[llvm] [AMDGPU][SDAG] Legalise v2i32 or/xor/and instructions to make use of 64-bit wide instructions (PR #140694)

[Janek van Oirschot via llvm-commits]

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@@ -4048,6 +4048,59 @@ SDValue AMDGPUTargetLowering::splitBinaryBitConstantOpImpl(
   return DAG.getNode(ISD::BITCAST, SL, MVT::i64, Vec);
 }
 
+// Part of the shift combines is to optimise for the case where its possible
+// to reduce e.g shl64 to shl32 if shift range is [63-32]. This
+// transforms: DST = shl i64 X, Y to [0, srl i32 X, (Y & 31) ]. The
+// '&' is then elided by ISel. The vector code for this was being
+// completely scalarised by the vector legalizer, but when v2i32 is
+// legal the vector legaliser only partially scalarises the
+// vector operations and the and is not elided. This function
+// scalarises the AND for this optimisation case.
+static SDValue getShiftForReduction(unsigned ShiftOpc, SDValue LHS, SDValue RHS,
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JanekvO wrote:

Wouldn't it be easier if you just passed the `SDNode *` directly? Now it looks like the `SDNode *` is being picked apart for these function arguments and the `SDLoc` below seems to use `RHS` for its constructor which I'm not sure is accurate.

https://github.com/llvm/llvm-project/pull/140694