diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 16591c4..4bbdd86 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -80,10 +80,10 @@
     setOperationAction(ISD::SIGN_EXTEND_INREG, VT, Expand);
 
   if (Subtarget.is64Bit()) {
-    setTargetDAGCombine(ISD::SHL);
-    setTargetDAGCombine(ISD::SRL);
-    setTargetDAGCombine(ISD::SRA);
     setTargetDAGCombine(ISD::ANY_EXTEND);
+    setOperationAction(ISD::SHL, MVT::i32, Custom);
+    setOperationAction(ISD::SRA, MVT::i32, Custom);
+    setOperationAction(ISD::SRL, MVT::i32, Custom);
   }
 
   if (!Subtarget.hasStdExtM()) {
@@ -512,15 +512,52 @@
   return DAG.getCopyFromReg(DAG.getEntryNode(), DL, Reg, XLenVT);
 }
 
-// Return true if the given node is a shift with a non-constant shift amount.
-static bool isVariableShift(SDValue Val) {
-  switch (Val.getOpcode()) {
+// Returns the opcode of the target-specific SDNode that implements the 32-bit
+// form of the given Opcode.
+static RISCVISD::NodeType getRISCVWOpcode(unsigned Opcode) {
+  switch (Opcode) {
   default:
-    return false;
+    llvm_unreachable("Unexpected opcode");
+  case ISD::SHL:
+    return RISCVISD::SLLW;
+  case ISD::SRA:
+    return RISCVISD::SRAW;
+  case ISD::SRL:
+    return RISCVISD::SRLW;
+  }
+}
+
+// Converts the given 32-bit operation to a target-specific SelectionDAG node.
+// Because i32 isn't a legal type for RV64, these operations would otherwise
+// be promoted to i64, making it difficult to select the SLLW/DIVUW/.../*W
+// later one because the fact the operation was originally of type i32 is
+// lost.
+static SDValue customLegalizeToWOp(SDNode *N, SelectionDAG &DAG) {
+  SDLoc DL(N);
+  RISCVISD::NodeType WOpcode = getRISCVWOpcode(N->getOpcode());
+  SDValue NewOp0 = DAG.getNode(ISD::ANY_EXTEND, DL, MVT::i64, N->getOperand(0));
+  SDValue NewOp1 = DAG.getNode(ISD::ANY_EXTEND, DL, MVT::i64, N->getOperand(1));
+  SDValue NewRes = DAG.getNode(WOpcode, DL, MVT::i64, NewOp0, NewOp1);
+  // ReplaceNodeResults requires we maintain the same type for the return value.
+  return DAG.getNode(ISD::TRUNCATE, DL, MVT::i32, NewRes);
+}
+
+void RISCVTargetLowering::ReplaceNodeResults(SDNode *N,
+                                             SmallVectorImpl<SDValue> &Results,
+                                             SelectionDAG &DAG) const {
+  SDLoc DL(N);
+  switch (N->getOpcode()) {
+  default:
+    llvm_unreachable("Don't know how to custom type legalize this operation!");
   case ISD::SHL:
   case ISD::SRA:
   case ISD::SRL:
-    return Val.getOperand(1).getOpcode() != ISD::Constant;
+    assert(N->getValueType(0) == MVT::i32 && Subtarget.is64Bit() &&
+           "Unexpected custom legalisation");
+    if (N->getOperand(1).getOpcode() == ISD::Constant)
+      return;
+    Results.push_back(customLegalizeToWOp(N, DAG));
+    break;
   }
 }
 
