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

[Richard Biener <richard.guenther@gmail.com>]

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?

Richard.

> Jeff