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

[Jeff Law <law@redhat.com>]

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

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.

Jeff