[RISCV] Account for ADDI immediate range in select of two constants w/ zicond (#155471)

When choosing to materialize a select of two constants using zicond, we
have a choice of which direction to compute the delta. The prior cost
was looking only at the cost of the values without accounting for the
fact it's actually the delta which is the highest cost and that
sometimes the addend can fold into an addi.

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -9287,11 +9287,18 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
         }
       }
 
-      const int TrueValCost = RISCVMatInt::getIntMatCost(
-          TrueVal, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
-      const int FalseValCost = RISCVMatInt::getIntMatCost(
-          FalseVal, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
-      bool IsCZERO_NEZ = TrueValCost <= FalseValCost;
+      auto getCost = [&](const APInt &Delta, const APInt &Addend) {
+        const int DeltaCost = RISCVMatInt::getIntMatCost(
+            Delta, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
+        // Does the addend fold into an ADDI
+        if (Addend.isSignedIntN(12))
+          return DeltaCost;
+        const int AddendCost = RISCVMatInt::getIntMatCost(
+            Addend, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
+        return AddendCost + DeltaCost;
+      };
+      bool IsCZERO_NEZ = getCost(FalseVal - TrueVal, TrueVal) <=
+                         getCost(TrueVal - FalseVal, FalseVal);
       SDValue LHSVal = DAG.getConstant(
           IsCZERO_NEZ ? FalseVal - TrueVal : TrueVal - FalseVal, DL, VT);
       SDValue RHSVal =

llvm/test/CodeGen/RISCV/select-const.ll

@@ -506,11 +506,10 @@ define i32 @select_nonnegative_lui_addi(i32 signext %x) {
 ; RV32ZICOND-LABEL: select_nonnegative_lui_addi:
 ; RV32ZICOND:       # %bb.0:
 ; RV32ZICOND-NEXT:    srli a0, a0, 31
-; RV32ZICOND-NEXT:    lui a1, 1048572
-; RV32ZICOND-NEXT:    addi a1, a1, 25
-; RV32ZICOND-NEXT:    czero.eqz a0, a1, a0
 ; RV32ZICOND-NEXT:    lui a1, 4
-; RV32ZICOND-NEXT:    add a0, a0, a1
+; RV32ZICOND-NEXT:    addi a1, a1, -25
+; RV32ZICOND-NEXT:    czero.nez a0, a1, a0
+; RV32ZICOND-NEXT:    addi a0, a0, 25
 ; RV32ZICOND-NEXT:    ret
 ;
 ; RV64I-LABEL: select_nonnegative_lui_addi:
@@ -536,11 +535,10 @@ define i32 @select_nonnegative_lui_addi(i32 signext %x) {
 ; RV64ZICOND-LABEL: select_nonnegative_lui_addi:
 ; RV64ZICOND:       # %bb.0:
 ; RV64ZICOND-NEXT:    srli a0, a0, 63
-; RV64ZICOND-NEXT:    lui a1, 1048572
-; RV64ZICOND-NEXT:    addi a1, a1, 25
-; RV64ZICOND-NEXT:    czero.eqz a0, a1, a0
 ; RV64ZICOND-NEXT:    lui a1, 4
-; RV64ZICOND-NEXT:    add a0, a0, a1
+; RV64ZICOND-NEXT:    addi a1, a1, -25
+; RV64ZICOND-NEXT:    czero.nez a0, a1, a0
+; RV64ZICOND-NEXT:    addi a0, a0, 25
 ; RV64ZICOND-NEXT:    ret
   %cmp = icmp sgt i32 %x, -1
   %cond = select i1 %cmp, i32 16384, i32 25