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

[Jeff Law <law@redhat.com>]

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.

JEff