On Fri, Nov 8, 2013 at 9:21 PM, Jeff Law <law@xxxxxxxxxx> wrote:
> On 11/08/13 02:28, Konstantin Vladimirov wrote:
>>
>> typedef struct
>> {
>> unsigned prev;
>> unsigned next;
>> } foo_t;
>>
>> void
>> foo( unsigned x, unsigned y)
>> {
>> foo_t *ptr = (foo_t *)((void *)x);
>>
>> if (y != 0)
>> {
>> ptr->prev = y;
>> ptr->next = x;
>> }
>> else
>> {
>> ptr->prev = 0; /* or explicitly ptr->prev = y; no difference */
>> ptr->next = 0;
>> }
>> }
>
> Umm, you can't hoist ptr->prev before the conditional because that would
> change the meaning of this code.
We test y != 0 and thus know that y == 0 in the else block. Thus we can
do
ptr->prev = y;
if (y != 0)
ptr->next = x;
else
ptr->next = 0;
note that we already transform
if (y != 0)
...
else
ptr->prev = y;
to ptr->prev = 0 AFAIK.
Richard.
>
> I think you wanted the conditional to test y == 0 which exposes the code
> hoisting opportunity for the ptr->prev assignment. Once you fix the
> testcase the code in jump2 will hoist the assignment resulting in:
>
>
>
>
>
> .cfi_startproc
>
> testl %esi, %esi
> movl %edi, %eax
> movl $0, (%edi)
>
> je .L5
> movl $0, 4(%rax)
> ret
> .p2align 4,,10
> .p2align 3
>
> .L5:
> movl %edi, 4(%rax)
> ret
> .cfi_endproc
>
>
> Jeff
