[lanai] Custom lowering of SHL_PARTS

[jpienaar]

Summary

Implement custom lowering of SHL_PARTS to enable lowering of left shift with larger than 32-bit shifts.

[majnemer]

lib/Target/Lanai/LanaiISelLowering.cpp

1248–1253

It'd be nice if there was more of a correspondence between this pseudo code and the variable names below.

[jpienaar]

Updated the pseudo code variable names to correspond better with actual variable names used.

[majnemer]

LGTM with issues addressed.

test/CodeGen/Lanai/lshift64.ll

5–7

Please check the sequence emitted.

[jpienaar]

Expand test to test lowering of ashldi.

lib/Target/Lanai/LanaiISelLowering.h

public:
   SDValue LowerFRAMEADDR(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerGlobalAddress(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerJumpTable(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerMUL(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerRETURNADDR(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerSELECT_CC(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerSETCC(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerSETCCE(SDValue Op, SelectionDAG &DAG) const;
+  SDValue LowerSHL_PARTS(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerSRL_PARTS(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerVASTART(SDValue Op, SelectionDAG &DAG) const;
 
   unsigned getRegisterByName(const char *RegName, EVT VT,
                              SelectionDAG &DAG) const override;
   std::pair<unsigned, const TargetRegisterClass *>
   getRegForInlineAsmConstraint(const TargetRegisterInfo *TRI,
                                StringRef Constraint, MVT VT) const override;

lib/Target/Lanai/LanaiISelLowering.cpp

LanaiTargetLowering::LanaiTargetLowering(const TargetMachine &TM,
   setOperationAction(ISD::MUL, MVT::i32, Custom);
   setOperationAction(ISD::MULHU, MVT::i32, Expand);
   setOperationAction(ISD::MULHS, MVT::i32, Expand);
   setOperationAction(ISD::UMUL_LOHI, MVT::i32, Expand);
   setOperationAction(ISD::SMUL_LOHI, MVT::i32, Expand);
 
   setOperationAction(ISD::ROTR, MVT::i32, Expand);
   setOperationAction(ISD::ROTL, MVT::i32, Expand);
-  setOperationAction(ISD::SHL_PARTS, MVT::i32, Expand);
+  setOperationAction(ISD::SHL_PARTS, MVT::i32, Custom);
   setOperationAction(ISD::SRL_PARTS, MVT::i32, Custom);
   setOperationAction(ISD::SRA_PARTS, MVT::i32, Expand);
 
   setOperationAction(ISD::BSWAP, MVT::i32, Expand);
   setOperationAction(ISD::CTPOP, MVT::i32, Legal);
   setOperationAction(ISD::CTLZ, MVT::i32, Legal);
   setOperationAction(ISD::CTTZ, MVT::i32, Legal);
 
SDValue LanaiTargetLowering::LowerOperation(SDValue Op,
   case ISD::JumpTable:
     return LowerJumpTable(Op, DAG);
   case ISD::SELECT_CC:
     return LowerSELECT_CC(Op, DAG);
   case ISD::SETCC:
     return LowerSETCC(Op, DAG);
   case ISD::SETCCE:
     return LowerSETCCE(Op, DAG);
+  case ISD::SHL_PARTS:
+    return LowerSHL_PARTS(Op, DAG);
   case ISD::SRL_PARTS:
     return LowerSRL_PARTS(Op, DAG);
   case ISD::VASTART:
     return LowerVASTART(Op, DAG);
   case ISD::DYNAMIC_STACKALLOC:
     return LowerDYNAMIC_STACKALLOC(Op, DAG);
   case ISD::RETURNADDR:
     return LowerRETURNADDR(Op, DAG);
SDValue Lo = DAG.getTargetJumpTable(
         JT->getIndex(), getPointerTy(DAG.getDataLayout()), OpFlagLo);
     Hi = DAG.getNode(LanaiISD::HI, DL, MVT::i32, Hi);
     Lo = DAG.getNode(LanaiISD::LO, DL, MVT::i32, Lo);
     SDValue Result = DAG.getNode(ISD::OR, DL, MVT::i32, Hi, Lo);
     return Result;
   }
 }
 
