Re: [RFA] Handle target with no length attributes sanely in bb-reorder.c

[Jeff Law <law@redhat.com>]

On 12/01/2016 05:04 AM, Segher Boessenkool wrote:
> On Thu, Dec 01, 2016 at 10:19:42AM +0100, Richard Biener wrote:
>>>> Thinking about this again maybe targets w/o insn-length should simply
>>>> always use the 'simple' algorithm instead of the STV one?  At least that
>>>> might be what your change effectively does in some way?
>>>
>>> From reading the comments I don't think STC will collapse down into the
>>> simple algorithm if block copying is disabled.  But Segher would know for
>>> sure.
>>>
>>> WRT the choice of simple vs STC, I doubt it matters much for the processors
>>> in question.
>>
>> I guess STC doesn't make much sense if we can't say anything about BB sizes.
>
> STC tries to make as large as possible consecutive "traces", mainly to
> help with instruction cache utilization and hit rate etc.  It cannot do
> a very good job if it isn't allowed to copy blocks.
>
> "simple" tries to (dynamically) have as many fall-throughs as possible,
> i.e. as few jumps as possible.  It never copies code; if that means it
> has to jump every second insn, so be it.  It provably is within a factor
> three of optimal (optimal is NP-hard), under a really weak assumption
> within a factor two, and it does better than that in practice.
>
> STC without block copying makes longer traces which is not a good idea
> for most architectures, only for those that have a short jump that is
> much shorter than longer jumps (and thus, cannot cover many jump
> targets).
>
> I do not know how STC behaves when it does not know the insn lengths.
mn103 &  m68k are definitely sensitive to jump distances, the former 
more so than the latter.  Some of the others probably are as well.

I think we've probably discussed this more than is really necessary.  We 
just need to pick an alternative and go with it, I think either 
alternative is reasonable (avoid copying when STC has no length 
information or fall back to simple when there is no length information).



jeff