[llvm] AMDGPU: Use OtherPredicates for v_dot2_bf16_bf16(f16_f16) pseudo (PR #84354)

[Changpeng Fang via llvm-commits]

changpeng wrote:

> > > > > > > LGTM, but this is really unfortunate. Can we try to address dpp predicates?
> > > > > > 
> > > > > > 
> > > > > > let SubtargetPredicate = HasDPP8;
> > > > > > I am thinking whether we can put hasDPP8 into OtherPredicates (using concat). But we need to be very careful of the ordering this assignment with copy from pseudo to real of OtherPredicates.
> > > > > 
> > > > > 
> > > > > let SubtargetPredicate = HasDPP8; // do we really need? let AssemblerPredicate = HasDPP8;
> > > > > Actually why do we need to set both SubtargetPredicate and AssemblerPredicate? One of them enough? "AssemblerPredicate = HasDPP8" may also have "overriden" issue on the path.
> > > > 
> > > > 
> > > > Imagine you have the same instruction in different targets, so you are using 'SubtargetPredicate = HasFeature' on the pseudo. But the encoding is different, so you this HasFeature is not sufficient, you need to use AssemblerPredicate to distinguish between targets for real instructions.
> > > 
> > > 
> > > Is it redundant to have HasDPP8 in both SubtargetPredicate and AssemblerPredicate for a real? I think it is SubtargetPredicate && AssemblerPredicate
> > 
> > 
> > Yes, both should not be needed.
> 
> Then we can remove "let SubtargetPredicate = ...", and copy SubtargetPredicate from pseudo to real for dpp16 and dpp8. I am going to look into this. Thanks

https://github.com/llvm/llvm-project/pull/84517

https://github.com/llvm/llvm-project/pull/84354