[RISCV] DAG Combine vcpop and vfirst with VL=0 to li imm

vcpop and vfirst are still useful when VL=0.
vcpop equivalents to li 0 and vfirst equivalents to li -1,
since no mask elements are active.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D120302

Author: ZCBing

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -1031,6 +1031,7 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
   setTargetDAGCombine(ISD::ROTL);
   setTargetDAGCombine(ISD::ROTR);
   setTargetDAGCombine(ISD::ANY_EXTEND);
+  setTargetDAGCombine(ISD::INTRINSIC_WO_CHAIN);
   if (Subtarget.hasStdExtZfh())
     setTargetDAGCombine(ISD::SIGN_EXTEND_INREG);
   if (Subtarget.hasStdExtF()) {
@@ -8474,6 +8475,32 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
 
     break;
   }
+  case ISD::INTRINSIC_WO_CHAIN: {
+    unsigned IntNo = N->getConstantOperandVal(0);
+    switch (IntNo) {
+      // By default we do not combine any intrinsic.
+    default:
+      return SDValue();
+    case Intrinsic::riscv_vcpop:
+    case Intrinsic::riscv_vcpop_mask:
+    case Intrinsic::riscv_vfirst:
+    case Intrinsic::riscv_vfirst_mask: {
+      SDValue VL = N->getOperand(2);
+      if (IntNo == Intrinsic::riscv_vcpop_mask ||
+          IntNo == Intrinsic::riscv_vfirst_mask)
+        VL = N->getOperand(3);
+      if (!isNullConstant(VL))
+        return SDValue();
+      // If VL is 0, vcpop -> li 0, vfirst -> li -1.
+      SDLoc DL(N);
+      EVT VT = N->getValueType(0);
+      if (IntNo == Intrinsic::riscv_vfirst ||
+          IntNo == Intrinsic::riscv_vfirst_mask)
+        return DAG.getConstant(-1, DL, VT);
+      return DAG.getConstant(0, DL, VT);
+    }
+    }
+  }
   }
 
   return SDValue();

llvm/test/CodeGen/RISCV/rvv/vcpop.ll

@@ -24,8 +24,7 @@ entry:
 define iXLen @intrinsic_vcpop_m_nxv1i1_zero(<vscale x 1 x i1> %0) nounwind {
 ; CHECK-LABEL: intrinsic_vcpop_m_nxv1i1_zero:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    vsetivli zero, 0, e8, mf8, ta, mu
-; CHECK-NEXT:    vcpop.m a0, v0
+; CHECK-NEXT:    li a0, 0
 ; CHECK-NEXT:    ret
 entry:
   %a = call iXLen @llvm.riscv.vcpop.iXLen.nxv1i1(
@@ -60,10 +59,7 @@ entry:
 define iXLen @intrinsic_vcpop_mask_m_nxv1i1_zero(<vscale x 1 x i1> %0, <vscale x 1 x i1> %1) nounwind {
 ; CHECK-LABEL: intrinsic_vcpop_mask_m_nxv1i1_zero:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    vmv1r.v v9, v0
-; CHECK-NEXT:    vsetivli zero, 0, e8, mf8, ta, mu
-; CHECK-NEXT:    vmv1r.v v0, v8
-; CHECK-NEXT:    vcpop.m a0, v9, v0.t
+; CHECK-NEXT:    li a0, 0
 ; CHECK-NEXT:    ret
 entry:
   %a = call iXLen @llvm.riscv.vcpop.mask.iXLen.nxv1i1(

llvm/test/CodeGen/RISCV/rvv/vfirst.ll

@@ -24,8 +24,7 @@ entry:
 define iXLen @intrinsic_vfirst_m_nxv1i1_zero(<vscale x 1 x i1> %0) nounwind {
 ; CHECK-LABEL: intrinsic_vfirst_m_nxv1i1_zero:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    vsetivli zero, 0, e8, mf8, ta, mu
-; CHECK-NEXT:    vfirst.m a0, v0
+; CHECK-NEXT:    li a0, -1
 ; CHECK-NEXT:    ret
 entry:
   %a = call iXLen @llvm.riscv.vfirst.iXLen.nxv1i1(
@@ -60,10 +59,7 @@ entry:
 define iXLen @intrinsic_vfirst_mask_m_nxv1i1_zero(<vscale x 1 x i1> %0, <vscale x 1 x i1> %1) nounwind {
 ; CHECK-LABEL: intrinsic_vfirst_mask_m_nxv1i1_zero:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    vmv1r.v v9, v0
-; CHECK-NEXT:    vsetivli zero, 0, e8, mf8, ta, mu
-; CHECK-NEXT:    vmv1r.v v0, v8
-; CHECK-NEXT:    vfirst.m a0, v9, v0.t
+; CHECK-NEXT:    li a0, -1
 ; CHECK-NEXT:    ret
 entry:
   %a = call iXLen @llvm.riscv.vfirst.mask.iXLen.nxv1i1(