[llvm] [RISCV] Lower SELECT's with one constant more efficiently using Zicond (PR #143581)

[Mikhail Gudim via llvm-commits]

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@@ -9088,6 +9088,32 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
       return DAG.getNode(ISD::ADD, DL, VT, CMOV, RHSVal);
     }
 
+    // (select c, c1, t) -> (add (czero_nez t - c1, c), c1)
+    // (select c, t, c1) -> (add (czero_eqz t - c1, c), c1)
+    if (isa<ConstantSDNode>(TrueV) != isa<ConstantSDNode>(FalseV)) {
+      bool IsCZERO_NEZ = isa<ConstantSDNode>(TrueV);
+      SDValue ConstVal = IsCZERO_NEZ ? TrueV : FalseV;
+      SDValue RegV = IsCZERO_NEZ ? FalseV : TrueV;
+      int64_t RawConstVal = cast<ConstantSDNode>(ConstVal)->getSExtValue();
+      // Fall back to XORI if Const == -0x800
+      if (RawConstVal == -0x800) {
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mgudim wrote:

why do we do this only for -0x800? 

is this not always true?
select cond, c1, t -> xor(c1, czero_nez(xor(t, c1), cond)) 

https://github.com/llvm/llvm-project/pull/143581