@@ -545,34 +582,14 @@
   switch (N->getOpcode()) {
   default:
     break;
-  case ISD::SHL:
-  case ISD::SRL:
-  case ISD::SRA: {
-    assert(Subtarget.getXLen() == 64 && "Combine should be 64-bit only");
-    if (!DCI.isBeforeLegalize())
-      break;
-    SDValue RHS = N->getOperand(1);
-    if (N->getValueType(0) != MVT::i32 || RHS->getOpcode() == ISD::Constant ||
-        (RHS->getOpcode() == ISD::AssertZext &&
-         cast<VTSDNode>(RHS->getOperand(1))->getVT().getSizeInBits() <= 5))
-      break;
-    SDValue LHS = N->getOperand(0);
-    SDLoc DL(N);
-    SDValue NewRHS =
-        DAG.getNode(ISD::AssertZext, DL, RHS.getValueType(), RHS,
-                    DAG.getValueType(EVT::getIntegerVT(*DAG.getContext(), 5)));
-    return DCI.CombineTo(
-        N, DAG.getNode(N->getOpcode(), DL, LHS.getValueType(), LHS, NewRHS));
-  }
   case ISD::ANY_EXTEND: {
-    // If any-extending an i32 variable-length shift or sdiv/udiv/urem to i64,
-    // then instead sign-extend in order to increase the chance of being able
-    // to select the sllw/srlw/sraw/divw/divuw/remuw instructions.
+    // If any-extending an i32 sdiv/udiv/urem to i64, then instead sign-extend
+    // in order to increase the chance of being able to select the
+    // divw/divuw/remuw instructions.
     SDValue Src = N->getOperand(0);
     if (N->getValueType(0) != MVT::i64 || Src.getValueType() != MVT::i32)
       break;
-    if (!isVariableShift(Src) &&
-        !(Subtarget.hasStdExtM() && isVariableSDivUDivURem(Src)))
+    if (!(Subtarget.hasStdExtM() && isVariableSDivUDivURem(Src)))
       break;
     SDLoc DL(N);
     // Don't add the new node to the DAGCombiner worklist, in order to avoid
@@ -589,11 +606,42 @@
       break;
     return DCI.CombineTo(N, Op0.getOperand(0), Op0.getOperand(1));
   }
+  case RISCVISD::SLLW:
+  case RISCVISD::SRAW:
+  case RISCVISD::SRLW: {
+    // Only the lower 32 bits of LHS and lower 5 bits of RHS are read.
+    SDValue LHS = N->getOperand(0);
+    SDValue RHS = N->getOperand(1);
+    APInt LHSMask = APInt::getLowBitsSet(LHS.getValueSizeInBits(), 32);
+    APInt RHSMask = APInt::getLowBitsSet(RHS.getValueSizeInBits(), 5);
+    if ((SimplifyDemandedBits(N->getOperand(0), LHSMask, DCI)) ||
+        (SimplifyDemandedBits(N->getOperand(1), RHSMask, DCI)))
+      return SDValue();
+    break;
+  }
   }
 
   return SDValue();
 }
 
+unsigned RISCVTargetLowering::ComputeNumSignBitsForTargetNode(
+    SDValue Op, const APInt &DemandedElts, const SelectionDAG &DAG,
+    unsigned Depth) const {
+  switch (Op.getOpcode()) {
+  default:
+    break;
+  case RISCVISD::SLLW:
+  case RISCVISD::SRAW:
+  case RISCVISD::SRLW:
+    // TODO: As the result is sign-extended, this is conservatively correct. A
+    // more precise answer could be calculated for SRAW depending on known
+    // bits in the shift amount.
+    return 33;
+  }
+
+  return 1;
+}
+
 static MachineBasicBlock *emitSplitF64Pseudo(MachineInstr &MI,
                                              MachineBasicBlock *BB) {
   assert(MI.getOpcode() == RISCV::SplitF64Pseudo && "Unexpected instruction");
@@ -1682,6 +1730,12 @@
     return "RISCVISD::SplitF64";
   case RISCVISD::TAIL:
     return "RISCVISD::TAIL";
+  case RISCVISD::SLLW:
+    return "RISCVISD::SLLW";
+  case RISCVISD::SRAW:
+    return "RISCVISD::SRAW";
+  case RISCVISD::SRLW:
+    return "RISCVISD::SRLW";
   }
   return nullptr;
 }
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.h b/llvm/lib/Target/RISCV/RISCVISelLowering.h
index 319c270..340993c 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -31,7 +31,12 @@
   SELECT_CC,
   BuildPairF64,
   SplitF64,
-  TAIL
+  TAIL,
+  // RV64I shifts, directly matching the semantics of the named RISC-V
+  // instructions.
+  SLLW,
+  SRAW,
+  SRLW
 };
 }
 
@@ -57,9 +62,16 @@
 
   // Provide custom lowering hooks for some operations.
   SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const override;
+  void ReplaceNodeResults(SDNode *N, SmallVectorImpl<SDValue> &Results,
+                          SelectionDAG &DAG) const override;
 
   SDValue PerformDAGCombine(SDNode *N, DAGCombinerInfo &DCI) const override;
 
