[llvm-dev] Handling node through TargetLowering::LowerOperation vs TargetLowering::ReplaceNodeResults

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 28 16:12:59 PST 2020

Node results are always legalized first. So ReplaceNodeResults will be
called first. The custom code generated must start and end with the illegal
types. Instructions like concat_vectors, extract_subvector, zero_extend,
sign_extend, truncate, and build_pair will be used to bridge between the
illegal types and the legal type your custom code uses. All of the nodes
that are custom emitted will also go through type legalization. So those
connecting nodes will also get legalized.

If the node that's being custom legalized is something that has the result
type and the input type always the same like an add instruction. Then
legalizing the output should be enough to fully replace the node. If some
instruction that doesn't have a fixed relationship between input and output
type and they are both illegal then its up to the custom code that handles
the result as to whether or not it wants to fix the input too. Once the new
code has been emitted from ReplaceNodeResults then the original node is
dead and will be not be legalized further. Only the custom code that
replace it will be looked at. It's fine to emit code from
ReplaceNodeResults with illegal types as long as some of the types are
legal or you emit different nodes than what you started with that can
undergo their own legalization.

I hope that helped.


On Tue, Jan 28, 2020 at 12:40 PM Przemyslaw Ossowski <
przemyslaw.ossowski at googlemail.com> wrote:

> Thank you Craig for explanation.
> Could be the same algorithm used for custom legalizing given node in
> LowerOperation and ReplaceNodeResults in case results and inputs of the
> node are illegal?
> Or actually such situation is impossible and for given node either
> LowerOperation or ReplaceNodeResults can be only called?
> Przemek
> wt., 28 sty 2020, 18:48 u┼╝ytkownik Craig Topper <craig.topper at gmail.com>
> napisał:
>> ReplaceNodeResults is called by the type legalizer for custom legalizing
>> any of the results of a node that have an illegal type.
>> LowerOperation is called by the type legalizer to custom legalize node
>> inputs that have an illegal type. I believe technically it calls
>> LowerOperationWrapper, but that forwards to LowerOperation by default.
>> LowerOperation is also called for custom legalizing nodes with legal
>> types after type legalization by both the vector op legalizer and the DAG
>> legalizer. In those cases LowerOperation is called directly and does not
>> call LowerOperationWrapper.
>> On Tue, Jan 28, 2020 at 9:10 AM Przemyslaw Ossowski via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>> Hi,
>>> I see that for different targets in classes which inherits from
>>> TargetLowering there are implemented both methods:
>>> LowerOperation and ReplaceNodeResults
>>> What decides that for one given ISD we have to add handling in
>>> LowerOperation and for other in ReplaceNodeResults, when for both
>>> SetOperationAction is configured to be Custom?
>>> Is it related with number of results of given operation and
>>> LowerOperation is called when there is only one SDValue?
>>> Thanks,
>>> Przemek
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> --
>> ~Craig
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