[llvm] [X86] Use GFNI for vXi8 shifts/rotates (PR #89115)

[Simon Pilgrim via llvm-commits]

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@@ -28988,6 +28988,29 @@ SDValue X86TargetLowering::LowerWin64_INT128_TO_FP(SDValue Op,
   return IsStrict ? DAG.getMergeValues({Result, Chain}, dl) : Result;
 }
 
+// Generate a GFNI gf2p8affine bitmask for vXi8 bitreverse/shift/rotate.
+uint64_t getGFNICtrlImm(unsigned Opcode, unsigned Amt = 0) {
+  assert((Amt < 8) && "Shift/Rotation amount out of range");
+  switch (Opcode) {
+  case ISD::BITREVERSE:
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RKSimon wrote:

We use the llvm_unreachable instead to stop MSVC complaining :| 

https://github.com/llvm/llvm-project/pull/89115