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

[Richard Biener]

On Wed, Nov 30, 2016 at 6:59 PM, Jeff Law <law@redhat.com> wrote:
> On 11/30/2016 01:38 AM, Richard Biener wrote:
>>
>> On Tue, Nov 29, 2016 at 5:07 PM, Jeff Law <law@redhat.com> wrote:
>>>
>>> On 11/29/2016 03:23 AM, Richard Biener wrote:
>>>>
>>>>
>>>> On Mon, Nov 28, 2016 at 10:23 PM, Jeff Law <law@redhat.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> I was digging into  issues around the patches for 78120 when I stumbled
>>>>> upon
>>>>> undesirable bb copying in bb-reorder.c on the m68k.
>>>>>
>>>>> The core issue is that the m68k does not define a length attribute and
>>>>> therefore generic code assumes that the length of all insns is 0 bytes.
>>>>
>>>>
>>>>
>>>> What other targets behave like this?
>>>
>>>
>>> ft32, nvptx, mmix, mn10300, m68k, c6x, rl78, vax, ia64, m32c
>>
>>
>> Ok.
>>
>>> cris has a hack to define a length, even though no attempt is made to
>>> make
>>> it accurate.  The hack specifically calls out that it's to make
>>> bb-reorder
>>> happy.
>>>
>>>>
>>>>> That in turn makes bb-reorder think it is infinitely cheap to copy
>>>>> basic
>>>>> blocks.  In the two codebases I looked at (GCC's runtime libraries and
>>>>> newlib) this leads to a 10% and 15% undesirable increase in code size.
>>>>>
>>>>> I've taken a slight variant of this patch and bootstrapped/regression
>>>>> tested
>>>>> it on x86_64-linux-gnu to verify sanity as well as built the m68k
>>>>> target
>>>>> libraries noted above.
>>>>>
>>>>> OK for the trunk?
>>>>
>>>>
>>>>
>>>> I wonder if it isn't better to default to a length of 1 instead of zero
>>>> when
>>>> there is no length attribute.  There are more users of the length
>>>> attribute
>>>> in bb-reorder.c (and elsewhere as well I suppose).
>>>
>>>
>>> I pondered that as well, but felt it was riskier given we've had a
>>> default
>>> length of 0 for ports that don't define lengths since the early 90s.
>>> It's
>>> certainly easy enough to change that default if you'd prefer.  I don't
>>> have
>>> a strong preference either way.
>>
>>
>> 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.

Richard.

>
> JEff