llvm dev - May 2016 - SelectionDAGISel::Select's API considered harmful

TLDR: Heads up for out of tree backends - you're going to need to update
your *DAGToDAGISel::Select method to unconditionally replace nodes
directly instead of returning the desired replacement.

So I'm working on fixing the undefined behaviour I described in
llvm.org/PR26808. As part of this, we need to stop looking into deleted
SDNodes to check if they were, in fact, deleted. A big place that we do
this is in SelectionDAGISel::DoInstructionSelection, where we can find
this helpful comment that came with the commit that added the check for
DELETED_NODE:

SelectionDAGISel.cpp says:
> // FIXME: This is pretty gross.  'Select' should be changed to not
return
> // anything at all and this code should be nuked with a tactical strike.
I'm just gonna go ahead and take this advice.

I'll phase this in a couple of steps:

1. Rename Select to SelectImpl in all targets, and implement "virtual
   void Select(SDNode *)" in SelectionDAGISel. I'll move the current
   sketchy behaviour into this version of Select.

2. Update backends one at a time to implement "void Select(SDNode *)"
   instead of SelectImpl.

3. Make SelectionDAGISel::Select pure virtual and remove SelectImpl
   entirely.

If you have an out of tree backend and you merge from trunk, I recommend
updating in between steps 1 and 3 to avoid breakage after 3 happens.

Update: All in tree backends now implement `void Select`. I'll be
removing the SelectImpl path on Monday.

Justin Bogner <mail at justinbogner.com> writes:
> TLDR: Heads up for out of tree backends - you're going to need to
update
> your *DAGToDAGISel::Select method to unconditionally replace nodes
> directly instead of returning the desired replacement.
>
> So I'm working on fixing the undefined behaviour I described in
> llvm.org/PR26808. As part of this, we need to stop looking into deleted
> SDNodes to check if they were, in fact, deleted. A big place that we do
> this is in SelectionDAGISel::DoInstructionSelection, where we can find
> this helpful comment that came with the commit that added the check for
> DELETED_NODE:
>
> SelectionDAGISel.cpp says:
>> // FIXME: This is pretty gross.  'Select' should be changed to
not return
>> // anything at all and this code should be nuked with a tactical
strike.
>
> I'm just gonna go ahead and take this advice.
>
> I'll phase this in a couple of steps:
>
> 1. Rename Select to SelectImpl in all targets, and implement "virtual
>    void Select(SDNode *)" in SelectionDAGISel. I'll move the
current
>    sketchy behaviour into this version of Select.
>
> 2. Update backends one at a time to implement "void Select(SDNode
*)"
>    instead of SelectImpl.
>
> 3. Make SelectionDAGISel::Select pure virtual and remove SelectImpl
>    entirely.
>
> If you have an out of tree backend and you merge from trunk, I recommend
> updating in between steps 1 and 3 to avoid breakage after 3 happens.

Can you put something in the release notes when this happens?

Thanks,
Hans

On Sat, May 21, 2016 at 10:57 AM, Justin Bogner via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Update: All in tree backends now implement `void Select`. I'll be
> removing the SelectImpl path on Monday.
>
> Justin Bogner <mail at justinbogner.com> writes:
>> TLDR: Heads up for out of tree backends - you're going to need to
update
>> your *DAGToDAGISel::Select method to unconditionally replace nodes
>> directly instead of returning the desired replacement.
>>
>> So I'm working on fixing the undefined behaviour I described in
>> llvm.org/PR26808. As part of this, we need to stop looking into deleted
>> SDNodes to check if they were, in fact, deleted. A big place that we do
>> this is in SelectionDAGISel::DoInstructionSelection, where we can find
>> this helpful comment that came with the commit that added the check for
>> DELETED_NODE:
>>
>> SelectionDAGISel.cpp says:
>>> // FIXME: This is pretty gross.  'Select' should be changed
to not return
>>> // anything at all and this code should be nuked with a tactical
strike.
>>
>> I'm just gonna go ahead and take this advice.
>>
>> I'll phase this in a couple of steps:
>>
>> 1. Rename Select to SelectImpl in all targets, and implement
"virtual
>>    void Select(SDNode *)" in SelectionDAGISel. I'll move the
current
>>    sketchy behaviour into this version of Select.
>>
>> 2. Update backends one at a time to implement "void Select(SDNode
*)"
>>    instead of SelectImpl.
>>
>> 3. Make SelectionDAGISel::Select pure virtual and remove SelectImpl
>>    entirely.
>>
>> If you have an out of tree backend and you merge from trunk, I
recommend
>> updating in between steps 1 and 3 to avoid breakage after 3 happens.
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