[RISCV] Add BREV8 to SimplifyDemandedBitsForTargetNode.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -20717,6 +20717,34 @@ unsigned RISCVTargetLowering::ComputeNumSignBitsForTargetNode(
   return 1;
 }
 
+bool RISCVTargetLowering::SimplifyDemandedBitsForTargetNode(
+    SDValue Op, const APInt &OriginalDemandedBits,
+    const APInt &OriginalDemandedElts, KnownBits &Known, TargetLoweringOpt &TLO,
+    unsigned Depth) const {
+  unsigned BitWidth = OriginalDemandedBits.getBitWidth();
+
+  switch (Op.getOpcode()) {
+  case RISCVISD::BREV8: {
+    KnownBits Known2;
+    APInt DemandedBits =
+        APInt(BitWidth, computeGREVOrGORC(OriginalDemandedBits.getZExtValue(),
+                                          7, /*IsGORC=*/false));
+    if (SimplifyDemandedBits(Op.getOperand(0), DemandedBits,
+                             OriginalDemandedElts, Known2, TLO, Depth + 1))
+      return true;
+
+    Known.Zero =
+        computeGREVOrGORC(Known2.Zero.getZExtValue(), 7, /*IsGORC=*/false);
+    Known.One =
+        computeGREVOrGORC(Known2.One.getZExtValue(), 7, /*IsGORC=*/false);
+    return false;
+  }
+  }
+
+  return TargetLowering::SimplifyDemandedBitsForTargetNode(
+      Op, OriginalDemandedBits, OriginalDemandedElts, Known, TLO, Depth);
+}
+
 bool RISCVTargetLowering::canCreateUndefOrPoisonForTargetNode(
     SDValue Op, const APInt &DemandedElts, const SelectionDAG &DAG,
     bool PoisonOnly, bool ConsiderFlags, unsigned Depth) const {

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -153,6 +153,12 @@ class RISCVTargetLowering : public TargetLowering {
                                            const SelectionDAG &DAG,
                                            unsigned Depth) const override;
 
+  bool SimplifyDemandedBitsForTargetNode(SDValue Op, const APInt &DemandedBits,
+                                         const APInt &DemandedElts,
+                                         KnownBits &Known,
+                                         TargetLoweringOpt &TLO,
+                                         unsigned Depth) const override;
+
   bool canCreateUndefOrPoisonForTargetNode(SDValue Op,
                                            const APInt &DemandedElts,
                                            const SelectionDAG &DAG,

llvm/test/CodeGen/RISCV/bswap-bitreverse.ll

@@ -245,14 +245,14 @@ define i8 @test_bitreverse_i8(i8 %a) nounwind {
 ;
 ; RV32ZBKB-LABEL: test_bitreverse_i8:
 ; RV32ZBKB:       # %bb.0:
-; RV32ZBKB-NEXT:    rev8 a0, a0
+; RV32ZBKB-NEXT:    slli a0, a0, 24
 ; RV32ZBKB-NEXT:    brev8 a0, a0
 ; RV32ZBKB-NEXT:    srli a0, a0, 24
 ; RV32ZBKB-NEXT:    ret
 ;
 ; RV64ZBKB-LABEL: test_bitreverse_i8:
 ; RV64ZBKB:       # %bb.0:
-; RV64ZBKB-NEXT:    rev8 a0, a0
+; RV64ZBKB-NEXT:    slli a0, a0, 56
 ; RV64ZBKB-NEXT:    brev8 a0, a0
 ; RV64ZBKB-NEXT:    srli a0, a0, 56
 ; RV64ZBKB-NEXT:    ret