+SDValue LanaiTargetLowering::LowerSHL_PARTS(SDValue Op,
+                                            SelectionDAG &DAG) const {
+  EVT VT = Op.getValueType();
+  unsigned VTBits = VT.getSizeInBits();
+  SDLoc dl(Op);
+  assert(Op.getNumOperands() == 3 && "Unexpected SHL!");
+  SDValue ShOpLo = Op.getOperand(0);
+  SDValue ShOpHi = Op.getOperand(1);
+  SDValue ShAmt = Op.getOperand(2);
+
+  // Performs the following for (ShOpLo + (ShOpHi << 32)) << ShAmt:
+  //   LoBitsForHi = (ShAmt == 0) ? 0 : (ShOpLo >> (32-ShAmt))
+  //   HiBitsForHi = ShOpHi << ShAmt
+  //   Hi = (ShAmt >= 32) ? (ShOpLo << (ShAmt-32)) : (LoBitsForHi | HiBitsForHi)
+  //   Lo = (ShAmt >= 32) ? 0 : (ShOpLo << ShAmt)
+  //   return (Hi << 32) | Lo;
+
+  SDValue RevShAmt = DAG.getNode(ISD::SUB, dl, MVT::i32,
+                                 DAG.getConstant(VTBits, dl, MVT::i32), ShAmt);
+  SDValue LoBitsForHi = DAG.getNode(ISD::SRL, dl, VT, ShOpLo, RevShAmt);
+
+  // If ShAmt == 0, we just calculated "(SRL ShOpLo, 32)" which is "undef". We
+  // wanted 0, so CSEL it directly.
+  SDValue Zero = DAG.getConstant(0, dl, MVT::i32);
+  SDValue SetCC = DAG.getSetCC(dl, MVT::i32, ShAmt, Zero, ISD::SETEQ);
+  LoBitsForHi = DAG.getSelect(dl, MVT::i32, SetCC, Zero, LoBitsForHi);
+
+  SDValue ExtraShAmt = DAG.getNode(ISD::SUB, dl, MVT::i32, ShAmt,
+                                   DAG.getConstant(VTBits, dl, MVT::i32));
+  SDValue HiBitsForHi = DAG.getNode(ISD::SHL, dl, VT, ShOpHi, ShAmt);
+  SDValue HiForNormalShift =
+      DAG.getNode(ISD::OR, dl, VT, LoBitsForHi, HiBitsForHi);
+
+  SDValue HiForBigShift = DAG.getNode(ISD::SHL, dl, VT, ShOpLo, ExtraShAmt);
+
+  SetCC = DAG.getSetCC(dl, MVT::i32, ExtraShAmt, Zero, ISD::SETGE);
+  SDValue Hi =
+      DAG.getSelect(dl, MVT::i32, SetCC, HiForBigShift, HiForNormalShift);
+
+  // Lanai shifts of larger than register sizes are wrapped rather than
+  // clamped, so we can't just emit "lo << b" if b is too big.
+  SDValue LoForNormalShift = DAG.getNode(ISD::SHL, dl, VT, ShOpLo, ShAmt);
+  SDValue Lo = DAG.getSelect(
+      dl, MVT::i32, SetCC, DAG.getConstant(0, dl, MVT::i32), LoForNormalShift);
+
+  SDValue Ops[2] = {Lo, Hi};
+  return DAG.getMergeValues(Ops, dl);
+}
+
 SDValue LanaiTargetLowering::LowerSRL_PARTS(SDValue Op,
                                             SelectionDAG &DAG) const {
   MVT VT = Op.getSimpleValueType();
   unsigned VTBits = VT.getSizeInBits();
   SDLoc dl(Op);
   SDValue ShOpLo = Op.getOperand(0);
   SDValue ShOpHi = Op.getOperand(1);
   SDValue ShAmt = Op.getOperand(2);

test/CodeGen/Lanai/lshift64.ll

+; RUN: llc < %s -mtriple=lanai-unknown-unknown | FileCheck %s
+
+; Test left-shift i64 lowering does not result in call being inserted.
+
+; CHECK-LABEL: shift
+; CHECK-NOT: bt __ashldi3
+; CHECK: %rv
+define i64 @shift(i64 inreg, i32 inreg) {
+  %3 = zext i32 %1 to i64
+  %4 = shl i64 %0, %3
+  ret i64 %4
+}