Re: [Qemu-devel] [PATCH 2/2] translate-all: Use proper type

[Eric Blake]

On 10/18/2016 01:34 PM, Emilio G. Cota wrote:
> +++ b/translate-all.c
> @@ -405,23 +405,23 @@ static void page_init(void)
>  static PageDesc *page_find_alloc(tb_page_addr_t index, int alloc)
>  {
>      PageDesc *pd;
>      void **lp;
>      int i;
> 
>      /* Level 1.  Always allocated.  */
>      lp = l1_map + ((index >> V_L1_SHIFT) & (V_L1_SIZE - 1));
> 
>      /* Level 2..N-1.  */
>      for (i = V_L1_SHIFT / V_L2_BITS - 1; i > 0; i--) {
> -        void **p = atomic_rcu_read(lp);
> +        void *p = atomic_rcu_read(lp);
> 
>          if (p == NULL) {
>              if (!alloc) {
>                  return NULL;
>              }
>              p = g_new0(void *, V_L2_SIZE);
>              atomic_rcu_set(lp, p);
>          }
> 
>          lp = p + ((index >> (i * V_L2_BITS)) & (V_L2_SIZE - 1));

Pointer addition of 'void *' plus an offset is undefined (gcc, and
presumably clang, have an extension that treats it the same as computing
an offset to a 'char *'; but some compilers choke); this is because
sizeof(void) is unknown, so you don't know what stride to make for each
offset.  Or put another way, 'p + offset' is the same as
'&((*p)[offset])', but (*p)[offset] is ill-defined when p is the opaque
type void.

Pointer addition of 'void **' plus an offset is well-defined, because
sizeof(void*) is well-defined and therefore the stride (4 or 8) makes
sense.  Or in array notation, computing '&((*p)[offset]) means we are
skipping to the offset array entry where p is the start of the array of
void* pointers.

In that regards, changing the type of p from 'void **' (where you stride
by the size of a pointer when computing lp) to 'void *' (where you
stride by 1 under gcc when computing lp) is WRONG.

> I prefer void **p since that matches lp's and l1_map's type.

Not just prefer, but require.

> 
> It's true that since we're dealing with void * the compiler won't
> complain either way.

It's a shame that void* relaxes typing so much, but this is one case
where we HAVE to use the right type.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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