going to be a single call...
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