[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 2/2] translate-all: Use proper type
From: |
Pranith Kumar |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] translate-all: Use proper type |
Date: |
Tue, 18 Oct 2016 15:14:42 -0400 |
Eric Blake writes:
> 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.
>
Indeed. I missed that crucial detail. I would prefer explicitly casting to
'void **' for p, since that is not the type of what is being returned by
atomic_rcu_read().
The joys of void pointer arithmetic, TIL.
--
Pranith