On Fri, Jul 27, 2012 at 11:08 AM, Steven Bosscher <stevenb....@gmail.com> wrote:
> On Fri, Jul 27, 2012 at 10:57 AM, Richard Guenther
> <richard.guent...@gmail.com> wrote:
>> On Thu, Jul 26, 2012 at 11:58 PM, Richard Henderson <r...@redhat.com> wrote:
>>> On 07/26/2012 02:41 PM, Richard Henderson wrote:
>>>> This is a patch...
>>>
>>> ... that I should have attached. Bah.
>>
>> Do we need to mark the labels so we preserve them? Consider
>>
>> goto foo;
>>
>> foo:
>> bar __attribute__((cold)):
>> ...
>>
>> so bar will be unused? What about BB merging if we end up with
>>
>> <BB 3>:
>> ..
>> fallthru
>> bar __attribute__((cold)):
>> ...
>>
>> should BB 3 inherit the coldness? I think we no longer disable
>> BB merging if the destination has user labels.
>
> Right. I don't like the use of this attribute on labels at all, for
> the reasons you list here. I think it would be much cleaner to add a
> branch hint on the label in the asm goto, to contain this extension
> and to also to make it clear that it's not the label that is cold but
> the jump that is unlikely to be executed (i.e. cause and effect: the
> jump is unlikely and therefore the basic block is cold).
As in the case where you have both an unlikely and likely jump to a
basic-block. But what I understand is that rth adds a way to mark
a basic-block as hot or cold, not a way to mark an edge as hot or cold
(that would be what the asm goto annotation would do). Both cases
are of course useful.
Richard.
> Something like this:
>
> asm-goto-operands:
> asm-got-branch-hint identifier
> asm-goto-operands , asm-got-branch-hint identifier
>
> asm-got-branch-hint: <empty | + | ->
> where + means branch-likely and - means branch-unlikely
>
> ->
>
> asm goto (""::::+l1);
> asm goto (""::::+l1);
> asm goto (""::::-l1);
> Ciao!
> Steven
