[llvm] [AMDGPU] select v_sat_pk from two i16 or v2i16 (PR #121124)

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Shoreshen wrote:

> > Hi @arsenm , maybe we can use TRUNCATE_SSAT_U node for the 2 of i16 case, but the v2i16 case maybe not, since the result of the truncation is v2i8, which causes compilation fail for the current backend I think.
> 
> The illegal type doesn't mean you have to throw away the whole thing and implement your own pattern matching. You can still custom lower the illegal type, you'll just need to process it into a wrapper node that does use a legal type plus a cast.

Hi @arsenm , maybe it is little bit hard for me to understand. 

At the pattern matching stage of DAG (which is `DoInstructionSelection`), we are not going to have any type of v2i8. These are all optimized out during the combination stage. 

So I think unless we either modify the pre-DAG optimization passes, otherwise only changing the TD file will not make use of v2i8.

https://github.com/llvm/llvm-project/pull/121124