[llvm-dev] Problem ScheduleDAG on PowerPC, X86 works fine.

[Amaury SECHET via llvm-dev]

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
>>>
>>>
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>>>
>>
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