On May 19, 2011, at 6:41 AM, Paul Koning wrote:
>
> On May 19, 2011, at 3:32 AM, Richard Sandiford wrote:
>
>> Paul Koning <[email protected]> writes:
>>> ...
>>
>>> 2. In the delay slot fill machinery (reorg.c), I don't see how a
>>> target can supply hooks to adjust the picking of one branch over
>>> another. In other words, if the architecture has branch-likely that
>>> has annul, but that instruction should not be used in a particular
>>> spot because the branch is in fact NOT likely, how would one do that?
>>
>> This isn't something that should be handled by target hooks. The way
>> that we record branch probabilities is target-independent, and reorg.c
>> already knows whether the target has both annulled and non-annulled
>> branches.
>
> I saw that. But it looks to me like that logic picks one or the other
> depending on how that fills branch delay slots, and it does not take into
> account that the annulling branch on MIPS is supposed to be used (if at all)
> only when the branch is likely to be taken.
That wasn't very clear, so let me restate it. The current logic appears to
choose between non-annulling and annulling branches without taking into account
branch probability. That may be fine on some platforms, but on MIPS this is
not supposed to be correct -- annulling branches (except in -Os) should be used
only when the branch is likely, and I don't see that.
Since the question of whether an annulling branch is appropriate depends on the
architecture -- the MIPS rule may not apply to other processors -- I was
thinking a target hook should be defined for this.
paul
