llvm dev - Feb 2017 - Problem ScheduleDAG on PowerPC, X86 works fine.

Would it not make sense to refactor the code so those don't use glue rather
than emitting them with glue and then getting rid of it. There are times
when we would like to emit these in separate blocks but can't (presumably
because of the glue).

On Tue, Feb 7, 2017 at 9:15 PM, James Y Knight via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> That's seems really odd that ADDC/ADDE uses glue there, instead of a
plain
> value.
>
> The x86 backend has code that converts the glue into a value, which is why
> it wasn't affected.... (LowerADDC_ADDE_SUBC_SUBE).
>
> On Tue, Feb 7, 2017 at 2:48 PM, Amaury SECHET via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Long story short: https://llvm.org/bugs/show_bug.cgi?id=31890
>>
>> The backend fails to schedule a given DAG, the reason being that there
is
>> an instruction and it glue that needs to be broken apart as they
can't be
>> scheduled consecutively. See attached file for a picture of the DAG.
>>
>> Not sure what's the best course of action is, and not sure why this
isn't
>> a problem for the X86 backend either. I'm looking for advice on the
best
>> course of actions. As I see it, the option are:
>>
>> 1/ add extra logic in the DAGCombiner to make sure this doesn't
happen. I
>> don't see a way this could be done cheaply and overall I don't
think this
>> is the best option/
>> 2/ Have the ScheduleDAG machinery detect this case and break up the
glue,
>> for instance via breaking up (adde X, Y, Carry) into (add (add X, Y)n
(adde
>> 0, 0, Carry)) or something alike when the situation present itself.
>> 3/ Do whatever the X86 backend does, which I'm not sure what it is.
>>
>> Advice ?
>>
>> Thanks in advance,
>>
>> Amaury SECHET
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170207/f85015b5/attachment.html>

Making this a value instead of a glue looks like a good longer term
solution, but it doesn't quite cut it as a short term one. It would require
to change a fair amount of code in the DAG stack, plus in each backend.

@James,
do you think doing something similar to what the X86 backend does in the
PowerPC one would cut it ?


2017-02-07 22:51 GMT+01:00 Nemanja Ivanovic <nemanja.i.ibm at gmail.com>:
> Would it not make sense to refactor the code so those don't use glue
> rather than emitting them with glue and then getting rid of it. There are
> times when we would like to emit these in separate blocks but can't
> (presumably because of the glue).
>
> On Tue, Feb 7, 2017 at 9:15 PM, James Y Knight via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> That's seems really odd that ADDC/ADDE uses glue there, instead of
a
>> plain value.
>>
>> The x86 backend has code that converts the glue into a value, which is
>> why it wasn't affected.... (LowerADDC_ADDE_SUBC_SUBE).
>>
>> On Tue, Feb 7, 2017 at 2:48 PM, Amaury SECHET via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Long story short: https://llvm.org/bugs/show_bug.cgi?id=31890
>>>
>>> The backend fails to schedule a given DAG, the reason being that
there
>>> is an instruction and it glue that needs to be broken apart as they
can't
>>> be scheduled consecutively. See attached file for a picture of the
DAG.
>>>
>>> Not sure what's the best course of action is, and not sure why
this
>>> isn't a problem for the X86 backend either. I'm looking for
advice on the
>>> best course of actions. As I see it, the option are:
>>>
>>> 1/ add extra logic in the DAGCombiner to make sure this doesn't
happen.
>>> I don't see a way this could be done cheaply and overall I
don't think this
>>> is the best option/
>>> 2/ Have the ScheduleDAG machinery detect this case and break up the
>>> glue, for instance via breaking up (adde X, Y, Carry) into (add
(add X, Y)n
>>> (adde 0, 0, Carry)) or something alike when the situation present
itself.
>>> 3/ Do whatever the X86 backend does, which I'm not sure what it
is.
>>>
>>> Advice ?
>>>
>>> Thanks in advance,
>>>
>>> Amaury SECHET
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170207/9195b97d/attachment.html>

I don't think that'd work, because it leaves all other backends broken.
AFAICT, your transform is simply not a legal transform, with the way the
ADDC/ADDE opcodes are currently defined, and to do it you really need to
fix the opcode definitions to not involve glue, first.

I also note that your transform doesn't actually trigger at all on this
particular test case on x86, because the dag ends up looking like (uaddo X,
(adde Y, 0, Carry)), which the transform doesn't match. That is not really
relevant to the issue here, because the x86 backend does work even if the
transform gets triggered (replacing the final store with "store i64 %add37,
i64* %r, align 8" will make this happen)


On Tue, Feb 7, 2017 at 5:44 PM, Amaury SECHET <deadalnix at gmail.com>
wrote:
> Making this a value instead of a glue looks like a good longer term
> solution, but it doesn't quite cut it as a short term one. It would
require
> to change a fair amount of code in the DAG stack, plus in each backend.
>
> @James,
> do you think doing something similar to what the X86 backend does in the
> PowerPC one would cut it ?
>
>
> 2017-02-07 22:51 GMT+01:00 Nemanja Ivanovic <nemanja.i.ibm at
gmail.com>:
>
>> Would it not make sense to refactor the code so those don't use
glue
>> rather than emitting them with glue and then getting rid of it. There
are
>> times when we would like to emit these in separate blocks but can't
>> (presumably because of the glue).
>>
>> On Tue, Feb 7, 2017 at 9:15 PM, James Y Knight via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> That's seems really odd that ADDC/ADDE uses glue there, instead
of a
>>> plain value.
>>>
>>> The x86 backend has code that converts the glue into a value, which
is
>>> why it wasn't affected.... (LowerADDC_ADDE_SUBC_SUBE).
>>>
>>> On Tue, Feb 7, 2017 at 2:48 PM, Amaury SECHET via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Long story short: https://llvm.org/bugs/show_bug.cgi?id=31890
>>>>
>>>> The backend fails to schedule a given DAG, the reason being
that there
>>>> is an instruction and it glue that needs to be broken apart as
they can't
>>>> be scheduled consecutively. See attached file for a picture of
the DAG.
>>>>
>>>> Not sure what's the best course of action is, and not sure
why this
>>>> isn't a problem for the X86 backend either. I'm looking
for advice on the
>>>> best course of actions. As I see it, the option are:
>>>>
>>>> 1/ add extra logic in the DAGCombiner to make sure this
doesn't happen.
>>>> I don't see a way this could be done cheaply and overall I
don't think this
>>>> is the best option/
>>>> 2/ Have the ScheduleDAG machinery detect this case and break up
the
>>>> glue, for instance via breaking up (adde X, Y, Carry) into (add
(add X, Y)n
>>>> (adde 0, 0, Carry)) or something alike when the situation
present itself.
>>>> 3/ Do whatever the X86 backend does, which I'm not sure
what it is.
>>>>
>>>> Advice ?
>>>>
>>>> Thanks in advance,
>>>>
>>>> Amaury SECHET
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170208/9afdeb97/attachment.html>