[llvm] [RISCV] and x (sub -1, y) -> andn x y (PR #129253)

[Craig Topper via llvm-commits]

topperc wrote:

> > The added tests already produce `andn`.
> 
> I get it now. At first, I did this incorrectly, and matched to `sub 0,y`. And that rightfully didn't work. After that I have fixed that to `-1`. And it already works.
> 
> Sorry for this.
> 
> After this I still don't get why the scenario shown in the issue doesn't work.
> 
> Do you have any idea for that?

I think you have to match at least 3 instructions.

https://github.com/llvm/llvm-project/pull/129253