[llvm-dev] SelectionDAGISel::Select's API considered harmful

Justin Bogner via llvm-dev llvm-dev at lists.llvm.org
Mon May 23 10:01:41 PDT 2016

Hans Wennborg <hans at chromium.org> writes:
> Can you put something in the release notes when this happens?

I already updated the release notes in r268693, when I added the void
Select option in the first place :)

> 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.

This is done in r270454.

>> 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
>>> 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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