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

Hans Wennborg via llvm-dev llvm-dev at lists.llvm.org
Mon May 23 08:59:10 PDT 2016

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


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
>> 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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