Re: Add hot/cold attributes for labels

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

On Fri, Jul 27, 2012 at 11:15 AM, Richard Guenther
<richard.guenther@gmail.com> wrote:
>> 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.

I don't see why it is useful to be able to mark a basic block as hot
or cold. This is something that the compiler can figure out for itself
if you provide the branch hints (__builtin_expect is also a kind of
branch hint). Marking basic blocks as likely or unlikely seems just
redundant and confusing to me. A basic block being hot or cold is an
effect of its incoming edges being unlikely-taken, not an inherent
property of the basic block itself.

Ciao!
Steven