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

[Ryan Buchner via llvm-commits]

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@@ -9088,6 +9088,31 @@ 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();
+      // Efficient only if the constant and its negation fit into `ADDI`
+      if (RawConstVal != INT64_MIN && std::abs(RawConstVal) < 0x800) {
+        SDValue SubOp = DAG.getNode(ISD::SUB, DL, VT, RegV, ConstVal);
+        SDValue CMOV =
+            DAG.getNode(IsCZERO_NEZ ? RISCVISD::CZERO_NEZ : RISCVISD::CZERO_EQZ,
+                        DL, VT, SubOp, CondV);
+        return DAG.getNode(ISD::ADD, DL, VT, CMOV, ConstVal);
+      }
+      // Can use XORI if Const == -0x800
+      else if (RawConstVal == -0x800) {
+        SDValue XorOp = DAG.getNode(ISD::XOR, DL, VT, RegV, ConstVal);
+        SDValue CMOV =
+            DAG.getNode(IsCZERO_NEZ ? RISCVISD::CZERO_NEZ : RISCVISD::CZERO_EQZ,
+                        DL, VT, XorOp, CondV);
+        return DAG.getNode(ISD::XOR, DL, VT, CMOV, ConstVal);
+      }
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bababuck wrote:

@topperc Is this too verbose to have this seperate clause? I could pull out the meat of these two clauses into a function that I pass the opcodes too, but that felt like it would be hard to read.

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