+  unsigned ComputeNumSignBitsForTargetNode(SDValue Op,
+                                           const APInt &DemandedElts,
+                                           const SelectionDAG &DAG,
+                                           unsigned Depth) const override;
+
   // This method returns the name of a target specific DAG node.
   const char *getTargetNodeName(unsigned Opcode) const override;
 
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.td b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
index 3ba86cf..7a874af 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
@@ -51,6 +51,9 @@
 def riscv_tail      : SDNode<"RISCVISD::TAIL", SDT_RISCVCall,
                              [SDNPHasChain, SDNPOptInGlue, SDNPOutGlue,
                               SDNPVariadic]>;
+def riscv_sllw      : SDNode<"RISCVISD::SLLW", SDTIntShiftOp>;
+def riscv_sraw      : SDNode<"RISCVISD::SRAW", SDTIntShiftOp>;
+def riscv_srlw      : SDNode<"RISCVISD::SRLW", SDTIntShiftOp>;
 
 //===----------------------------------------------------------------------===//
 // Operand and SDNode transformation definitions.
@@ -672,21 +675,9 @@
 def assertzexti32 : PatFrag<(ops node:$src), (assertzext node:$src), [{
   return cast<VTSDNode>(N->getOperand(1))->getVT() == MVT::i32;
 }]>;
-def assertzexti5 : PatFrag<(ops node:$src), (assertzext node:$src), [{
-  return cast<VTSDNode>(N->getOperand(1))->getVT().getSizeInBits() <= 5;
-}]>;
 def zexti32 : PatFrags<(ops node:$src),
                        [(and node:$src, 0xffffffff),
                         (assertzexti32 node:$src)]>;
-// Defines a legal mask for (assertzexti5 (and src, mask)) to be combinable
-// with a shiftw operation. The mask mustn't modify the lower 5 bits or the
-// upper 32 bits.
-def shiftwamt_mask : ImmLeaf<XLenVT, [{
-  return countTrailingOnes<uint64_t>(Imm) >= 5 && isUInt<32>(Imm);
-}]>;
-def shiftwamt : PatFrags<(ops node:$src),
-                         [(assertzexti5 (and node:$src, shiftwamt_mask)),
-                          (assertzexti5 node:$src)]>;
 
 /// Immediates
 
@@ -946,28 +937,9 @@
 def : Pat<(sra (sext_inreg GPR:$rs1, i32), uimm5:$shamt),
           (SRAIW GPR:$rs1, uimm5:$shamt)>;
 
-// For variable-length shifts, we rely on assertzexti5 being inserted during
-// lowering (see RISCVTargetLowering::PerformDAGCombine). This enables us to
-// guarantee that selecting a 32-bit variable shift is legal (as the variable
-// shift is known to be <= 32). We must also be careful not to create
-// semantically incorrect patterns. For instance, selecting SRLW for
-// (srl (zexti32 GPR:$rs1), (shiftwamt GPR:$rs2)),
-// is not guaranteed to be safe, as we don't know whether the upper 32-bits of
-// the result are used or not (in the case where rs2=0, this is a
-// sign-extension operation).
-
-def : Pat<(sext_inreg (shl GPR:$rs1, (shiftwamt GPR:$rs2)), i32),
-          (SLLW GPR:$rs1, GPR:$rs2)>;
-def : Pat<(zexti32 (shl GPR:$rs1, (shiftwamt GPR:$rs2))),
-          (SRLI (SLLI (SLLW GPR:$rs1, GPR:$rs2), 32), 32)>;
-
-def : Pat<(sext_inreg (srl (zexti32 GPR:$rs1), (shiftwamt GPR:$rs2)), i32),
-          (SRLW GPR:$rs1, GPR:$rs2)>;
-def : Pat<(zexti32 (srl (zexti32 GPR:$rs1), (shiftwamt GPR:$rs2))),
-          (SRLI (SLLI (SRLW GPR:$rs1, GPR:$rs2), 32), 32)>;
-
-def : Pat<(sra (sexti32 GPR:$rs1), (shiftwamt GPR:$rs2)),
-          (SRAW GPR:$rs1, GPR:$rs2)>;
+def : PatGprGpr<riscv_sllw, SLLW>;
+def : PatGprGpr<riscv_srlw, SRLW>;
+def : PatGprGpr<riscv_sraw, SRAW>;
 
 /// Loads