[PATCH] D96506: [AVR] Optimize 16-bit shifts

[Ayke via Phabricator via llvm-commits]

aykevl added a comment.

I should have explained this a while ago, sorry for neglecting this.

In D90092#2368009 <https://reviews.llvm.org/D90092#2368009>, @benshi001 wrote:

> In D90092#2366711 <https://reviews.llvm.org/D90092#2366711>, @aykevl wrote:
>
>> See my comment D89047#2366709 <https://reviews.llvm.org/D89047#2366709>.
>> I think this optimization would be implemented better in SelectionDAG instead of using pseudo-instructions. That would also make it possible to optimize things like `(short)x << 12`.
>
> Could you please explain more about `be implemented better in SelectionDAG instead of using pseudo-instructions` ? I can not figure out a way using SelectionDAG. Because I think SelectionDAG requires data flow dependency and looks like a tree structure. But The mov/clr pair seems can not be put into  a tree structure.

Sorry I meant in the MachineIR phase. I think the ideal way to expand bit shift operations is as follows:

1. Lower lsl/lshr/ashr operations in SelectionDAG to pseudo instructions that have a constant immediate operand. You will only need six pseudo instructions this way: for lsl/lshr/ashr and for both 8 bit and 16 bit versions. These pseudo instructions should have the `usesCustomInserter` flag set.
2. Expand these instructions in `AVRTargetLowering::EmitInstrWithCustomInserter`. There are already a few existing expansions in there.

The big advantage is that you can do shifts before register allocation and I think it is also easier to structure the code. Also, this means you don't need as many instructions to expand in the pseudo instruction expansion pass.

Recently I have been thinking a lot about constant shifts on AVR and I've written a blog post here: https://aykevl.nl/2021/02/avr-bitshift
I think you will find it useful.

To be clear: I really appreciate your work on the AVR backend. There are many things that need to be improved, such as these bit shift operations. But I think there is a different and possibly better way to implement these expansions.


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