[llvm] [AMDGPU] selecting v_sat_pk instruction, version 2 (PR #123297)

[Matt Arsenault via llvm-commits]

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@@ -1982,8 +2004,10 @@ bool SITargetLowering::isTypeDesirableForOp(unsigned Op, EVT VT) const {
   if (VT == MVT::i1 && Op == ISD::SETCC)
     return false;
 
-  // v2i8 is illegal and only allowed in specific cases
-  if (VT == MVT::v2i8 && Op == ISD::TRUNCATE_SSAT_U)
+  // Special case for vNi8 handling where N is even
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arsenm wrote:

> Hi @arsenm , this function checks the destination type, while TLI.isOperationLegalOrCustom checks the source type.

This just sounds buggy. The interpretation of which type is the one that matters for the opcode needs to be globally consistent 

> The backend haven't add register class for vNi8. We maybe can add the relevant register class, 

Please no, this is a huge amount of work. 

https://github.com/llvm/llvm-project/pull/123297